[GitHub] [hadoop] mukund-thakur commented on a change in pull request #2587: HADOOP-13327 Output Stream Specification.

[GitBox <gi...@apache.org>]

mukund-thakur commented on a change in pull request #2587:
URL: https://github.com/apache/hadoop/pull/2587#discussion_r572182556



##########
File path: hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/impl/StoreImplementationUtils.java
##########
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+package org.apache.hadoop.fs.impl;
+
+import java.io.InputStream;
+import java.io.OutputStream;
+
+import org.apache.hadoop.classification.InterfaceAudience;
+import org.apache.hadoop.classification.InterfaceStability;
+import org.apache.hadoop.fs.StreamCapabilities;
+
+import static org.apache.hadoop.fs.StreamCapabilities.HFLUSH;
+import static org.apache.hadoop.fs.StreamCapabilities.HSYNC;
+
+/**
+ * Utility classes to help implementing filesystems and streams.
+ */
+@InterfaceAudience.Private
+@InterfaceStability.Unstable
+public final class StoreImplementationUtils {
+
+  private StoreImplementationUtils() {
+  }
+
+  /**
+   * Check the probe capability being for {@link StreamCapabilities#HSYNC}
+   * or {@link StreamCapabilities#HFLUSH}
+   * {@code Syncable.hsync()} and {@code Syncable.hflush()} functionality.
+   * @param capability capability string.
+   * @return true if either refers to one of the Syncable operations.
+   */
+  public static boolean isProbeForSyncable(String capability) {
+    return capability.equalsIgnoreCase(HSYNC) ||
+        capability.equalsIgnoreCase(HFLUSH);
+  }
+
+  /**
+   * Probe for an object having a capability; returns true
+   * if the stream implements {@link StreamCapabilities} and its
+   * {@code hasCapabilities()} method returns true for the capability.
+   * This is a package private method intended to provided a common
+   * implementation for input and output streams.
+   * {@link StreamCapabilities#hasCapability(String)} call is for public use.
+   * @param object object to probe.
+   * @param capability capability to probe for
+   * @return true if the object implements stream capabilities and
+   * declares that it supports the capability.
+   */
+  static boolean objectHasCapability(Object object, String capability) {
+    if (object instanceof StreamCapabilities) {
+      return ((StreamCapabilities) object).hasCapability(capability);
+    }
+    return false;
+  }
+
+  /**
+   * Probe for an output stream having a capability; returns true
+   * if the stream implements {@link StreamCapabilities} and its
+   * {@code hasCapabilities()} method returns true for the capability.
+   * @param out output stream
+   * @param capability capability to probe for
+   * @return true if the stream declares that it supports the capability.
+   */
+  public static boolean hasCapability(OutputStream out, String capability) {
+    return objectHasCapability(out, capability);
+  }
+
+  /**
+   * Probe for an input stream having a capability; returns true
+   * if the stream implements {@link StreamCapabilities} and its
+   * {@code hasCapabilities()} method returns true for the capability.
+   * @param out output stream

Review comment:
       input stream

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
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+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions

Review comment:
       are the actions?

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.
+The `Syncable` operations differ in their durability
+guarantees, not visibility of data.
+
+### State of Stream and File System after `Filesystem.create()`
+
+The output stream returned by a `FileSystem.create(path)` or
+`FileSystem.createFile(path).build()` within a filesystem `FS`,
+can be modeled as a triple containing an empty array of no data:
+
+```python
+Stream' = (path, true, [])
+```
+
+The filesystem `FS'` MUST contain a 0-byte file at the path:
+
+```python
+FS' = FS where data(FS', path) == []
+```
+
+Thus, the initial state of `Stream'.buffer` is implicitly
+consistent with the data at the filesystem.
+
+
+*Object Stores*: see caveats in the "Object Stores" section below.
+
+### State of Stream and File System after `Filesystem.append()`
+
+The output stream returned from a call of
+ `FileSystem.append(path, buffersize, progress)` within a filesystem `FS`,
+can be modelled as a stream whose `buffer` is intialized to that of
+the original file:
+
+```python
+Stream' = (path, true, data(FS, path))
+```
+
+####  Persisting data
+
+When the stream writes data back to its store, be it in any
+supported flush operation, in the `close()` operation, or at any other
+time the stream chooses to do so, the contents of the file
+are replaced with the current buffer
+
+```python
+Stream' = (path, true, buffer)
+FS' = FS where data(FS', path) == buffer
+```
+
+After a call to `close()`, the stream is closed for all operations other
+than `close()`; they MAY fail with `IOException` or `RuntimeException`.
+
+```python
+Stream' = (path, false, [])
+```
+
+The `close()` operation MUST be idempotent with the sole attempt to write the
+data made in the first invocation.
+
+1. If `close()` succeeds, subsequent calls are no-ops.
+1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow
+the previous exception, but they MUST NOT retry the write.
+
+<!--  ============================================================= -->
+<!--  CLASS: FSDataOutputStream -->
+<!--  ============================================================= -->
+
+## <a name="fsdataoutputstream"></a>Class `FSDataOutputStream`
+
+```java
+public class FSDataOutputStream
+  extends DataOutputStream
+  implements Syncable, CanSetDropBehind, StreamCapabilities {
+ // ...
+}
+```
+
+The `FileSystem.create()`, `FileSystem.append()` and
+`FSDataOutputStreamBuilder.build()` calls return an instance
+of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`.
+
+The base class wraps an `OutputStream` instance, one which may implement `Streamable`,

Review comment:
       Syncable not Streamable right?

