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http://git-wip-us.apache.org/repos/asf/hbase/blob/e80b3092/src/main/docbkx/cp.xml
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-<?xml version="1.0" encoding="UTF-8"?>
-<chapter
- version="5.0"
- xml:id="cp"
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- xmlns:html="http://www.w3.org/1999/xhtml"
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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.
- */
--->
- <title>Apache HBase Coprocessors</title>
- <para> HBase coprocessors are modeled after the coprocessors which are part of Google's BigTable
- (<link xlink:href="http://www.scribd.com/doc/21631448/Dean-Keynote-Ladis2009"/>, pages
- 66-67.). Coprocessors function in a similar way to Linux kernel modules. They provide a way to
- run server-level code against locally-stored data. The functionality they provide is very
- powerful, but also carries great risk and can have adverse effects on the system, at the level
- of the operating system. The information in this chapter is primarily sourced and heavily reused
- from Mingjie Lai's blog post at <link
- xlink:href="https://blogs.apache.org/hbase/entry/coprocessor_introduction"/>. </para>
-
- <para> Coprocessors are not designed to be used by end users of HBase, but by HBase developers who
- need to add specialized functionality to HBase. One example of the use of coprocessors is
- pluggable compaction and scan policies, which are provided as coprocessors in <link
- xlink:href="HBASE-6427">HBASE-6427</link>. </para>
-
- <section>
- <title>Coprocessor Framework</title>
- <para>The implementation of HBase coprocessors diverges from the BigTable implementation. The
- HBase framework provides a library and runtime environment for executing user code within the
- HBase region server and master processes. </para>
- <para> The framework API is provided in the <link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/package-summary.html">coprocessor</link>
- package.</para>
- <para>Two different types of coprocessors are provided by the framework, based on their
- scope.</para>
- <variablelist>
- <title>Types of Coprocessors</title>
- <varlistentry>
- <term>System Coprocessors</term>
- <listitem>
- <para>System coprocessors are loaded globally on all tables and regions hosted by a region
- server.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Table Coprocessors</term>
- <listitem>
- <para>You can specify which coprocessors should be loaded on all regions for a table on a
- per-table basis.</para>
- </listitem>
- </varlistentry>
- </variablelist>
-
- <para>The framework provides two different aspects of extensions as well:
- <firstterm>observers</firstterm> and <firstterm>endpoints</firstterm>.</para>
- <variablelist>
- <varlistentry>
- <term>Observers</term>
- <listitem>
- <para>Observers are analogous to triggers in conventional databases. They allow you to
- insert user code by overriding upcall methods provided by the coprocessor framework.
- Callback functions are executed from core HBase code when events occur. Callbacks are
- handled by the framework, and the coprocessor itself only needs to insert the extended
- or alternate functionality.</para>
- <variablelist>
- <title>Provided Observer Interfaces</title>
- <varlistentry>
- <term><link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/RegionObserver.html">RegionObserver</link></term>
- <listitem>
- <para>A RegionObserver provides hooks for data manipulation events, such as Get,
- Put, Delete, Scan. An instance of a RegionObserver coprocessor exists for each
- table region. The scope of the observations a RegionObserver can make is
- constrained to that region. </para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/RegionServerObserver.html">RegionServerObserver</link></term>
- <listitem>
- <para>A RegionServerObserver provides for operations related to the RegionServer,
- such as stopping the RegionServer and performing operations before or after
- merges, commits, or rollbacks.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/WALObserver.html">WALObserver</link></term>
- <listitem>
- <para>A WALObserver provides hooks for operations related to the write-ahead log
- (WAL). You can observe or intercept WAL writing and reconstruction events. A
- WALObserver runs in the context of WAL processing. A single WALObserver exists on
- a single region server.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/MasterObserver.html">MasterObserver</link></term>
- <listitem>
- <para>A MasterObserver provides hooks for DDL-type operation, such as create,
- delete, modify table. The MasterObserver runs within the context of the HBase
- master. </para>
- </listitem>
- </varlistentry>
- </variablelist>
- <para>More than one observer of a given type can be loaded at once. Multiple observers are
- chained to execute sequentially by order of assigned priority. Nothing prevents a
- coprocessor implementor from communicating internally among its installed