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.
+The `Syncable` operations differ in their durability
+guarantees, not visibility of data.
+
+### State of Stream and File System after `Filesystem.create()`
+
+The output stream returned by a `FileSystem.create(path)` or
+`FileSystem.createFile(path).build()` within a filesystem `FS`,
+can be modeled as a triple containing an empty array of no data:
+
+```python
+Stream' = (path, true, [])
+```
+
+The filesystem `FS'` MUST contain a 0-byte file at the path:
+
+```python
+FS' = FS where data(FS', path) == []
+```
+
+Thus, the initial state of `Stream'.buffer` is implicitly
+consistent with the data at the filesystem.
+
+
+*Object Stores*: see caveats in the "Object Stores" section below.
+
+### State of Stream and File System after `Filesystem.append()`
+
+The output stream returned from a call of
+ `FileSystem.append(path, buffersize, progress)` within a filesystem `FS`,
+can be modelled as a stream whose `buffer` is intialized to that of
+the original file:
+
+```python
+Stream' = (path, true, data(FS, path))
+```
+
+####  Persisting data
+
+When the stream writes data back to its store, be it in any
+supported flush operation, in the `close()` operation, or at any other
+time the stream chooses to do so, the contents of the file
+are replaced with the current buffer
+
+```python
+Stream' = (path, true, buffer)
+FS' = FS where data(FS', path) == buffer
+```
+
+After a call to `close()`, the stream is closed for all operations other
+than `close()`; they MAY fail with `IOException` or `RuntimeException`.
+
+```python
+Stream' = (path, false, [])
+```
+
+The `close()` operation MUST be idempotent with the sole attempt to write the
+data made in the first invocation.
+
+1. If `close()` succeeds, subsequent calls are no-ops.
+1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow
+the previous exception, but they MUST NOT retry the write.
+
+<!--  ============================================================= -->
+<!--  CLASS: FSDataOutputStream -->
+<!--  ============================================================= -->
+
+## <a name="fsdataoutputstream"></a>Class `FSDataOutputStream`
+
+```java
+public class FSDataOutputStream
+  extends DataOutputStream
+  implements Syncable, CanSetDropBehind, StreamCapabilities {
+ // ...
+}
+```
+
+The `FileSystem.create()`, `FileSystem.append()` and
+`FSDataOutputStreamBuilder.build()` calls return an instance
+of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`.
+
+The base class wraps an `OutputStream` instance, one which may implement `Streamable`,
+`CanSetDropBehind` and `StreamCapabilities`.
+
+This document covers the requirements of such implementations.
+
+HDFS's `FileSystem` implementation, `DistributedFileSystem`, returns an instance
+of `HdfsDataOutputStream`. This implementation has at least two behaviors
+which are not explicitly declared by the base Java implmentation
+
+1. Writes are synchronized: more than one thread can write to the same
+output stream. This is a use pattern which HBase relies on.
+
+1. `OutputStream.flush()` is a no-op when the file is closed. Apache Druid
+has made such a call on this in the past
+[HADOOP-14346](https://issues.apache.org/jira/browse/HADOOP-14346).
+
+
+As the HDFS implementation is considered the de-facto specification of
+the FileSystem APIs, the fact that `write()` is thread-safe is significant.
+
+For compatibility, not only SHOULD other FS clients be thread-safe,
+but new HDFS features, such as encryption and Erasure Coding SHOULD also
+implement consistent behavior with the core HDFS output stream.
+
+Put differently:
+
+*It isn't enough for Output Streams to implement the core semantics
+of `java.io.OutputStream`: they need to implement the extra semantics
+of `HdfsDataOutputStream`, especially for HBase to work correctly.*
+
+The concurrent `write()` call is the most significant tightening of
+the Java specification.
+
+## <a name="outputstream"></a>Class `java.io.OutputStream`
+
+A Java `OutputStream` allows applications to write a sequence of bytes to a destination.
+In a Hadoop filesystem, that destination is the data under a path in the filesystem.
+
+```java
+public abstract class OutputStream implements Closeable, Flushable {
+  public abstract void write(int b) throws IOException;
+  public void write(byte b[]) throws IOException;
+  public void write(byte b[], int off, int len) throws IOException;
+  public void flush() throws IOException;
+  public void close() throws IOException;
+}
+```
+### <a name="write(data: int)"></a>`write(Stream, data)`
+
+Writes a byte of data to the stream.
+
+#### Preconditions
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+```
+
+The exception `java.nio.channels.ClosedChannelExceptionn` is
+raised in the HDFS output streams when trying to write to a closed file.
+This exception does not include the destination path; and
+`Exception.getMessage()` is `null`. It is therefore of limited value in stack
+traces. Implementors may wish to raise exceptions with more detail, such
+as a `PathIOException`.
+
+
+#### Postconditions
+
+The buffer has the lower 8 bits of the data argument appended to it.
+
+```python
+Stream'.buffer = Stream.buffer + [data & 0xff]
+```
+
+There may be an explicit limit on the size of cached data, or an implicit
+limit based by the available capacity of the destination filesystem.
+When a limit is reached, `write()` SHOULD fail with an `IOException`.
+
+### <a name="write(buffer,offset,len)"></a>`write(Stream, byte[] data, int offset, int len)`
+
+
+#### Preconditions
+
+The preconditions are all defined in `OutputStream.write()`
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+data != null else raise NullPointerException
+offset >= 0 else raise IndexOutOfBoundsException
+len >= 0 else raise IndexOutOfBoundsException
+offset < data.length else raise IndexOutOfBoundsException
+offset + len < data.length else raise IndexOutOfBoundsException
+```
+
+After the operation has returned, the buffer may be re-used. The outcome
+of updates to the buffer while the `write()` operation is in progress is undefined.
+
+#### Postconditions
+
+```python
+Stream'.buffer = Stream.buffer + data[offset...(offset + len)]
+```
+
+### <a name="write(buffer)"></a>`write(byte[] data)`
+
+This is defined as the equivalent of:
+
+```python
+write(data, 0, data.length)
+```
+
+### <a name="flush()"></a>`flush()`
+
+Requests that the data is flushed. The specification of `ObjectStream.flush()`
+declares that this SHOULD write data to the "intended destination".
+
+It explicitly precludes any guarantees about durability.
+
+For that reason, this document doesn't provide any normative
+specifications of behaviour.
+
+#### Preconditions
+
+None.
+
+#### Postconditions
+
+None.
+
+If the implementation chooses to implement a stream-flushing operation,
+the data may be saved to the file system such that it becomes visible to
+others"
+
+```python
+FS' = FS where data(FS', path) == buffer
+```
+
+When a stream is closed, `flush()` SHOULD downgrade to being a no-op, if it was not
+one already. This is to work with applications and libraries which can invoke
+it in exactly this way.
+
+
+*Issue*: Should `flush()` forward to `hflush()`?
+
+No. Or at least, make it optional.
+
+There's a lot of application code which assumes that `flush()` is low cost
+and should be invoked after writing every single line of output, after
+writing small 4KB blocks or similar.
+
+Forwarding this to a full flush across a distributed filesystem, or worse,
+a distant object store, is very inefficient.
+Filesystem clients which do uprate a `flush()` to an `hflush()` will eventually
+have to roll back that feature: 
+[HADOOP-16548](https://issues.apache.org/jira/browse/HADOOP-16548).
+
+### <a name="close"></a>`close()`
+
+The `close()` operation saves all data to the filesystem and
+releases any resources used for writing data.
+
+The `close()` call is expected to block
+until the write has completed (as with `Syncable.hflush()`), possibly
+until it has been written to durable storage.
+
+After `close()` completes, the data in a file MUST be visible and consistent
+with the data most recently written. The metadata of the file MUST be consistent
+with the data and the write history itself (i.e. any modification time fields
+updated).
+
+After `close()` is invoked, all subsequent `write()` calls on the stream
+MUST fail with an `IOException`.
+
+Any locking/leaseholding mechanism MUST release its lock/lease.
+
+```python
+Stream'.open = false
+FS' = FS where data(FS', path) == buffer
+```
+
+The `close()` call MAY fail during its operation.
+
+1. Callers of the API MUST expect for some calls to  `close()` to fail and SHOULD code appropriately.
+Catching and swallowing exceptions, while common, is not always the ideal solution.
+1. Even after a failure, `close()` MUST place the stream into a closed state.
+Follow-on calls to `close()` are ignored, and calls to other methods
+rejected. That is: caller's cannot be expected to call `close()` repeatedly
+until it succeeds.
+1. The duration of the `call()` operation is undefined. Operations which rely
+on acknowledgements from remote systems to meet the persistence guarantees
+implicitly have to await these acknowledgements. Some Object Store output streams
+upload the entire data file in the `close()` operation. This can take a large amount
+of time. The fact that many user applications assume that `close()` is both fast
+and does not fail means that this behavior is dangerous.
+
+Recommendations for safe use by callers
+
+* Do plan for exceptions being raised, either in catching and logging or
+by throwing the exception further up. Catching and silently swallowing exceptions
+may hide serious problems.
+* Heartbeat operations SHOULD take place on a separate thread, so that a long
+delay in `close()` does not block the thread so long that the heartbeat times
+out.
+
+Implementors:
+
+* Have a look at [HADOOP-16785](https://issues.apache.org/jira/browse/HADOOP-16785)
+to see examples of complications in close.
+* Incrementally writing blocks before a close operation results in a behavior which
+matches client expectations better: write failures to surface earlier and close
+to be more housekeeping than the actual upload.
+* If block uploads are executed in separate threads, the output stream `close()`
+call MUST block until all the asynchronous uploads have completed; any error raised
+MUST be reported.
+If multiple errors were raised, the stream can choose which to propagate.
+What is important is: when `close()` returns without an error, applications expect
+the data to have been successfully written.
+
+### HDFS and `OutputStream.close()`
+
+HDFS does not immediately `sync()` the output of a written file to disk on
+`OutputStream.close()` unless configured with `dfs.datanode.synconclose` 
+is true. This has caused [problems in some applications](https://issues.apache.org/jira/browse/ACCUMULO-1364).
+
+Applications which absolutely require the guarantee that a file has been persisted
+MUST call `Syncable.hsync()` *before* the file is closed.
+
+
+## <a name="syncable"></a>`org.apache.hadoop.fs.Syncable`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Stable
+public interface Syncable {
+
+
+  /** Flush out the data in client's user buffer. After the return of
+   * this call, new readers will see the data.
+   * @throws IOException if any error occurs
+   */
+  void hflush() throws IOException;
+
+  /** Similar to posix fsync, flush out the data in client's user buffer
+   * all the way to the disk device (but the disk may have it in its cache).
+   * @throws IOException if error occurs
+   */
+  void hsync() throws IOException;
+}
+```
+
+The purpose of `Syncable` interface is to provide guarantees that data is written
+to a filesystem for both visibility and durability.
+
+*SYNC-1*: An `OutputStream` which implements `Syncable` is
+making an explicit declaration that it can meet those guarantees.
+
+*SYNC-2*: The interface MUST NOT be declared as implemented by an `OutputStream` unless
+those guarantees can be met.
+
+The `Syncable` interface has been implemented by other classes than
+subclasses of `OutputStream`, such as `org.apache.hadoop.io.SequenceFile.Writer`.
+
+*SYNC-3* The fact that a class implements `Syncable` does not guarantee
+that `extends OutputStream` holds.
+
+That is, for any class `C`: `(C instanceof Syncable)` does not imply
+`(C instanceof OutputStream)`
+
+This specification only covers the required behavior of `OutputStream` subclasses
+which implement `Syncable`.
+
+
+*SYNC-4:* The return value of `FileSystem.create(Path)` is an instance
+of `FSDataOutputStream`.
+
+*SYNC-5:* `FSDataOutputStream implements Syncable`
+
+
+SYNC-5 and SYNC-1 imply that all output streams which can be created
+with `FileSystem.create(Path)` must support the semantics of `Syncable`.
+This is demonstrably not true: `FSDataOutputStream` simply downgrades
+to a `flush()` if its wrapped stream is not `Syncable`.
+Therefore the declarations SYNC-1 and SYNC-2 do not hold: you cannot trust `Syncable`.
+
+Put differently: *callers MUST NOT rely on the presence of the interface
+as evidence that the semantics of `Syncable` are supported*. Instead
+they MUST be dynamically probed for using the `StreamCapabilities`
+interface, where available.
+
+
+### <a name="syncable.hflush"></a>`Syncable.hflush()`
+
+Flush out the data in client's user buffer. After the return of
+this call, new readers will see the data. The `hflush()` operation
+does not contain any guarantees as to the durability of the data. only
+its visibility.
+
+Thus implementations may cache the written data in memory
+—visible to all, but not yet persisted.
+
+#### Preconditions
+
+```python
+hasCapability(Stream, "hflush")
+Stream.open else raise IOException
+```
+
+
+#### Postconditions
+
+```python
+FS' = FS where data(path) == cache
+```
+
+
+After the call returns, the data MUST be visible to all new callers
+of `FileSystem.open(path)` and `FileSystem.openFile(path).build()`.
+
+There is no requirement or guarantee that clients with an existing
+`DataInputStream` created by a call to `(FS, path)` will see the updated
+data, nor is there a guarantee that they *will not* in a current or subsequent
+read.
+
+Implementation note: as a correct `hsync()` implementation MUST also
+offer all the semantics of an `hflush()` call, implementations of `hflush()`
+may just invoke `hsync()`:
+
+```java
+public void hflush() throws IOException {
+  hsync();
+}
+```
+
+#### `hflush()` Performance
+
+The `hflush()` call MUST block until the store has acknowledge that the
+data has been received and is now visible to others. This can be slow,
+as it will include the time to upload any outstanding data from the
+client, and for the filesystem itself to process it.
+
+Often Filesystems only offer the `Syncable.hsync()` guarantees: persistence as
+well as visibility. This means the time to return can be even greater.
+
+Application code MUST NOT call `hflush()` or `hsync()` at the end of every line
+or, unless they are writing a WAL, at the end of every record. Use with care.
+
+
+### <a name="syncable.hsync"></a> `Syncable.hsync()`
+
+Similar to POSIX `fsync()`, this call saves the data in client's user buffer
+all the way to the disk device (but the disk may have it in its cache).
+
+That is: it is a requirement for the underlying FS To save all the data to
+the disk hardware itself, where it is expected to be durable.
+
+#### Preconditions
+
+```python
+hasCapability(Stream, "hsync")
+Stream.open else raise IOException
+```
+
+#### Postconditions
+
+```python
+FS' = FS where data(path) == buffer
+```
+
+_Implementations are required to block until that write has been
+acknowledged by the store._
+
+This is so the caller can be confident that once the call has
+returned successfully, the data has been written.
+
+
+
+## <a name="streamcapabilities"></a>Interface `StreamCapabilities`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Evolving
+```
+
+The `org.apache.hadoop.fs.StreamCapabilities` interface exists to allow callers to dynamically
+determine the behavior of a stream.
+
+```java
+  public boolean hasCapability(String capability) {
+    switch (capability.toLowerCase(Locale.ENGLISH)) {
+      case StreamCapabilities.HSYNC:
+      case StreamCapabilities.HFLUSH:
+        return supportFlush;
+      default:
+        return false;
+    }
+  }
+```
+
+Once a stream has been closed, a `hasCapability()` call MUST do one of
+
+* return the capabilities of the open stream.
+* return false.
+
+That is: it MUST NOT raise an exception about the file being closed;
+
+See [pathcapabilities](pathcapabilities.html) for specifics on the `PathCapabilities` API;
+the requirements are similar: a stream MUST NOT return true for a capability
+for which it lacks support, be it because
+
+* The capability is unknown.
+* The capability is known and known to be unsupported.
+
+Standard stream capabilities are defined in `StreamCapabilities`;
+consult the javadocs for the complete set of options.
+
+| Name  | Probes for support of |
+|-------|---------|
+| `dropbehind` | `CanSetDropBehind.setDropBehind()` |
+| `hsync` | `Syncable.hsync()` |
+| `hflush` | `Syncable.hflush()`. Deprecated: probe for `HSYNC` only. |
+| `in:readahead` | `CanSetReadahead.setReadahead()` |
+| `in:unbuffer"` | `CanUnbuffer.unbuffer()` |
+| `in:readbytebuffer` | `ByteBufferReadable#read(ByteBuffer)` |
+| `in:preadbytebuffer` | `ByteBufferPositionedReadable#read(long, ByteBuffer)` |
+
+Stream implementations MAY add their own custom options.
+These MUST be prefixed with `fs.SCHEMA.`, where `SCHEMA` is the schema of the filesystem.
+
+## <a name="cansetdropbehind"></a> interface `CanSetDropBehind`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Evolving
+public interface CanSetDropBehind {
+  /**
+   * Configure whether the stream should drop the cache.
+   *
+   * @param dropCache     Whether to drop the cache.  null means to use the
+   *                      default value.
+   * @throws IOException  If there was an error changing the dropBehind
+   *                      setting.
+   *         UnsupportedOperationException  If this stream doesn't support
+   *                                        setting the drop-behind.
+   */
+  void setDropBehind(Boolean dropCache)
+      throws IOException, UnsupportedOperationException;
+}
+```
+
+This interface allows callers to change policies used inside HDFS.
+
+Implementations MUST return `true` for the call
+
+```java
+StreamCapabilities.hasCapability("dropbehind");
+```
+
+
+## <a name="durability-of-output"></a>Durability, Concurrency, Consistency and Visibility of stream output.
+
+These are the aspects of the system behaviour which are not directly
+covered in this (very simplistic) filesystem model, but which are visible
+in production.
+
+
+### <a name="durability"></a> Durability
+
+1. `OutputStream.write()` MAY persist the data, synchronously or asynchronously
+1. `OutputStream.flush()` flushes data to the destination. There
+are no strict persistence requirements.
+1. `Syncable.hflush()` synchronously sends all outstaning data to the destination
+filesystem. After returning to the caller, the data MUST be visible to other readers,
+it MAY be durable. That is: it does not have to be persisted, merely guaranteed
+to be consistently visible to all clients attempting to open a new stream reading
+data at the path.
+1. `Syncable.hsync()` MUST transmit the data as per `hflush` the data and persist

Review comment:
       remove the second "the data" ?

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.
+The `Syncable` operations differ in their durability
+guarantees, not visibility of data.
+
+### State of Stream and File System after `Filesystem.create()`
+
+The output stream returned by a `FileSystem.create(path)` or
+`FileSystem.createFile(path).build()` within a filesystem `FS`,
+can be modeled as a triple containing an empty array of no data:
+
+```python
+Stream' = (path, true, [])
+```
+
+The filesystem `FS'` MUST contain a 0-byte file at the path:
+
+```python
+FS' = FS where data(FS', path) == []
+```
+
+Thus, the initial state of `Stream'.buffer` is implicitly
+consistent with the data at the filesystem.
+
+
+*Object Stores*: see caveats in the "Object Stores" section below.
+
+### State of Stream and File System after `Filesystem.append()`
+
+The output stream returned from a call of
+ `FileSystem.append(path, buffersize, progress)` within a filesystem `FS`,
+can be modelled as a stream whose `buffer` is intialized to that of
+the original file:
+
+```python
+Stream' = (path, true, data(FS, path))
+```
+
+####  Persisting data
+
+When the stream writes data back to its store, be it in any
+supported flush operation, in the `close()` operation, or at any other
+time the stream chooses to do so, the contents of the file
+are replaced with the current buffer
+
+```python
+Stream' = (path, true, buffer)
+FS' = FS where data(FS', path) == buffer
+```
+
+After a call to `close()`, the stream is closed for all operations other
+than `close()`; they MAY fail with `IOException` or `RuntimeException`.
+
+```python
+Stream' = (path, false, [])
+```
+
+The `close()` operation MUST be idempotent with the sole attempt to write the
+data made in the first invocation.
+
+1. If `close()` succeeds, subsequent calls are no-ops.
+1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow
+the previous exception, but they MUST NOT retry the write.
+
+<!--  ============================================================= -->
+<!--  CLASS: FSDataOutputStream -->
+<!--  ============================================================= -->
+
+## <a name="fsdataoutputstream"></a>Class `FSDataOutputStream`
+
+```java
+public class FSDataOutputStream
+  extends DataOutputStream
+  implements Syncable, CanSetDropBehind, StreamCapabilities {
+ // ...
+}
+```
+
+The `FileSystem.create()`, `FileSystem.append()` and
+`FSDataOutputStreamBuilder.build()` calls return an instance
+of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`.
+
+The base class wraps an `OutputStream` instance, one which may implement `Streamable`,
+`CanSetDropBehind` and `StreamCapabilities`.
+
+This document covers the requirements of such implementations.
+
+HDFS's `FileSystem` implementation, `DistributedFileSystem`, returns an instance
+of `HdfsDataOutputStream`. This implementation has at least two behaviors
+which are not explicitly declared by the base Java implmentation
+
+1. Writes are synchronized: more than one thread can write to the same
+output stream. This is a use pattern which HBase relies on.
+
+1. `OutputStream.flush()` is a no-op when the file is closed. Apache Druid
+has made such a call on this in the past
+[HADOOP-14346](https://issues.apache.org/jira/browse/HADOOP-14346).
+
+
+As the HDFS implementation is considered the de-facto specification of
+the FileSystem APIs, the fact that `write()` is thread-safe is significant.
+
+For compatibility, not only SHOULD other FS clients be thread-safe,
+but new HDFS features, such as encryption and Erasure Coding SHOULD also
+implement consistent behavior with the core HDFS output stream.
+
+Put differently:
+
+*It isn't enough for Output Streams to implement the core semantics
+of `java.io.OutputStream`: they need to implement the extra semantics
+of `HdfsDataOutputStream`, especially for HBase to work correctly.*
+
+The concurrent `write()` call is the most significant tightening of
+the Java specification.
+
+## <a name="outputstream"></a>Class `java.io.OutputStream`
+
+A Java `OutputStream` allows applications to write a sequence of bytes to a destination.
+In a Hadoop filesystem, that destination is the data under a path in the filesystem.
+
+```java
+public abstract class OutputStream implements Closeable, Flushable {
+  public abstract void write(int b) throws IOException;
+  public void write(byte b[]) throws IOException;
+  public void write(byte b[], int off, int len) throws IOException;
+  public void flush() throws IOException;
+  public void close() throws IOException;
+}
+```
+### <a name="write(data: int)"></a>`write(Stream, data)`
+
+Writes a byte of data to the stream.
+
+#### Preconditions
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+```
+
+The exception `java.nio.channels.ClosedChannelExceptionn` is
+raised in the HDFS output streams when trying to write to a closed file.
+This exception does not include the destination path; and
+`Exception.getMessage()` is `null`. It is therefore of limited value in stack
+traces. Implementors may wish to raise exceptions with more detail, such
+as a `PathIOException`.
+
+
+#### Postconditions
+
+The buffer has the lower 8 bits of the data argument appended to it.
+
+```python
+Stream'.buffer = Stream.buffer + [data & 0xff]
+```
+
+There may be an explicit limit on the size of cached data, or an implicit
+limit based by the available capacity of the destination filesystem.
+When a limit is reached, `write()` SHOULD fail with an `IOException`.
+
+### <a name="write(buffer,offset,len)"></a>`write(Stream, byte[] data, int offset, int len)`
+
+
+#### Preconditions
+
+The preconditions are all defined in `OutputStream.write()`
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+data != null else raise NullPointerException
+offset >= 0 else raise IndexOutOfBoundsException
+len >= 0 else raise IndexOutOfBoundsException
+offset < data.length else raise IndexOutOfBoundsException
+offset + len < data.length else raise IndexOutOfBoundsException
+```
+
+After the operation has returned, the buffer may be re-used. The outcome
+of updates to the buffer while the `write()` operation is in progress is undefined.
+
+#### Postconditions
+
+```python
+Stream'.buffer = Stream.buffer + data[offset...(offset + len)]
+```
+
+### <a name="write(buffer)"></a>`write(byte[] data)`
+
+This is defined as the equivalent of:
+
+```python
+write(data, 0, data.length)
+```
+
+### <a name="flush()"></a>`flush()`
+
+Requests that the data is flushed. The specification of `ObjectStream.flush()`
+declares that this SHOULD write data to the "intended destination".
+
+It explicitly precludes any guarantees about durability.
+
+For that reason, this document doesn't provide any normative
+specifications of behaviour.
+
+#### Preconditions
+
+None.
+
+#### Postconditions
+
+None.
+
+If the implementation chooses to implement a stream-flushing operation,
+the data may be saved to the file system such that it becomes visible to
+others"
+
+```python
+FS' = FS where data(FS', path) == buffer
+```
+
+When a stream is closed, `flush()` SHOULD downgrade to being a no-op, if it was not
+one already. This is to work with applications and libraries which can invoke
+it in exactly this way.
+
+
+*Issue*: Should `flush()` forward to `hflush()`?
+
+No. Or at least, make it optional.
+
+There's a lot of application code which assumes that `flush()` is low cost
+and should be invoked after writing every single line of output, after
+writing small 4KB blocks or similar.
+
+Forwarding this to a full flush across a distributed filesystem, or worse,
+a distant object store, is very inefficient.
+Filesystem clients which do uprate a `flush()` to an `hflush()` will eventually
+have to roll back that feature: 
+[HADOOP-16548](https://issues.apache.org/jira/browse/HADOOP-16548).
+
+### <a name="close"></a>`close()`
+
+The `close()` operation saves all data to the filesystem and
+releases any resources used for writing data.
+
+The `close()` call is expected to block
+until the write has completed (as with `Syncable.hflush()`), possibly
+until it has been written to durable storage.
+
+After `close()` completes, the data in a file MUST be visible and consistent
+with the data most recently written. The metadata of the file MUST be consistent
+with the data and the write history itself (i.e. any modification time fields
+updated).
+
+After `close()` is invoked, all subsequent `write()` calls on the stream
+MUST fail with an `IOException`.
+
+Any locking/leaseholding mechanism MUST release its lock/lease.
+
+```python
+Stream'.open = false
+FS' = FS where data(FS', path) == buffer
+```
+
+The `close()` call MAY fail during its operation.
+
+1. Callers of the API MUST expect for some calls to  `close()` to fail and SHOULD code appropriately.
+Catching and swallowing exceptions, while common, is not always the ideal solution.
+1. Even after a failure, `close()` MUST place the stream into a closed state.
+Follow-on calls to `close()` are ignored, and calls to other methods
+rejected. That is: caller's cannot be expected to call `close()` repeatedly
+until it succeeds.
+1. The duration of the `call()` operation is undefined. Operations which rely
+on acknowledgements from remote systems to meet the persistence guarantees
+implicitly have to await these acknowledgements. Some Object Store output streams
+upload the entire data file in the `close()` operation. This can take a large amount
+of time. The fact that many user applications assume that `close()` is both fast
+and does not fail means that this behavior is dangerous.
+
+Recommendations for safe use by callers
+
+* Do plan for exceptions being raised, either in catching and logging or
+by throwing the exception further up. Catching and silently swallowing exceptions
+may hide serious problems.
+* Heartbeat operations SHOULD take place on a separate thread, so that a long
+delay in `close()` does not block the thread so long that the heartbeat times
+out.
+
+Implementors:
+
+* Have a look at [HADOOP-16785](https://issues.apache.org/jira/browse/HADOOP-16785)
+to see examples of complications in close.
+* Incrementally writing blocks before a close operation results in a behavior which
+matches client expectations better: write failures to surface earlier and close
+to be more housekeeping than the actual upload.
+* If block uploads are executed in separate threads, the output stream `close()`
+call MUST block until all the asynchronous uploads have completed; any error raised
+MUST be reported.
+If multiple errors were raised, the stream can choose which to propagate.
+What is important is: when `close()` returns without an error, applications expect
+the data to have been successfully written.
+
+### HDFS and `OutputStream.close()`
+
+HDFS does not immediately `sync()` the output of a written file to disk on
+`OutputStream.close()` unless configured with `dfs.datanode.synconclose` 
+is true. This has caused [problems in some applications](https://issues.apache.org/jira/browse/ACCUMULO-1364).
+
+Applications which absolutely require the guarantee that a file has been persisted
+MUST call `Syncable.hsync()` *before* the file is closed.
+
+
+## <a name="syncable"></a>`org.apache.hadoop.fs.Syncable`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Stable
+public interface Syncable {
+
+
+  /** Flush out the data in client's user buffer. After the return of
+   * this call, new readers will see the data.
+   * @throws IOException if any error occurs
+   */
+  void hflush() throws IOException;
+
+  /** Similar to posix fsync, flush out the data in client's user buffer
+   * all the way to the disk device (but the disk may have it in its cache).
+   * @throws IOException if error occurs
+   */
+  void hsync() throws IOException;
+}
+```
+
+The purpose of `Syncable` interface is to provide guarantees that data is written
+to a filesystem for both visibility and durability.
+
+*SYNC-1*: An `OutputStream` which implements `Syncable` is
+making an explicit declaration that it can meet those guarantees.
+
+*SYNC-2*: The interface MUST NOT be declared as implemented by an `OutputStream` unless
+those guarantees can be met.
+
+The `Syncable` interface has been implemented by other classes than
+subclasses of `OutputStream`, such as `org.apache.hadoop.io.SequenceFile.Writer`.
+
+*SYNC-3* The fact that a class implements `Syncable` does not guarantee
+that `extends OutputStream` holds.
+
+That is, for any class `C`: `(C instanceof Syncable)` does not imply
+`(C instanceof OutputStream)`
+
+This specification only covers the required behavior of `OutputStream` subclasses
+which implement `Syncable`.
+
+
+*SYNC-4:* The return value of `FileSystem.create(Path)` is an instance
+of `FSDataOutputStream`.
+
+*SYNC-5:* `FSDataOutputStream implements Syncable`
+
+
+SYNC-5 and SYNC-1 imply that all output streams which can be created
+with `FileSystem.create(Path)` must support the semantics of `Syncable`.
+This is demonstrably not true: `FSDataOutputStream` simply downgrades
+to a `flush()` if its wrapped stream is not `Syncable`.
+Therefore the declarations SYNC-1 and SYNC-2 do not hold: you cannot trust `Syncable`.
+
+Put differently: *callers MUST NOT rely on the presence of the interface
+as evidence that the semantics of `Syncable` are supported*. Instead
+they MUST be dynamically probed for using the `StreamCapabilities`
+interface, where available.
+
+
+### <a name="syncable.hflush"></a>`Syncable.hflush()`
+
+Flush out the data in client's user buffer. After the return of
+this call, new readers will see the data. The `hflush()` operation
+does not contain any guarantees as to the durability of the data. only
+its visibility.
+
+Thus implementations may cache the written data in memory