- observers.</para>
- <para>An observer of a higher priority can preempt lower-priority observers by throwing an
- IOException or a subclass of IOException.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Endpoints (HBase 0.96.x and later)</term>
- <listitem>
- <para>The implementation for endpoints changed significantly in HBase 0.96.x due to the
- introduction of protocol buffers (protobufs) (<link
- xlink:href="https://issues.apache.org/jira/browse/HBASE-5448">HBASE-5488</link>). If
- you created endpoints before 0.96.x, you will need to rewrite them. Endpoints are now
- defined and callable as protobuf services, rather than endpoint invocations passed
- through as Writable blobs</para>
- <para>Dynamic RPC endpoints resemble stored procedures. An endpoint can be invoked at any
- time from the client. When it is invoked, it is executed remotely at the target region
- or regions, and results of the executions are returned to the client.</para>
- <para>The endpoint implementation is installed on the server and is invoked using HBase
- RPC. The client library provides convenience methods for invoking these dynamic
- interfaces. </para>
- <para>An endpoint, like an observer, can communicate with any installed observers. This
- allows you to plug new features into HBase without modifying or recompiling HBase
- itself.</para>
- <itemizedlist>
- <title>Steps to Implement an Endpoint</title>
- <listitem><para>Define the coprocessor service and related messages in a <filename>.proto</filename> file</para></listitem>
- <listitem><para>Run the <command>protoc</command> command to generate the code.</para></listitem>
- <listitem><para>Write code to implement the following:</para>
- <itemizedlist>
- <listitem><para>the generated protobuf Service interface</para></listitem>
- <listitem>
- <para>the new <link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/HTable.html#coprocessorService(byte[])">org.apache.hadoop.hbase.coprocessor.CoprocessorService</link>
- interface (required for the <link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.html">RegionCoprocessorHost</link>
- to register the exposed service)</para></listitem>
- </itemizedlist>
- </listitem>
- <listitem><para>The client calls the new HTable.coprocessorService() methods to perform the endpoint RPCs.</para></listitem>
- </itemizedlist>
-
- <para>For more information and examples, refer to the API documentation for the <link
- xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/coprocessor/package-summary.html">coprocessor</link>
- package, as well as the included RowCount example in the
- <filename>/hbase-examples/src/test/java/org/apache/hadoop/hbase/coprocessor/example/</filename>
- of the HBase source.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Endpoints (HBase 0.94.x and earlier)</term>
- <listitem>
- <para>Dynamic RPC endpoints resemble stored procedures. An endpoint can be invoked at any
- time from the client. When it is invoked, it is executed remotely at the target region
- or regions, and results of the executions are returned to the client.</para>
- <para>The endpoint implementation is installed on the server and is invoked using HBase
- RPC. The client library provides convenience methods for invoking these dynamic
- interfaces. </para>
- <para>An endpoint, like an observer, can communicate with any installed observers. This
- allows you to plug new features into HBase without modifying or recompiling HBase
- itself.</para>
- <itemizedlist>
- <title>Steps to Implement an Endpoint</title>
- <listitem>
- <bridgehead>Server-Side</bridgehead>
- <itemizedlist>
- <listitem>
- <para>Create new protocol interface which extends CoprocessorProtocol.</para>
- </listitem>
- <listitem>
- <para>Implement the Endpoint interface. The implementation will be loaded into and
- executed from the region context.</para>
- </listitem>
- <listitem>
- <para>Extend the abstract class BaseEndpointCoprocessor. This convenience class
- hides some internal details that the implementer does not need to be concerned
- about, ˆ such as coprocessor framework class loading.</para>
- </listitem>
- </itemizedlist>
- </listitem>
- <listitem>
- <bridgehead>Client-Side</bridgehead>
- <para>Endpoint can be invoked by two new HBase client APIs:</para>
- <itemizedlist>
- <listitem>
- <para><code>HTableInterface.coprocessorProxy(Class<T> protocol, byte[]
- row)</code> for executing against a single region</para>
- </listitem>
- <listitem>
- <para><code>HTableInterface.coprocessorExec(Class<T> protocol, byte[]
- startKey, byte[] endKey, Batch.Call<T,R> callable)</code> for executing
- over a range of regions</para>
- </listitem>
- </itemizedlist>
- </listitem>
- </itemizedlist>
- </listitem>
- </varlistentry>
- </variablelist>
- </section>
-
- <section>
- <title>Examples</title>
- <para>An example of an observer is included in
- <filename>hbase-examples/src/test/java/org/apache/hadoop/hbase/coprocessor/example/TestZooKeeperScanPolicyObserver.java</filename>.