+—visible to all, but not yet persisted.
+
+#### Preconditions
+
+```python
+hasCapability(Stream, "hflush")
+Stream.open else raise IOException
+```
+
+
+#### Postconditions
+
+```python
+FS' = FS where data(path) == cache
+```
+
+
+After the call returns, the data MUST be visible to all new callers
+of `FileSystem.open(path)` and `FileSystem.openFile(path).build()`.
+
+There is no requirement or guarantee that clients with an existing
+`DataInputStream` created by a call to `(FS, path)` will see the updated
+data, nor is there a guarantee that they *will not* in a current or subsequent
+read.
+
+Implementation note: as a correct `hsync()` implementation MUST also
+offer all the semantics of an `hflush()` call, implementations of `hflush()`
+may just invoke `hsync()`:
+
+```java
+public void hflush() throws IOException {
+  hsync();
+}
+```
+
+#### `hflush()` Performance
+
+The `hflush()` call MUST block until the store has acknowledge that the
+data has been received and is now visible to others. This can be slow,
+as it will include the time to upload any outstanding data from the
+client, and for the filesystem itself to process it.
+
+Often Filesystems only offer the `Syncable.hsync()` guarantees: persistence as
+well as visibility. This means the time to return can be even greater.
+
+Application code MUST NOT call `hflush()` or `hsync()` at the end of every line
+or, unless they are writing a WAL, at the end of every record. Use with care.
+
+
+### <a name="syncable.hsync"></a> `Syncable.hsync()`
+
+Similar to POSIX `fsync()`, this call saves the data in client's user buffer
+all the way to the disk device (but the disk may have it in its cache).
+
+That is: it is a requirement for the underlying FS To save all the data to
+the disk hardware itself, where it is expected to be durable.
+
+#### Preconditions
+
+```python
+hasCapability(Stream, "hsync")
+Stream.open else raise IOException
+```
+
+#### Postconditions
+
+```python
+FS' = FS where data(path) == buffer
+```
+
+_Implementations are required to block until that write has been
+acknowledged by the store._
+
+This is so the caller can be confident that once the call has
+returned successfully, the data has been written.
+
+
+
+## <a name="streamcapabilities"></a>Interface `StreamCapabilities`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Evolving
+```
+
+The `org.apache.hadoop.fs.StreamCapabilities` interface exists to allow callers to dynamically
+determine the behavior of a stream.
+
+```java
+  public boolean hasCapability(String capability) {
+    switch (capability.toLowerCase(Locale.ENGLISH)) {
+      case StreamCapabilities.HSYNC:
+      case StreamCapabilities.HFLUSH:
+        return supportFlush;
+      default:
+        return false;
+    }
+  }
+```
+
+Once a stream has been closed, a `hasCapability()` call MUST do one of
+
+* return the capabilities of the open stream.
+* return false.
+
+That is: it MUST NOT raise an exception about the file being closed;
+
+See [pathcapabilities](pathcapabilities.html) for specifics on the `PathCapabilities` API;
+the requirements are similar: a stream MUST NOT return true for a capability
+for which it lacks support, be it because
+
+* The capability is unknown.
+* The capability is known and known to be unsupported.
+
+Standard stream capabilities are defined in `StreamCapabilities`;
+consult the javadocs for the complete set of options.
+
+| Name  | Probes for support of |
+|-------|---------|
+| `dropbehind` | `CanSetDropBehind.setDropBehind()` |
+| `hsync` | `Syncable.hsync()` |
+| `hflush` | `Syncable.hflush()`. Deprecated: probe for `HSYNC` only. |
+| `in:readahead` | `CanSetReadahead.setReadahead()` |
+| `in:unbuffer"` | `CanUnbuffer.unbuffer()` |
+| `in:readbytebuffer` | `ByteBufferReadable#read(ByteBuffer)` |
+| `in:preadbytebuffer` | `ByteBufferPositionedReadable#read(long, ByteBuffer)` |
+
+Stream implementations MAY add their own custom options.
+These MUST be prefixed with `fs.SCHEMA.`, where `SCHEMA` is the schema of the filesystem.
+
+## <a name="cansetdropbehind"></a> interface `CanSetDropBehind`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Evolving
+public interface CanSetDropBehind {
+  /**
+   * Configure whether the stream should drop the cache.
+   *
+   * @param dropCache     Whether to drop the cache.  null means to use the
+   *                      default value.
+   * @throws IOException  If there was an error changing the dropBehind
+   *                      setting.
+   *         UnsupportedOperationException  If this stream doesn't support
+   *                                        setting the drop-behind.
+   */
+  void setDropBehind(Boolean dropCache)
+      throws IOException, UnsupportedOperationException;
+}
+```
+
+This interface allows callers to change policies used inside HDFS.
+
+Implementations MUST return `true` for the call
+
+```java
+StreamCapabilities.hasCapability("dropbehind");
+```
+
+
+## <a name="durability-of-output"></a>Durability, Concurrency, Consistency and Visibility of stream output.
+
+These are the aspects of the system behaviour which are not directly
+covered in this (very simplistic) filesystem model, but which are visible
+in production.
+
+
+### <a name="durability"></a> Durability
+
+1. `OutputStream.write()` MAY persist the data, synchronously or asynchronously
+1. `OutputStream.flush()` flushes data to the destination. There
+are no strict persistence requirements.
+1. `Syncable.hflush()` synchronously sends all outstaning data to the destination
+filesystem. After returning to the caller, the data MUST be visible to other readers,
+it MAY be durable. That is: it does not have to be persisted, merely guaranteed
+to be consistently visible to all clients attempting to open a new stream reading
+data at the path.
+1. `Syncable.hsync()` MUST transmit the data as per `hflush` the data and persist
+   that data to the underlying durable storage.
+1. `close()` The first call to `close()` MUST flush out all remaining data in
+the buffers, and persist it, as a call to `hsync()`.
+
+
+Many applications call `flush()` far too often -such as at the end of every line written.
+If this triggered an update of the data in persistent storage and any accompanying
+metadata, distributed stores would overload fast.
+Thus: `flush()` is often treated at most as a cue to flush data to the network
+buffers -but not commit to writing any data.
+
+It is only the `Syncable` interface which offers guarantees.
+
+The two `Syncable` operations `hsync()` and `hflush()` differ purely by the extra guarantee of `hsync()`: the data must be persisted.
+If `hsync()` is implemented, then `hflush()` can be implemented simply
+by invoking `hsync()`
+
+```java
+public void hflush() throws IOException {
+  hsync();
+}
+```
+
+This is perfectly acceptable as an implementation: the semantics of `hflush()`
+are satisifed.
+What is not acceptable is downgrading `hsync()` to `hflush()`, as the durability guarantee is no longer met.
+
+
+### <a name="concurrency"></a> Concurrency
+
+1. The outcome of more than one process writing to the same file is undefined.
+
+1. An input stream opened to read a file *before the file was opened for writing*
+MAY fetch data updated by writes to an OutputStream.
+Because of buffering and caching, this is not a requirement
+—and if an input stream does pick up updated data, the point at
+which the updated data is read is undefined. This surfaces in object stores
+where a `seek()` call which closes and re-opens the connection may pick up
+updated data, while forward stream reads do not. Similarly, in block-oriented
+filesystems, the data may be cached a block at a time —and changes only picked
+up when a different block is read.
+
+1. A filesystem MAY allow the destination path to be manipulated while a stream
+is writing to it —for example, `rename()` of the path or a parent; `delete()` of
+a path or parent. In such a case, the outcome of future write operations on
+the output stream is undefined. Some filesystems MAY implement locking to
+prevent conflict. However, this tends to be rare on distributed filesystems,
+for reasons well known in the literature.
+
+1. The Java API specification of `java.io.OutputStream` does not require
+an instance of the class to be thread safe.
+However, `org.apache.hadoop.hdfs.DFSOutputStream`
+has a stronger thread safety model (possibly unintentionally). This fact is
+relied upon in Apache HBase, as discovered in HADOOP-11708. Implementations
+SHOULD be thread safe. *Note*: even the `DFSOutputStream` synchronization
+model permits the output stream to have `close()` invoked while awaiting an
+acknowledgement from datanode or namenode writes in an `hsync()` operation.
+
+### <a name="consistency"></a>Consistency and Visibility
+
+There is no requirement for the data to be immediately visible to other applications
+—not until a specific call to flush buffers or persist it to the underlying storage
+medium are made.
+
+If an output stream is created with `FileSystem.create(path, overwrite==true)`
+and there is an existing file at the path, that is `exists(FS, path)` holds,
+then, the existing data is immediately unavailable; the data at the end of the
+path MUST consist of an empty byte sequence `[]`, with consistent metadata.
+
+
+```python
+exists(FS, path)
+(Stream', FS') = create(FS, path)
+exists(FS', path)
+getFileStatus(FS', path).getLen() = 0
+```
+
+The metadata of a file (`length(FS, path)` in particular) SHOULD be consistent
+with the contents of the file after `flush()` and `sync()`.
+
+```python
+(Stream', FS') = create(FS, path)
+(Stream'', FS'') = write(Stream', data)
+(Stream''', FS''') hsync(Stream'')
+exists(FS''', path)
+getFileStatus(FS''', path).getLen() = len(data)
+```
+
+*HDFS does not do this except when the write crosses a block boundary*; to do
+otherwise would overload the Namenode. Other stores MAY copy this behavior. 
+
+As a result, while a file is being written
+`length(Filesystem, Path)` MAY be less than the length of `data(Filesystem, Path)`.
+
+The metadata MUST be consistent with the contents of a file after the `close()`
+operation.
+
+After the contents of an output stream have been persisted (`hflush()/hsync()`)
+all new `open(FS, Path)` operations MUST return the updated data.
+
+After `close()` has been invoked on an output stream,
+a call to `getFileStatus(path)` MUST return the final metadata of the written file,
+including length and modification time.
+The metadata of the file returned in any of the FileSystem `list` operations
+MUST be consistent with this metadata.
+
+The value of `getFileStatus(path).getModificationTime()` is not defined
+while a stream is being written to.
+The timestamp MAY be updated while a file is being written,
+especially after a `Syncable.hsync()` call.
+The timestamps MUST be updated after the file is closed
+to that of a clock value observed by the server during the `close()` call.
+It is *likely* to be in the time and time zone of the filesystem, rather
+than that of the client.
+
+Formally, if a `close()` operation triggers an interaction with a server
+which starts at server-side time `t1` and completes at time `t2` with a successfully
+written file, then the last modification time SHOULD be a time `t` where
+`t1 <= t <= t2`
+
+## <a name="issues"></a> Issues with the Hadoop Output Stream model.
+
+There are some known issues with the output stream model as offered by Hadoop,
+specifically about the guarantees about when data is written and persisted
+—and when the metadata is synchronized.
+These are where implementation aspects of HDFS and the "Local" filesystem
+do not follow the simple model of the filesystem used in this specification.
+
+### <a name="hdfs-issues"></a> HDFS
+
+#### HDFS: `hsync()` only syncs the latest block
+
+The reference implementation, `DFSOutputStream` will block until an
+acknowledgement is received from the datanodes: that is, all hosts in the
+replica write chain have successfully written the file.
+
+That means that the expectation callers may have is that the return of the
+method call contains visibility and durability guarantees which other
+implementations must maintain.
+
+Note, however, that the reference `DFSOutputStream.hsync()` call only actually
+persists *the current block*. If there have been a series of writes since the
+last sync, such that a block boundary has been crossed. The `hsync()` call
+claims only to write the most recent.
+
+From the javadocs of `DFSOutputStream.hsync(EnumSet<SyncFlag> syncFlags)`
+
+> Note that only the current block is flushed to the disk device.
+> To guarantee durable sync across block boundaries the stream should
+> be created with {@link CreateFlag#SYNC_BLOCK}.
+
+
+This is an important HDFS implementation detail which must not be ignored by
+anyone relying on HDFS to provide a Write-Ahead-Log or other database structure
+where the requirement of the application is that
+"all preceeding bytes MUST have been persisted before the commit flag in the WAL
+is flushed"
+
+See [Stonebraker81], Michael Stonebraker, _Operating System Support for Database Management_,
+1981, for a discussion on this topic.
+
+If you do need `hsync()` to have synced every block in a very large write, call
+it regularly.
+
+#### HDFS: delayed visibility of metadata updates.
+
+That HDFS file metadata often lags the content of a file being written
+to is not something everyone expects, nor convenient for any program trying
+to pick up updated data in a file being written. Most visible is the length
+of a file returned in the various `list` commands and `getFileStatus` —this
+is often out of data.

Review comment:
       data or date?

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.
+The `Syncable` operations differ in their durability
+guarantees, not visibility of data.
+
+### State of Stream and File System after `Filesystem.create()`
+
+The output stream returned by a `FileSystem.create(path)` or
+`FileSystem.createFile(path).build()` within a filesystem `FS`,
+can be modeled as a triple containing an empty array of no data:
+
+```python
+Stream' = (path, true, [])
+```
+
+The filesystem `FS'` MUST contain a 0-byte file at the path:
+
+```python
+FS' = FS where data(FS', path) == []
+```
+
+Thus, the initial state of `Stream'.buffer` is implicitly
+consistent with the data at the filesystem.
+
+
+*Object Stores*: see caveats in the "Object Stores" section below.
+
+### State of Stream and File System after `Filesystem.append()`
+
+The output stream returned from a call of
+ `FileSystem.append(path, buffersize, progress)` within a filesystem `FS`,
+can be modelled as a stream whose `buffer` is intialized to that of
+the original file:
+
+```python
+Stream' = (path, true, data(FS, path))
+```
+
+####  Persisting data
+
+When the stream writes data back to its store, be it in any
+supported flush operation, in the `close()` operation, or at any other
+time the stream chooses to do so, the contents of the file
+are replaced with the current buffer
+
+```python
+Stream' = (path, true, buffer)
+FS' = FS where data(FS', path) == buffer
+```
+
+After a call to `close()`, the stream is closed for all operations other
+than `close()`; they MAY fail with `IOException` or `RuntimeException`.
+
+```python
+Stream' = (path, false, [])
+```
+
+The `close()` operation MUST be idempotent with the sole attempt to write the
+data made in the first invocation.
+
+1. If `close()` succeeds, subsequent calls are no-ops.
+1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow
+the previous exception, but they MUST NOT retry the write.
+
+<!--  ============================================================= -->
+<!--  CLASS: FSDataOutputStream -->
+<!--  ============================================================= -->
+
+## <a name="fsdataoutputstream"></a>Class `FSDataOutputStream`
+
+```java
+public class FSDataOutputStream
+  extends DataOutputStream
+  implements Syncable, CanSetDropBehind, StreamCapabilities {
+ // ...
+}
+```
+
+The `FileSystem.create()`, `FileSystem.append()` and
+`FSDataOutputStreamBuilder.build()` calls return an instance
+of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`.
+
+The base class wraps an `OutputStream` instance, one which may implement `Streamable`,
+`CanSetDropBehind` and `StreamCapabilities`.
+
+This document covers the requirements of such implementations.
+
+HDFS's `FileSystem` implementation, `DistributedFileSystem`, returns an instance
+of `HdfsDataOutputStream`. This implementation has at least two behaviors
+which are not explicitly declared by the base Java implmentation
+
+1. Writes are synchronized: more than one thread can write to the same
+output stream. This is a use pattern which HBase relies on.
+
+1. `OutputStream.flush()` is a no-op when the file is closed. Apache Druid
+has made such a call on this in the past
+[HADOOP-14346](https://issues.apache.org/jira/browse/HADOOP-14346).
+
+
+As the HDFS implementation is considered the de-facto specification of
+the FileSystem APIs, the fact that `write()` is thread-safe is significant.
+
+For compatibility, not only SHOULD other FS clients be thread-safe,
+but new HDFS features, such as encryption and Erasure Coding SHOULD also
+implement consistent behavior with the core HDFS output stream.
+
+Put differently:
+
+*It isn't enough for Output Streams to implement the core semantics
+of `java.io.OutputStream`: they need to implement the extra semantics
+of `HdfsDataOutputStream`, especially for HBase to work correctly.*
+
+The concurrent `write()` call is the most significant tightening of
+the Java specification.
+
+## <a name="outputstream"></a>Class `java.io.OutputStream`
+
+A Java `OutputStream` allows applications to write a sequence of bytes to a destination.
+In a Hadoop filesystem, that destination is the data under a path in the filesystem.
+
+```java
+public abstract class OutputStream implements Closeable, Flushable {
+  public abstract void write(int b) throws IOException;
+  public void write(byte b[]) throws IOException;
+  public void write(byte b[], int off, int len) throws IOException;
+  public void flush() throws IOException;
+  public void close() throws IOException;
+}
+```
+### <a name="write(data: int)"></a>`write(Stream, data)`
+
+Writes a byte of data to the stream.
+
+#### Preconditions
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+```
+
+The exception `java.nio.channels.ClosedChannelExceptionn` is
+raised in the HDFS output streams when trying to write to a closed file.
+This exception does not include the destination path; and
+`Exception.getMessage()` is `null`. It is therefore of limited value in stack
+traces. Implementors may wish to raise exceptions with more detail, such
+as a `PathIOException`.
+
+
+#### Postconditions
+
+The buffer has the lower 8 bits of the data argument appended to it.
+
+```python
+Stream'.buffer = Stream.buffer + [data & 0xff]
+```
+
+There may be an explicit limit on the size of cached data, or an implicit
+limit based by the available capacity of the destination filesystem.
+When a limit is reached, `write()` SHOULD fail with an `IOException`.
+
+### <a name="write(buffer,offset,len)"></a>`write(Stream, byte[] data, int offset, int len)`
+
+
+#### Preconditions
+
+The preconditions are all defined in `OutputStream.write()`
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+data != null else raise NullPointerException
+offset >= 0 else raise IndexOutOfBoundsException
+len >= 0 else raise IndexOutOfBoundsException
+offset < data.length else raise IndexOutOfBoundsException
+offset + len < data.length else raise IndexOutOfBoundsException
+```
+
+After the operation has returned, the buffer may be re-used. The outcome
+of updates to the buffer while the `write()` operation is in progress is undefined.
+
+#### Postconditions
+
+```python
+Stream'.buffer = Stream.buffer + data[offset...(offset + len)]
+```
+
+### <a name="write(buffer)"></a>`write(byte[] data)`
+
+This is defined as the equivalent of:
+
+```python
+write(data, 0, data.length)
+```
+
+### <a name="flush()"></a>`flush()`
+
+Requests that the data is flushed. The specification of `ObjectStream.flush()`
+declares that this SHOULD write data to the "intended destination".
+
+It explicitly precludes any guarantees about durability.
+
+For that reason, this document doesn't provide any normative
+specifications of behaviour.
+
+#### Preconditions
+
+None.
+
+#### Postconditions
+
+None.
+
+If the implementation chooses to implement a stream-flushing operation,
+the data may be saved to the file system such that it becomes visible to
+others"
+
+```python
+FS' = FS where data(FS', path) == buffer
+```
+
+When a stream is closed, `flush()` SHOULD downgrade to being a no-op, if it was not
+one already. This is to work with applications and libraries which can invoke
+it in exactly this way.
+
+
+*Issue*: Should `flush()` forward to `hflush()`?
+
+No. Or at least, make it optional.
+
+There's a lot of application code which assumes that `flush()` is low cost
+and should be invoked after writing every single line of output, after
+writing small 4KB blocks or similar.
+
+Forwarding this to a full flush across a distributed filesystem, or worse,
+a distant object store, is very inefficient.
+Filesystem clients which do uprate a `flush()` to an `hflush()` will eventually
+have to roll back that feature: 
+[HADOOP-16548](https://issues.apache.org/jira/browse/HADOOP-16548).
+
+### <a name="close"></a>`close()`
+
+The `close()` operation saves all data to the filesystem and
+releases any resources used for writing data.
+
+The `close()` call is expected to block
+until the write has completed (as with `Syncable.hflush()`), possibly
+until it has been written to durable storage.
+
+After `close()` completes, the data in a file MUST be visible and consistent
+with the data most recently written. The metadata of the file MUST be consistent
+with the data and the write history itself (i.e. any modification time fields
+updated).
+
+After `close()` is invoked, all subsequent `write()` calls on the stream
+MUST fail with an `IOException`.
+
+Any locking/leaseholding mechanism MUST release its lock/lease.
+
+```python
+Stream'.open = false
+FS' = FS where data(FS', path) == buffer
+```
+
+The `close()` call MAY fail during its operation.
+
+1. Callers of the API MUST expect for some calls to  `close()` to fail and SHOULD code appropriately.
+Catching and swallowing exceptions, while common, is not always the ideal solution.
+1. Even after a failure, `close()` MUST place the stream into a closed state.
+Follow-on calls to `close()` are ignored, and calls to other methods
+rejected. That is: caller's cannot be expected to call `close()` repeatedly
+until it succeeds.
+1. The duration of the `call()` operation is undefined. Operations which rely
+on acknowledgements from remote systems to meet the persistence guarantees
+implicitly have to await these acknowledgements. Some Object Store output streams
+upload the entire data file in the `close()` operation. This can take a large amount
+of time. The fact that many user applications assume that `close()` is both fast
+and does not fail means that this behavior is dangerous.
+
+Recommendations for safe use by callers
+
+* Do plan for exceptions being raised, either in catching and logging or
+by throwing the exception further up. Catching and silently swallowing exceptions
+may hide serious problems.
+* Heartbeat operations SHOULD take place on a separate thread, so that a long
+delay in `close()` does not block the thread so long that the heartbeat times
+out.
+
+Implementors:
+
+* Have a look at [HADOOP-16785](https://issues.apache.org/jira/browse/HADOOP-16785)
+to see examples of complications in close.
+* Incrementally writing blocks before a close operation results in a behavior which
+matches client expectations better: write failures to surface earlier and close
+to be more housekeeping than the actual upload.
+* If block uploads are executed in separate threads, the output stream `close()`
+call MUST block until all the asynchronous uploads have completed; any error raised
+MUST be reported.
+If multiple errors were raised, the stream can choose which to propagate.
+What is important is: when `close()` returns without an error, applications expect
+the data to have been successfully written.
+
+### HDFS and `OutputStream.close()`
+
+HDFS does not immediately `sync()` the output of a written file to disk on
+`OutputStream.close()` unless configured with `dfs.datanode.synconclose` 
+is true. This has caused [problems in some applications](https://issues.apache.org/jira/browse/ACCUMULO-1364).
+
+Applications which absolutely require the guarantee that a file has been persisted
+MUST call `Syncable.hsync()` *before* the file is closed.
+
+
+## <a name="syncable"></a>`org.apache.hadoop.fs.Syncable`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Stable
+public interface Syncable {
+
+
+  /** Flush out the data in client's user buffer. After the return of
+   * this call, new readers will see the data.
+   * @throws IOException if any error occurs
+   */
+  void hflush() throws IOException;
+
+  /** Similar to posix fsync, flush out the data in client's user buffer
+   * all the way to the disk device (but the disk may have it in its cache).
+   * @throws IOException if error occurs
+   */
+  void hsync() throws IOException;
+}
+```
+
+The purpose of `Syncable` interface is to provide guarantees that data is written
+to a filesystem for both visibility and durability.
+
+*SYNC-1*: An `OutputStream` which implements `Syncable` is
+making an explicit declaration that it can meet those guarantees.
+
+*SYNC-2*: The interface MUST NOT be declared as implemented by an `OutputStream` unless
+those guarantees can be met.
+
+The `Syncable` interface has been implemented by other classes than
+subclasses of `OutputStream`, such as `org.apache.hadoop.io.SequenceFile.Writer`.
+
+*SYNC-3* The fact that a class implements `Syncable` does not guarantee
+that `extends OutputStream` holds.
+
+That is, for any class `C`: `(C instanceof Syncable)` does not imply
+`(C instanceof OutputStream)`
+
+This specification only covers the required behavior of `OutputStream` subclasses
+which implement `Syncable`.
+
+
+*SYNC-4:* The return value of `FileSystem.create(Path)` is an instance
+of `FSDataOutputStream`.
+
+*SYNC-5:* `FSDataOutputStream implements Syncable`
+
+
+SYNC-5 and SYNC-1 imply that all output streams which can be created
+with `FileSystem.create(Path)` must support the semantics of `Syncable`.
+This is demonstrably not true: `FSDataOutputStream` simply downgrades
+to a `flush()` if its wrapped stream is not `Syncable`.
+Therefore the declarations SYNC-1 and SYNC-2 do not hold: you cannot trust `Syncable`.
+
+Put differently: *callers MUST NOT rely on the presence of the interface
+as evidence that the semantics of `Syncable` are supported*. Instead
+they MUST be dynamically probed for using the `StreamCapabilities`
+interface, where available.
+
+
+### <a name="syncable.hflush"></a>`Syncable.hflush()`
+
+Flush out the data in client's user buffer. After the return of
+this call, new readers will see the data. The `hflush()` operation
+does not contain any guarantees as to the durability of the data. only
+its visibility.
+
+Thus implementations may cache the written data in memory
+—visible to all, but not yet persisted.
+
+#### Preconditions
+
+```python
+hasCapability(Stream, "hflush")
+Stream.open else raise IOException
+```
+
+
+#### Postconditions
+
+```python
+FS' = FS where data(path) == cache
+```
+
+
+After the call returns, the data MUST be visible to all new callers
+of `FileSystem.open(path)` and `FileSystem.openFile(path).build()`.
+
+There is no requirement or guarantee that clients with an existing
+`DataInputStream` created by a call to `(FS, path)` will see the updated
+data, nor is there a guarantee that they *will not* in a current or subsequent
+read.
+
+Implementation note: as a correct `hsync()` implementation MUST also
+offer all the semantics of an `hflush()` call, implementations of `hflush()`
+may just invoke `hsync()`:
+
+```java
+public void hflush() throws IOException {
+  hsync();
+}
+```
+
+#### `hflush()` Performance
+
+The `hflush()` call MUST block until the store has acknowledge that the
+data has been received and is now visible to others. This can be slow,
+as it will include the time to upload any outstanding data from the
+client, and for the filesystem itself to process it.
+
+Often Filesystems only offer the `Syncable.hsync()` guarantees: persistence as
+well as visibility. This means the time to return can be even greater.
+
+Application code MUST NOT call `hflush()` or `hsync()` at the end of every line
+or, unless they are writing a WAL, at the end of every record. Use with care.
+
+
+### <a name="syncable.hsync"></a> `Syncable.hsync()`
+
+Similar to POSIX `fsync()`, this call saves the data in client's user buffer
+all the way to the disk device (but the disk may have it in its cache).
+
+That is: it is a requirement for the underlying FS To save all the data to
+the disk hardware itself, where it is expected to be durable.
+
+#### Preconditions
+
+```python
+hasCapability(Stream, "hsync")
+Stream.open else raise IOException
+```
+
+#### Postconditions
+
+```python
+FS' = FS where data(path) == buffer
+```
+
+_Implementations are required to block until that write has been
+acknowledged by the store._
+
+This is so the caller can be confident that once the call has
+returned successfully, the data has been written.
+
+
+
+## <a name="streamcapabilities"></a>Interface `StreamCapabilities`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Evolving
+```
+
+The `org.apache.hadoop.fs.StreamCapabilities` interface exists to allow callers to dynamically
+determine the behavior of a stream.
+
+```java
+  public boolean hasCapability(String capability) {
+    switch (capability.toLowerCase(Locale.ENGLISH)) {
+      case StreamCapabilities.HSYNC:
+      case StreamCapabilities.HFLUSH:
+        return supportFlush;
+      default:
+        return false;
+    }
+  }
+```
+
+Once a stream has been closed, a `hasCapability()` call MUST do one of
+
+* return the capabilities of the open stream.
+* return false.
+
+That is: it MUST NOT raise an exception about the file being closed;
+
+See [pathcapabilities](pathcapabilities.html) for specifics on the `PathCapabilities` API;
+the requirements are similar: a stream MUST NOT return true for a capability
+for which it lacks support, be it because
+
+* The capability is unknown.
+* The capability is known and known to be unsupported.
+
+Standard stream capabilities are defined in `StreamCapabilities`;
+consult the javadocs for the complete set of options.
+
+| Name  | Probes for support of |
+|-------|---------|
+| `dropbehind` | `CanSetDropBehind.setDropBehind()` |
+| `hsync` | `Syncable.hsync()` |
+| `hflush` | `Syncable.hflush()`. Deprecated: probe for `HSYNC` only. |
+| `in:readahead` | `CanSetReadahead.setReadahead()` |