- Several endpoint examples are included in the same directory.</para>
- </section>
-
-
-
- <section>
- <title>Building A Coprocessor</title>
-
- <para>Before you can build a processor, it must be developed, compiled, and packaged in a JAR
- file. The next step is to configure the coprocessor framework to use your coprocessor. You can
- load the coprocessor from your HBase configuration, so that the coprocessor starts with HBase,
- or you can configure the coprocessor from the HBase shell, as a table attribute, so that it is
- loaded dynamically when the table is opened or reopened.</para>
- <section>
- <title>Load from Configuration</title>
- <para> To configure a coprocessor to be loaded when HBase starts, modify the RegionServer's
- <filename>hbase-site.xml</filename> and configure one of the following properties, based
- on the type of observer you are configuring: </para>
- <itemizedlist>
- <listitem>
- <para><code>hbase.coprocessor.region.classes</code>for RegionObservers and
- Endpoints</para>
- </listitem>
- <listitem>
- <para><code>hbase.coprocessor.wal.classes</code>for WALObservers</para>
- </listitem>
- <listitem>
- <para><code>hbase.coprocessor.master.classes</code>for MasterObservers</para>
- </listitem>
- </itemizedlist>
- <example>
- <title>Example RegionObserver Configuration</title>
- <para>In this example, one RegionObserver is configured for all the HBase tables.</para>
- <screen language="xml"><![CDATA[
-<property>
- <name>hbase.coprocessor.region.classes</name>
- <value>org.apache.hadoop.hbase.coprocessor.AggregateImplementation</value>
- </property> ]]>
- </screen>
- </example>
-
- <para> If multiple classes are specified for loading, the class names must be comma-separated.
- The framework attempts to load all the configured classes using the default class loader.
- Therefore, the jar file must reside on the server-side HBase classpath.</para>
-
- <para>Coprocessors which are loaded in this way will be active on all regions of
- all tables. These are the system coprocessor introduced earlier. The first listed
- coprocessors will be assigned the priority <literal>Coprocessor.Priority.SYSTEM</literal>.
- Each subsequent coprocessor in the list will have its priority value incremented by one
- (which reduces its priority, because priorities have the natural sort order of Integers). </para>
- <para>When calling out to registered observers, the framework executes their callbacks methods
- in the sorted order of their priority. Ties are broken arbitrarily.</para>
- </section>
-
- <section>
- <title>Load from the HBase Shell</title>
- <para> You can load a coprocessor on a specific table via a table attribute. The following
- example will load the <systemitem>FooRegionObserver</systemitem> observer when table
- <systemitem>t1</systemitem> is read or re-read. </para>
- <example>
- <title>Load a Coprocessor On a Table Using HBase Shell</title>
- <screen>
-hbase(main):005:0> <userinput>alter 't1', METHOD => 'table_att',
- 'coprocessor'=>'hdfs:///foo.jar|com.foo.FooRegionObserver|1001|arg1=1,arg2=2'</userinput>
-<computeroutput>Updating all regions with the new schema...
-1/1 regions updated.
-Done.
-0 row(s) in 1.0730 seconds</computeroutput>
-
-hbase(main):006:0> <userinput>describe 't1'</userinput>
-<computeroutput>DESCRIPTION ENABLED
- {NAME => 't1', coprocessor$1 => 'hdfs:///foo.jar|com.foo.FooRegio false
- nObserver|1001|arg1=1,arg2=2', FAMILIES => [{NAME => 'c1', DATA_B
- LOCK_ENCODING => 'NONE', BLOOMFILTER => 'NONE', REPLICATION_SCOPE
- => '0', VERSIONS => '3', COMPRESSION => 'NONE', MIN_VERSIONS =>
- '0', TTL => '2147483647', KEEP_DELETED_CELLS => 'false', BLOCKSIZ
- E => '65536', IN_MEMORY => 'false', ENCODE_ON_DISK => 'true', BLO
- CKCACHE => 'true'}, {NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE',
- BLOOMFILTER => 'NONE', REPLICATION_SCOPE => '0', VERSIONS => '3'
- , COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => '2147483647'
- , KEEP_DELETED_CELLS => 'false', BLOCKSIZE => '65536', IN_MEMORY
- => 'false', ENCODE_ON_DISK => 'true', BLOCKCACHE => 'true'}]}
-1 row(s) in 0.0190 seconds</computeroutput>
- </screen>
- </example>
-
- <para>The coprocessor framework will try to read the class information from the coprocessor
- table attribute value. The value contains four pieces of information which are separated by
- the <literal>|</literal> character.</para>
-
- <itemizedlist>
- <listitem>
- <para>File path: The jar file containing the coprocessor implementation must be in a
- location where all region servers can read it. You could copy the file onto the local
- disk on each region server, but it is recommended to store it in HDFS.</para>
- </listitem>
- <listitem>
- <para>Class name: The full class name of the coprocessor.</para>
- </listitem>
- <listitem>
- <para>Priority: An integer. The framework will determine the execution sequence of all
- configured observers registered at the same hook using priorities. This field can be
- left blank. In that case the framework will assign a default priority value.</para>
- </listitem>
- <listitem>
- <para>Arguments: This field is passed to the coprocessor implementation.</para>
- </listitem>
- </itemizedlist>
- <example>
- <title>Unload a Coprocessor From a Table Using HBase Shell</title>
- <screen>
-hbase(main):007:0> <userinput>alter 't1', METHOD => 'table_att_unset',</userinput>
-hbase(main):008:0* <userinput>NAME => 'coprocessor$1'</userinput>
-<computeroutput>Updating all regions with the new schema...