+| `in:unbuffer"` | `CanUnbuffer.unbuffer()` |
+| `in:readbytebuffer` | `ByteBufferReadable#read(ByteBuffer)` |
+| `in:preadbytebuffer` | `ByteBufferPositionedReadable#read(long, ByteBuffer)` |
+
+Stream implementations MAY add their own custom options.
+These MUST be prefixed with `fs.SCHEMA.`, where `SCHEMA` is the schema of the filesystem.
+
+## <a name="cansetdropbehind"></a> interface `CanSetDropBehind`
+
+```java
+@InterfaceAudience.Public
+@InterfaceStability.Evolving
+public interface CanSetDropBehind {
+  /**
+   * Configure whether the stream should drop the cache.
+   *
+   * @param dropCache     Whether to drop the cache.  null means to use the
+   *                      default value.
+   * @throws IOException  If there was an error changing the dropBehind
+   *                      setting.
+   *         UnsupportedOperationException  If this stream doesn't support
+   *                                        setting the drop-behind.
+   */
+  void setDropBehind(Boolean dropCache)
+      throws IOException, UnsupportedOperationException;
+}
+```
+
+This interface allows callers to change policies used inside HDFS.
+
+Implementations MUST return `true` for the call
+
+```java
+StreamCapabilities.hasCapability("dropbehind");
+```
+
+
+## <a name="durability-of-output"></a>Durability, Concurrency, Consistency and Visibility of stream output.
+
+These are the aspects of the system behaviour which are not directly
+covered in this (very simplistic) filesystem model, but which are visible
+in production.
+
+
+### <a name="durability"></a> Durability
+
+1. `OutputStream.write()` MAY persist the data, synchronously or asynchronously
+1. `OutputStream.flush()` flushes data to the destination. There
+are no strict persistence requirements.
+1. `Syncable.hflush()` synchronously sends all outstaning data to the destination
+filesystem. After returning to the caller, the data MUST be visible to other readers,
+it MAY be durable. That is: it does not have to be persisted, merely guaranteed
+to be consistently visible to all clients attempting to open a new stream reading
+data at the path.
+1. `Syncable.hsync()` MUST transmit the data as per `hflush` the data and persist
+   that data to the underlying durable storage.
+1. `close()` The first call to `close()` MUST flush out all remaining data in
+the buffers, and persist it, as a call to `hsync()`.
+
+
+Many applications call `flush()` far too often -such as at the end of every line written.
+If this triggered an update of the data in persistent storage and any accompanying
+metadata, distributed stores would overload fast.
+Thus: `flush()` is often treated at most as a cue to flush data to the network
+buffers -but not commit to writing any data.
+
+It is only the `Syncable` interface which offers guarantees.
+
+The two `Syncable` operations `hsync()` and `hflush()` differ purely by the extra guarantee of `hsync()`: the data must be persisted.
+If `hsync()` is implemented, then `hflush()` can be implemented simply
+by invoking `hsync()`
+
+```java
+public void hflush() throws IOException {
+  hsync();
+}
+```
+
+This is perfectly acceptable as an implementation: the semantics of `hflush()`
+are satisifed.
+What is not acceptable is downgrading `hsync()` to `hflush()`, as the durability guarantee is no longer met.
+
+
+### <a name="concurrency"></a> Concurrency
+
+1. The outcome of more than one process writing to the same file is undefined.
+
+1. An input stream opened to read a file *before the file was opened for writing*
+MAY fetch data updated by writes to an OutputStream.
+Because of buffering and caching, this is not a requirement
+—and if an input stream does pick up updated data, the point at
+which the updated data is read is undefined. This surfaces in object stores
+where a `seek()` call which closes and re-opens the connection may pick up
+updated data, while forward stream reads do not. Similarly, in block-oriented
+filesystems, the data may be cached a block at a time —and changes only picked
+up when a different block is read.
+
+1. A filesystem MAY allow the destination path to be manipulated while a stream
+is writing to it —for example, `rename()` of the path or a parent; `delete()` of
+a path or parent. In such a case, the outcome of future write operations on
+the output stream is undefined. Some filesystems MAY implement locking to
+prevent conflict. However, this tends to be rare on distributed filesystems,
+for reasons well known in the literature.
+
+1. The Java API specification of `java.io.OutputStream` does not require
+an instance of the class to be thread safe.
+However, `org.apache.hadoop.hdfs.DFSOutputStream`
+has a stronger thread safety model (possibly unintentionally). This fact is
+relied upon in Apache HBase, as discovered in HADOOP-11708. Implementations
+SHOULD be thread safe. *Note*: even the `DFSOutputStream` synchronization
+model permits the output stream to have `close()` invoked while awaiting an
+acknowledgement from datanode or namenode writes in an `hsync()` operation.
+
+### <a name="consistency"></a>Consistency and Visibility
+
+There is no requirement for the data to be immediately visible to other applications
+—not until a specific call to flush buffers or persist it to the underlying storage
+medium are made.
+
+If an output stream is created with `FileSystem.create(path, overwrite==true)`
+and there is an existing file at the path, that is `exists(FS, path)` holds,
+then, the existing data is immediately unavailable; the data at the end of the
+path MUST consist of an empty byte sequence `[]`, with consistent metadata.
+
+
+```python
+exists(FS, path)
+(Stream', FS') = create(FS, path)
+exists(FS', path)
+getFileStatus(FS', path).getLen() = 0
+```
+
+The metadata of a file (`length(FS, path)` in particular) SHOULD be consistent
+with the contents of the file after `flush()` and `sync()`.
+
+```python
+(Stream', FS') = create(FS, path)
+(Stream'', FS'') = write(Stream', data)
+(Stream''', FS''') hsync(Stream'')
+exists(FS''', path)
+getFileStatus(FS''', path).getLen() = len(data)
+```
+
+*HDFS does not do this except when the write crosses a block boundary*; to do
+otherwise would overload the Namenode. Other stores MAY copy this behavior. 
+
+As a result, while a file is being written
+`length(Filesystem, Path)` MAY be less than the length of `data(Filesystem, Path)`.
+
+The metadata MUST be consistent with the contents of a file after the `close()`
+operation.
+
+After the contents of an output stream have been persisted (`hflush()/hsync()`)
+all new `open(FS, Path)` operations MUST return the updated data.
+
+After `close()` has been invoked on an output stream,
+a call to `getFileStatus(path)` MUST return the final metadata of the written file,
+including length and modification time.
+The metadata of the file returned in any of the FileSystem `list` operations
+MUST be consistent with this metadata.
+
+The value of `getFileStatus(path).getModificationTime()` is not defined
+while a stream is being written to.
+The timestamp MAY be updated while a file is being written,
+especially after a `Syncable.hsync()` call.
+The timestamps MUST be updated after the file is closed
+to that of a clock value observed by the server during the `close()` call.
+It is *likely* to be in the time and time zone of the filesystem, rather
+than that of the client.
+
+Formally, if a `close()` operation triggers an interaction with a server
+which starts at server-side time `t1` and completes at time `t2` with a successfully
+written file, then the last modification time SHOULD be a time `t` where
+`t1 <= t <= t2`
+
+## <a name="issues"></a> Issues with the Hadoop Output Stream model.
+
+There are some known issues with the output stream model as offered by Hadoop,
+specifically about the guarantees about when data is written and persisted
+—and when the metadata is synchronized.
+These are where implementation aspects of HDFS and the "Local" filesystem
+do not follow the simple model of the filesystem used in this specification.
+
+### <a name="hdfs-issues"></a> HDFS
+
+#### HDFS: `hsync()` only syncs the latest block
+
+The reference implementation, `DFSOutputStream` will block until an
+acknowledgement is received from the datanodes: that is, all hosts in the
+replica write chain have successfully written the file.
+
+That means that the expectation callers may have is that the return of the
+method call contains visibility and durability guarantees which other
+implementations must maintain.
+
+Note, however, that the reference `DFSOutputStream.hsync()` call only actually
+persists *the current block*. If there have been a series of writes since the
+last sync, such that a block boundary has been crossed. The `hsync()` call
+claims only to write the most recent.
+
+From the javadocs of `DFSOutputStream.hsync(EnumSet<SyncFlag> syncFlags)`
+
+> Note that only the current block is flushed to the disk device.
+> To guarantee durable sync across block boundaries the stream should
+> be created with {@link CreateFlag#SYNC_BLOCK}.
+
+
+This is an important HDFS implementation detail which must not be ignored by
+anyone relying on HDFS to provide a Write-Ahead-Log or other database structure
+where the requirement of the application is that
+"all preceeding bytes MUST have been persisted before the commit flag in the WAL
+is flushed"
+
+See [Stonebraker81], Michael Stonebraker, _Operating System Support for Database Management_,
+1981, for a discussion on this topic.
+
+If you do need `hsync()` to have synced every block in a very large write, call
+it regularly.
+
+#### HDFS: delayed visibility of metadata updates.
+
+That HDFS file metadata often lags the content of a file being written
+to is not something everyone expects, nor convenient for any program trying
+to pick up updated data in a file being written. Most visible is the length
+of a file returned in the various `list` commands and `getFileStatus` —this
+is often out of data.
+
+As HDFS only supports file growth in its output operations, this means
+that the size of the file as listed in the metadata may be less than or equal
+to the number of available bytes —but never larger. This is a guarantee which
+is also held
+
+One algorithm to determine whether a file in HDFS is updated is:
+
+1. Remember the last read position `pos` in the file, using `0` if this is the initial
+read.
+1. Use `getFileStatus(FS, Path)` to query the updated length of the file as
+recorded in the metadata.
+1. If `Status.length &gt; pos`, the file has grown.
+1. If the number has not changed, then
+    1. Reopen the file.
+    1. `seek(pos)` to that location
+    1. If `read() != -1`, there is new data.
+
+This algorithm works for filesystems which are consistent with metadata and
+data, as well as HDFS. What is important to know is that, for an open file
+`getFileStatus(FS, path).getLen() == 0` does not imply that `data(FS, path)` is
+empty.
+
+When an output stream in HDFS is closed; the newly written data is not immediately
+written to disk unless HDFS is deployed with `dfs.datanode.synconclose` set to
+true. Otherwise it is cached and written to disk later.
+
+### <a name="local-issues"></a>Local Filesystem, `file:`
+
+`LocalFileSystem`, `file:`, (or any other `FileSystem` implementation based on
+`ChecksumFileSystem`) has a different issue. If an output stream
+is obtained from `create()` and `FileSystem.setWriteChecksum(false)` has
+*not* been called on the filesystem, then the stream only flushes as much
+local data as can be written to full checksummed blocks of data.
+
+That is, the hsync/hflush operations are not guaranteed to write all the pending
+data until the file is finally closed.
+
+For this reason, the local fileystem accessed via `file://` URLs
+does not support `Syncable` unless `setWriteChecksum(false)` was
+called on that FileSystem instance so as do disable checksum creation.
+After which, obviously, checksums are not generated for any file.
+
+
+### <a name="checksummed-fs-issues"></a> Checksummed output streams
+
+Because  `org.apache.hadoop.fs.FSOutputSummer` and
+`org.apache.hadoop.fs.ChecksumFileSystem.ChecksumFSOutputSummer`
+implement the underlying checksummed output stream used by HDFS and
+other filesystems, it provides some of the core semantics of the output
+stream behavior.
+
+1. The `close()` call is unsynchronized, re-entrant and may attempt
+to close the stream more than once.
+1. It is possible to call `write(int)` on a closed stream (but not
+`write(byte[], int, int)`).
+1. It is possible to call `flush()` on a closed stream.
+
+Behaviors 1 and 2 really have to be considered bugs to fix, albeit with care.
+
+
+### <a name="object-store-issues"></a> Object Stores
+
+Object store streams MAY buffer the entire stream's output
+until the final `close()` operation triggers a single `PUT` of the data
+and materialization of the final output.
+
+This significantly change's their behaviour compared to that of
+POSIX filesystems and that specified in this document.
+
+#### Visibility of newly created objects
+
+There is no guarantee that any file will be visible at the path of an output
+stream after the output stream is created .
+
+That is: while `create(FS, path, boolean)` returns a new stream
+
+```python
+Stream' = (path, true, [])
+```
+
+The other postcondition of the operation, `data(FS', path) == []` MAY NOT
+hold, in which case:
+
+1. `exists(FS, p)` MAY return false.
+1. If a file was created with `overwrite = True`, the existing data MAY still
+be visible: `data(FS', path) = data(FS, path)`.
+
+1. The check for existing data in a `create()` call with `overwrite=False`, may
+take place in the `create()` call itself, in the `close()` call prior to/during
+the write, or at some point in between. In the special case that the
+object store supports an atomic `PUT` operation, the check for existence of
+existing data and the subsequent creation of data at the path contains a race
+condition: other clients may create data at the path between the existence check
+and the subsequent write.
+
+1. Calls to `create(FS, Path, overwrite=false)` MAY succeed, returning a new
+`OutputStream`, even while another stream is open and writing to the destination
+path.
+
+This allows for the following sequence of operations, which would
+raise an exception in the second `open()` call if invoked against HDFS:
+
+```python
+Stream1 = open(FS, path, false)
+sleep(200)
+Stream2 = open(FS, path, false)
+Stream.write('a')
+Stream1.close()
+Stream2.close()
+```
+
+For anyone wondering why the clients don't create a 0-byte file in the `create()` call,
+it would cause problems after `close()` —the marker file could get
+returned in `open()` calls instead of the final data.
+
+#### Visibility of the output of a stream after `close()`
+
+One guarantee which Object Stores SHOULD make is the same as those of POSIX
+filesystems: After a stream `close()` call returns, the data MUST be persisted
+durably and visible to all callers. Unfortunately, even that guarantee is
+not always met:
+
+1. Existing data on a path MAY be visible for an indeterminate period of time.
+
+1. If the store has any form of create inconsistency or buffering of negative
+existence probes, then even after the stream's `close()` operation has returned,
+`getFileStatus(FS, path)` and `open(FS, path)` may fail with a `FileNotFoundException`.
+
+In their favour, the atomicity of the store's PUT operations do offer their
+own guarantee: a newly created object is either absent or all of its data
+is present: the act of instantiating the object, while potentially exhibiting
+create inconsistency, is atomic. Applications may be able to use that fact
+to their advantage.
+
+## <a name="implementors"></a> Implementors notes.
+
+### `StreamCapabilities`
+
+Implementors of filesystem clients SHOULD implement the `StreamCapabilities`
+interface and its `hasCapabilities()` method to to declare whether or not 
+an output streams offer the visibility and durability guarantees of `Syncable`.
+
+Implementors of `StreamCapabilities.hasCapabilities()` MUST NOT declare that
+they support the  `hflush` and `hsync` capabilities on streams where this is not true.
+
+Sometimes streams pass their data to store, but the far end may not
+sync it all the way to disk. That is not something the client can determine.
+Here: if the client code is making the hflush/hsync passes these requests
+on to the distributed FS, it SHOULD declare that it supports them.
+
+### Metadata updates
+
+Implementors MAY NOT update a file's metadata (length, date, ...) after
+every `hsync()` call. HDFS doesn't, except when the written data crosses
+a block boundary.
+
+
+### Implemting `OutputStream.flush()`
+
+Implementors SHOULD NOT forward `OutputStream.flush()` to `Syncable.hflush()`.
+Too much code calls `flush()` at the end of writing every line of text;
+blocking to upload this to a remote store and waiting for the results
+significantly hurts performance.
+Given that there are no guarantees of what `flush()` does, this is
+needless.
+
+Implementors who do forward the call to `OutputStream.flush()`
+to `Syncable.hflush()` do end up having to turn this feature
+off [HADOOP-16548](https://issues.apache.org/jira/browse/HADOOP-16548).
+
+### Does `close()` sync data?
+
+By default, HDFS does not sync data to disk when a stream is closed; it will
+be asynchronously saved to disk.
+
+This does not mean that users do not expect it.

Review comment:
       This one is the best :)

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.

Review comment:
       As per the java doc on hsync and hflush,they are about the guarantees on data being actually writted to disk or final storage. So just a question here - How it is made sure that data will be visible after close call for the readers?

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.
+The `Syncable` operations differ in their durability
+guarantees, not visibility of data.
+
+### State of Stream and File System after `Filesystem.create()`
+
+The output stream returned by a `FileSystem.create(path)` or
+`FileSystem.createFile(path).build()` within a filesystem `FS`,
+can be modeled as a triple containing an empty array of no data:
+
+```python
+Stream' = (path, true, [])
+```
+
+The filesystem `FS'` MUST contain a 0-byte file at the path:
+
+```python
+FS' = FS where data(FS', path) == []
+```
+
+Thus, the initial state of `Stream'.buffer` is implicitly
+consistent with the data at the filesystem.
+
+
+*Object Stores*: see caveats in the "Object Stores" section below.
+
+### State of Stream and File System after `Filesystem.append()`
+
+The output stream returned from a call of
+ `FileSystem.append(path, buffersize, progress)` within a filesystem `FS`,
+can be modelled as a stream whose `buffer` is intialized to that of
+the original file:
+
+```python
+Stream' = (path, true, data(FS, path))
+```
+
+####  Persisting data
+
+When the stream writes data back to its store, be it in any
+supported flush operation, in the `close()` operation, or at any other
+time the stream chooses to do so, the contents of the file
+are replaced with the current buffer
+
+```python
+Stream' = (path, true, buffer)
+FS' = FS where data(FS', path) == buffer
+```
+
+After a call to `close()`, the stream is closed for all operations other
+than `close()`; they MAY fail with `IOException` or `RuntimeException`.
+
+```python
+Stream' = (path, false, [])
+```
+
+The `close()` operation MUST be idempotent with the sole attempt to write the
+data made in the first invocation.
+
+1. If `close()` succeeds, subsequent calls are no-ops.
+1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow
+the previous exception, but they MUST NOT retry the write.
+
+<!--  ============================================================= -->
+<!--  CLASS: FSDataOutputStream -->
+<!--  ============================================================= -->
+
+## <a name="fsdataoutputstream"></a>Class `FSDataOutputStream`
+
+```java
+public class FSDataOutputStream
+  extends DataOutputStream
+  implements Syncable, CanSetDropBehind, StreamCapabilities {
+ // ...
+}
+```
+
+The `FileSystem.create()`, `FileSystem.append()` and
+`FSDataOutputStreamBuilder.build()` calls return an instance
+of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`.
+
+The base class wraps an `OutputStream` instance, one which may implement `Streamable`,
+`CanSetDropBehind` and `StreamCapabilities`.
+
+This document covers the requirements of such implementations.
+
+HDFS's `FileSystem` implementation, `DistributedFileSystem`, returns an instance
+of `HdfsDataOutputStream`. This implementation has at least two behaviors
+which are not explicitly declared by the base Java implmentation
+
+1. Writes are synchronized: more than one thread can write to the same
+output stream. This is a use pattern which HBase relies on.
+
+1. `OutputStream.flush()` is a no-op when the file is closed. Apache Druid
+has made such a call on this in the past
+[HADOOP-14346](https://issues.apache.org/jira/browse/HADOOP-14346).
+
+
+As the HDFS implementation is considered the de-facto specification of
+the FileSystem APIs, the fact that `write()` is thread-safe is significant.
+
+For compatibility, not only SHOULD other FS clients be thread-safe,
+but new HDFS features, such as encryption and Erasure Coding SHOULD also
+implement consistent behavior with the core HDFS output stream.
+
+Put differently:
+
+*It isn't enough for Output Streams to implement the core semantics
+of `java.io.OutputStream`: they need to implement the extra semantics
+of `HdfsDataOutputStream`, especially for HBase to work correctly.*
+
+The concurrent `write()` call is the most significant tightening of
+the Java specification.
+
+## <a name="outputstream"></a>Class `java.io.OutputStream`
+
+A Java `OutputStream` allows applications to write a sequence of bytes to a destination.
+In a Hadoop filesystem, that destination is the data under a path in the filesystem.
+
+```java
+public abstract class OutputStream implements Closeable, Flushable {
+  public abstract void write(int b) throws IOException;
+  public void write(byte b[]) throws IOException;
+  public void write(byte b[], int off, int len) throws IOException;
+  public void flush() throws IOException;
+  public void close() throws IOException;
+}
+```
+### <a name="write(data: int)"></a>`write(Stream, data)`
+
+Writes a byte of data to the stream.
+
+#### Preconditions
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+```
+
+The exception `java.nio.channels.ClosedChannelExceptionn` is
+raised in the HDFS output streams when trying to write to a closed file.
+This exception does not include the destination path; and
+`Exception.getMessage()` is `null`. It is therefore of limited value in stack
+traces. Implementors may wish to raise exceptions with more detail, such
+as a `PathIOException`.
+
+
+#### Postconditions
+
+The buffer has the lower 8 bits of the data argument appended to it.
+
+```python
+Stream'.buffer = Stream.buffer + [data & 0xff]
+```
+
+There may be an explicit limit on the size of cached data, or an implicit
+limit based by the available capacity of the destination filesystem.
+When a limit is reached, `write()` SHOULD fail with an `IOException`.
+
+### <a name="write(buffer,offset,len)"></a>`write(Stream, byte[] data, int offset, int len)`
+
+
+#### Preconditions
+
+The preconditions are all defined in `OutputStream.write()`
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+data != null else raise NullPointerException
+offset >= 0 else raise IndexOutOfBoundsException
+len >= 0 else raise IndexOutOfBoundsException
+offset < data.length else raise IndexOutOfBoundsException
+offset + len < data.length else raise IndexOutOfBoundsException
+```
+
+After the operation has returned, the buffer may be re-used. The outcome
+of updates to the buffer while the `write()` operation is in progress is undefined.
+
+#### Postconditions
+
+```python
+Stream'.buffer = Stream.buffer + data[offset...(offset + len)]
+```
+
+### <a name="write(buffer)"></a>`write(byte[] data)`
+
+This is defined as the equivalent of:
+
+```python
+write(data, 0, data.length)
+```
+
+### <a name="flush()"></a>`flush()`
+
+Requests that the data is flushed. The specification of `ObjectStream.flush()`
+declares that this SHOULD write data to the "intended destination".
+
+It explicitly precludes any guarantees about durability.
+
+For that reason, this document doesn't provide any normative
+specifications of behaviour.
+
+#### Preconditions
+
+None.
+
+#### Postconditions
+
+None.
+
+If the implementation chooses to implement a stream-flushing operation,
+the data may be saved to the file system such that it becomes visible to
+others"
+
+```python
+FS' = FS where data(FS', path) == buffer
+```
+
+When a stream is closed, `flush()` SHOULD downgrade to being a no-op, if it was not
+one already. This is to work with applications and libraries which can invoke
+it in exactly this way.
+
+
+*Issue*: Should `flush()` forward to `hflush()`?
+
+No. Or at least, make it optional.
+
+There's a lot of application code which assumes that `flush()` is low cost
+and should be invoked after writing every single line of output, after
+writing small 4KB blocks or similar.
+
+Forwarding this to a full flush across a distributed filesystem, or worse,
+a distant object store, is very inefficient.
+Filesystem clients which do uprate a `flush()` to an `hflush()` will eventually
+have to roll back that feature: 
+[HADOOP-16548](https://issues.apache.org/jira/browse/HADOOP-16548).
+
+### <a name="close"></a>`close()`
+
+The `close()` operation saves all data to the filesystem and
+releases any resources used for writing data.
+
+The `close()` call is expected to block
+until the write has completed (as with `Syncable.hflush()`), possibly
+until it has been written to durable storage.
+
+After `close()` completes, the data in a file MUST be visible and consistent
+with the data most recently written. The metadata of the file MUST be consistent
+with the data and the write history itself (i.e. any modification time fields
+updated).
+
+After `close()` is invoked, all subsequent `write()` calls on the stream
+MUST fail with an `IOException`.
+
+Any locking/leaseholding mechanism MUST release its lock/lease.
+
+```python
+Stream'.open = false
+FS' = FS where data(FS', path) == buffer
+```
+
+The `close()` call MAY fail during its operation.
+
+1. Callers of the API MUST expect for some calls to  `close()` to fail and SHOULD code appropriately.
+Catching and swallowing exceptions, while common, is not always the ideal solution.
+1. Even after a failure, `close()` MUST place the stream into a closed state.
+Follow-on calls to `close()` are ignored, and calls to other methods
+rejected. That is: caller's cannot be expected to call `close()` repeatedly
+until it succeeds.
+1. The duration of the `call()` operation is undefined. Operations which rely