-1/1 regions updated.
-Done.
-0 row(s) in 1.1130 seconds</computeroutput>
-
-hbase(main):009:0> <userinput>describe 't1'</userinput>
-<computeroutput>DESCRIPTION ENABLED
- {NAME => 't1', FAMILIES => [{NAME => 'c1', DATA_BLOCK_ENCODING => false
- 'NONE', BLOOMFILTER => 'NONE', REPLICATION_SCOPE => '0', VERSION
- S => '3', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => '214
- 7483647', KEEP_DELETED_CELLS => 'false', BLOCKSIZE => '65536', IN
- _MEMORY => 'false', ENCODE_ON_DISK => 'true', BLOCKCACHE => 'true
- '}, {NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER =>
- 'NONE', REPLICATION_SCOPE => '0', VERSIONS => '3', COMPRESSION =>
- 'NONE', MIN_VERSIONS => '0', TTL => '2147483647', KEEP_DELETED_C
- ELLS => 'false', BLOCKSIZE => '65536', IN_MEMORY => 'false', ENCO
- DE_ON_DISK => 'true', BLOCKCACHE => 'true'}]}
-1 row(s) in 0.0180 seconds </computeroutput>
- </screen>
- </example>
- <warning>
- <para>There is no guarantee that the framework will load a given coprocessor successfully.
- For example, the shell command neither guarantees a jar file exists at a particular
- location nor verifies whether the given class is actually contained in the jar file.
- </para>
- </warning>
- </section>
- </section>
- <section>
- <title>Check the Status of a Coprocessor</title>
- <para>To check the status of a coprocessor after it has been configured, use the
- <command>status</command> HBase Shell command.</para>
- <screen>
-hbase(main):020:0> <userinput>status 'detailed'</userinput>
-<computeroutput>version 0.92-tm-6
-0 regionsInTransition
-master coprocessors: []
-1 live servers
- localhost:52761 1328082515520
- requestsPerSecond=3, numberOfOnlineRegions=3, usedHeapMB=32, maxHeapMB=995
- -ROOT-,,0
- numberOfStores=1, numberOfStorefiles=1, storefileUncompressedSizeMB=0, storefileSizeMB=0, memstoreSizeMB=0,
-storefileIndexSizeMB=0, readRequestsCount=54, writeRequestsCount=1, rootIndexSizeKB=0, totalStaticIndexSizeKB=0,
-totalStaticBloomSizeKB=0, totalCompactingKVs=0, currentCompactedKVs=0, compactionProgressPct=NaN, coprocessors=[]
- .META.,,1
- numberOfStores=1, numberOfStorefiles=0, storefileUncompressedSizeMB=0, storefileSizeMB=0, memstoreSizeMB=0,
-storefileIndexSizeMB=0, readRequestsCount=97, writeRequestsCount=4, rootIndexSizeKB=0, totalStaticIndexSizeKB=0,
-totalStaticBloomSizeKB=0, totalCompactingKVs=0, currentCompactedKVs=0, compactionProgressPct=NaN, coprocessors=[]
- t1,,1328082575190.c0491168a27620ffe653ec6c04c9b4d1.