Review comment:
       close() not call()?

##########
File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md
##########
@@ -0,0 +1,1002 @@
+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+
+<!-- MACRO{toc|fromDepth=1|toDepth=3} -->
+
+# Output: `OutputStream`, `Syncable` and `StreamCapabilities`
+
+## Introduction
+
+This document covers the Output Streams within the context of the
+[Hadoop File System Specification](index.html).
+
+It uses the filesystem model defined in [A Model of a Hadoop Filesystem](model.html)
+with the notation defined in [notation](Notation.md).
+
+The target audiences are:
+1. Users of the APIs. While `java.io.OutputStream` is a standard interfaces,
+this document clarifies how it is implemented in HDFS and elsewhere.
+The Hadoop-specific interfaces `Syncable` and `StreamCapabilities` are new;
+`Syncable` is notable in offering durability and visibility guarantees which
+exceed that of `OutputStream`.
+1. Implementors of File Systems and clients.
+
+## How data is written to a filesystem
+
+The core mechanism to write data to files through the Hadoop FileSystem APIs
+is through `OutputStream` subclasses obtained through calls to
+`FileSystem.create()`, `FileSystem.append()`,
+or `FSDataOutputStreamBuilder.build()`.
+
+These all return instances of `FSDataOutputStream`, through which data
+can be written through various `write()` methods.
+After a stream's `close()` method is called, all data written to the
+stream MUST BE persisted to the fileysystem and visible to oll other
+clients attempting to read data from that path via `FileSystem.open()`.
+
+As well as operations to write the data, Hadoop's `OutputStream` implementations
+provide methods to flush buffered data back to the filesystem,
+so as to ensure that the data is reliably persisted and/or visible
+to other callers. This is done via the `Syncable` interface. It was
+originally intended that the presence of this interface could be interpreted
+as a guarantee that the stream supported its methods. However, this has proven
+impossible to guarantee as the static nature of the interface is incompatible
+with filesystems whose syncability semantics may vary on a store/path basis.
+As an example, erasure coded files in HDFS do not support the Sync operations,
+even though they are implemented as subclass of an output stream which is `Syncable`.
+
+A new interface: `StreamCapabilities`. This allows callers
+to probe the exact capabilities of a stream, even transitively
+through a chain of streams.
+
+## Output Stream Model
+
+For this specification, an output stream can be viewed as a list of bytes
+stored in the client -the `hsync()` and `hflush()` operations the actions
+which propagate the data to be visible to other readers of the file and/or
+made durable.
+
+```python
+buffer: List[byte]
+```
+
+A flag, `open` tracks whether the stream is open: after the stream
+is closed no more data may be written to it:
+
+```python
+open: bool
+buffer: List[byte]
+```
+
+The destination path of the stream, `path`, can be tracked to form a triple
+`path, open, buffer`
+
+```python
+Stream = (path: Path, open: Boolean, buffer: byte[])
+```
+
+#### Visibility of Flushed Data
+
+(Immediately) after `Syncable` operations which flush data to the filesystem,
+the data at the stream's destination path MUST match that of
+`buffer`. That is, the following condition MUST hold:
+
+```python
+FS'.Files(path) == buffer
+```
+
+Any client reading the data at the path MUST see the new data.
+The `Syncable` operations differ in their durability
+guarantees, not visibility of data.
+
+### State of Stream and File System after `Filesystem.create()`
+
+The output stream returned by a `FileSystem.create(path)` or
+`FileSystem.createFile(path).build()` within a filesystem `FS`,
+can be modeled as a triple containing an empty array of no data:
+
+```python
+Stream' = (path, true, [])
+```
+
+The filesystem `FS'` MUST contain a 0-byte file at the path:
+
+```python
+FS' = FS where data(FS', path) == []
+```
+
+Thus, the initial state of `Stream'.buffer` is implicitly
+consistent with the data at the filesystem.
+
+
+*Object Stores*: see caveats in the "Object Stores" section below.
+
+### State of Stream and File System after `Filesystem.append()`
+
+The output stream returned from a call of
+ `FileSystem.append(path, buffersize, progress)` within a filesystem `FS`,
+can be modelled as a stream whose `buffer` is intialized to that of
+the original file:
+
+```python
+Stream' = (path, true, data(FS, path))
+```
+
+####  Persisting data
+
+When the stream writes data back to its store, be it in any
+supported flush operation, in the `close()` operation, or at any other
+time the stream chooses to do so, the contents of the file
+are replaced with the current buffer
+
+```python
+Stream' = (path, true, buffer)
+FS' = FS where data(FS', path) == buffer
+```
+
+After a call to `close()`, the stream is closed for all operations other
+than `close()`; they MAY fail with `IOException` or `RuntimeException`.
+
+```python
+Stream' = (path, false, [])
+```
+
+The `close()` operation MUST be idempotent with the sole attempt to write the
+data made in the first invocation.
+
+1. If `close()` succeeds, subsequent calls are no-ops.
+1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow
+the previous exception, but they MUST NOT retry the write.
+
+<!--  ============================================================= -->
+<!--  CLASS: FSDataOutputStream -->
+<!--  ============================================================= -->
+
+## <a name="fsdataoutputstream"></a>Class `FSDataOutputStream`
+
+```java
+public class FSDataOutputStream
+  extends DataOutputStream
+  implements Syncable, CanSetDropBehind, StreamCapabilities {
+ // ...
+}
+```
+
+The `FileSystem.create()`, `FileSystem.append()` and
+`FSDataOutputStreamBuilder.build()` calls return an instance
+of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`.
+
+The base class wraps an `OutputStream` instance, one which may implement `Streamable`,
+`CanSetDropBehind` and `StreamCapabilities`.
+
+This document covers the requirements of such implementations.
+
+HDFS's `FileSystem` implementation, `DistributedFileSystem`, returns an instance
+of `HdfsDataOutputStream`. This implementation has at least two behaviors
+which are not explicitly declared by the base Java implmentation
+
+1. Writes are synchronized: more than one thread can write to the same
+output stream. This is a use pattern which HBase relies on.
+
+1. `OutputStream.flush()` is a no-op when the file is closed. Apache Druid
+has made such a call on this in the past
+[HADOOP-14346](https://issues.apache.org/jira/browse/HADOOP-14346).
+
+
+As the HDFS implementation is considered the de-facto specification of
+the FileSystem APIs, the fact that `write()` is thread-safe is significant.
+
+For compatibility, not only SHOULD other FS clients be thread-safe,
+but new HDFS features, such as encryption and Erasure Coding SHOULD also
+implement consistent behavior with the core HDFS output stream.
+
+Put differently:
+
+*It isn't enough for Output Streams to implement the core semantics
+of `java.io.OutputStream`: they need to implement the extra semantics
+of `HdfsDataOutputStream`, especially for HBase to work correctly.*
+
+The concurrent `write()` call is the most significant tightening of
+the Java specification.
+
+## <a name="outputstream"></a>Class `java.io.OutputStream`
+
+A Java `OutputStream` allows applications to write a sequence of bytes to a destination.
+In a Hadoop filesystem, that destination is the data under a path in the filesystem.
+
+```java
+public abstract class OutputStream implements Closeable, Flushable {
+  public abstract void write(int b) throws IOException;
+  public void write(byte b[]) throws IOException;
+  public void write(byte b[], int off, int len) throws IOException;
+  public void flush() throws IOException;
+  public void close() throws IOException;
+}
+```
+### <a name="write(data: int)"></a>`write(Stream, data)`
+
+Writes a byte of data to the stream.
+
+#### Preconditions
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+```
+
+The exception `java.nio.channels.ClosedChannelExceptionn` is
+raised in the HDFS output streams when trying to write to a closed file.
+This exception does not include the destination path; and
+`Exception.getMessage()` is `null`. It is therefore of limited value in stack
+traces. Implementors may wish to raise exceptions with more detail, such
+as a `PathIOException`.
+
+
+#### Postconditions
+
+The buffer has the lower 8 bits of the data argument appended to it.
+
+```python
+Stream'.buffer = Stream.buffer + [data & 0xff]
+```
+
+There may be an explicit limit on the size of cached data, or an implicit
+limit based by the available capacity of the destination filesystem.
+When a limit is reached, `write()` SHOULD fail with an `IOException`.
+
+### <a name="write(buffer,offset,len)"></a>`write(Stream, byte[] data, int offset, int len)`
+
+
+#### Preconditions
+
+The preconditions are all defined in `OutputStream.write()`
+
+```python
+Stream.open else raise ClosedChannelException, PathIOException, IOException
+data != null else raise NullPointerException
+offset >= 0 else raise IndexOutOfBoundsException
+len >= 0 else raise IndexOutOfBoundsException
+offset < data.length else raise IndexOutOfBoundsException
+offset + len < data.length else raise IndexOutOfBoundsException
+```
+
+After the operation has returned, the buffer may be re-used. The outcome
+of updates to the buffer while the `write()` operation is in progress is undefined.
+
+#### Postconditions
+
+```python
+Stream'.buffer = Stream.buffer + data[offset...(offset + len)]
+```
+
+### <a name="write(buffer)"></a>`write(byte[] data)`
+
+This is defined as the equivalent of:
+
+```python
+write(data, 0, data.length)
+```
+
+### <a name="flush()"></a>`flush()`
+
+Requests that the data is flushed. The specification of `ObjectStream.flush()`
+declares that this SHOULD write data to the "intended destination".
+
+It explicitly precludes any guarantees about durability.
+
+For that reason, this document doesn't provide any normative
+specifications of behaviour.
+
+#### Preconditions
+
+None.
+
+#### Postconditions
+
+None.
+
+If the implementation chooses to implement a stream-flushing operation,
+the data may be saved to the file system such that it becomes visible to
+others"
+
+```python
+FS' = FS where data(FS', path) == buffer
+```
+
+When a stream is closed, `flush()` SHOULD downgrade to being a no-op, if it was not
+one already. This is to work with applications and libraries which can invoke
+it in exactly this way.
+
+
+*Issue*: Should `flush()` forward to `hflush()`?
+
+No. Or at least, make it optional.
+
+There's a lot of application code which assumes that `flush()` is low cost
+and should be invoked after writing every single line of output, after
+writing small 4KB blocks or similar.
+
+Forwarding this to a full flush across a distributed filesystem, or worse,
+a distant object store, is very inefficient.
+Filesystem clients which do uprate a `flush()` to an `hflush()` will eventually

Review comment:
       update




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