- numberOfStores=2, numberOfStorefiles=1, storefileUncompressedSizeMB=0, storefileSizeMB=0, memstoreSizeMB=0,
-storefileIndexSizeMB=0, readRequestsCount=0, writeRequestsCount=0, rootIndexSizeKB=0, totalStaticIndexSizeKB=0,
-totalStaticBloomSizeKB=0, totalCompactingKVs=0, currentCompactedKVs=0, compactionProgressPct=NaN,
-coprocessors=[AggregateImplementation]
-0 dead servers </computeroutput>
- </screen>
- </section>
- <section>
- <title>Monitor Time Spent in Coprocessors</title>
- <para>HBase 0.98.5 introduced the ability to monitor some statistics relating to the amount of
- time spent executing a given coprocessor. You can see these statistics via the HBase Metrics
- framework (see <xref linkend="hbase_metrics"/> or the Web UI for a given Region Server, via
- the <guilabel>Coprocessor Metrics</guilabel> tab. These statistics are valuable for debugging
- and benchmarking the performance impact of a given coprocessor on your cluster. Tracked
- statistics include min, max, average, and 90th, 95th, and 99th percentile. All times are shown
- in milliseconds. The statistics are calculated over coprocessor
- execution samples recorded during the reporting interval, which is 10 seconds by default. The
- metrics sampling rate as described in <xref linkend="hbase_metrics" />.</para>
- <figure>
- <title>Coprocessor Metrics UI</title>
- <mediaobject>
- <imageobject>
- <imagedata fileref="coprocessor_stats.png" width="100%"/>
- </imageobject>
- <caption>
- <para>The Coprocessor Metrics UI shows statistics about time spent executing a given
- coprocessor, including min, max, average, and 90th, 95th, and 99th percentile.</para>
- </caption>
- </mediaobject>
- </figure>
- </section>
- <section>
- <title>Status of Coprocessors in HBase</title>
- <para> Coprocessors and the coprocessor framework are evolving rapidly and work is ongoing on
- several different JIRAs. </para>
- </section>
-</chapter>
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- * 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,
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- <xsl:attribute name="font-family">
- <xsl:value-of select="$monospace.font.family"/>
- </xsl:attribute>
- <xsl:attribute name="font-size">0.8em</xsl:attribute>
- <xsl:attribute name="wrap-option">wrap</xsl:attribute>
- <xsl:attribute name="hyphenate">true</xsl:attribute>
- </xsl:attribute-set>
-
-
- <!-- add page break after abstract block -->
- <xsl:attribute-set name="abstract.properties">
- <xsl:attribute name="break-after">page</xsl:attribute>
- </xsl:attribute-set>
-
- <!-- add page break after toc -->
- <xsl:attribute-set name="toc.margin.properties">
- <xsl:attribute name="break-after">page</xsl:attribute>
- </xsl:attribute-set>
-
- <!-- add page break after first level sections -->
- <xsl:attribute-set name="section.level1.properties">
- <xsl:attribute name="break-after">page</xsl:attribute>
- </xsl:attribute-set>
-
- <!-- Show only Sections up to level 3 in the TOCs -->
- <xsl:param name="toc.section.depth">2</xsl:param>
-
- <!-- Dot and Whitespace as separator in TOC between Label and Title-->
- <xsl:param name="autotoc.label.separator" select="'. '"/>
-
- <!-- program listings / examples formatting -->
- <xsl:attribute-set name="monospace.verbatim.properties">
- <xsl:attribute name="font-family">Courier</xsl:attribute>
- <xsl:attribute name="font-size">8pt</xsl:attribute>
- <xsl:attribute name="keep-together.within-column">always</xsl:attribute>
- </xsl:attribute-set>
-
- <xsl:param name="shade.verbatim" select="1" />
-
- <xsl:attribute-set name="shade.verbatim.style">
- <xsl:attribute name="background-color">#E8E8E8</xsl:attribute>
- <xsl:attribute name="border-width">0.5pt</xsl:attribute>
- <xsl:attribute name="border-style">solid</xsl:attribute>
- <xsl:attribute name="border-color">#575757</xsl:attribute>
- <xsl:attribute name="padding">3pt</xsl:attribute>
- </xsl:attribute-set>
-
- <!-- callouts customization -->
- <xsl:param name="callout.unicode" select="1" />
- <xsl:param name="callout.graphics" select="0" />
- <xsl:param name="callout.defaultcolumn">90</xsl:param>
-
- <!-- Syntax Highlighting -->
-
-
-</xsl:stylesheet>
http://git-wip-us.apache.org/repos/asf/hbase/blob/e80b3092/src/main/docbkx/customization.xsl
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diff --git a/src/main/docbkx/customization.xsl b/src/main/docbkx/customization.xsl
deleted file mode 100644
index 5d0ec2c..0000000
--- a/src/main/docbkx/customization.xsl
+++ /dev/null
@@ -1,49 +0,0 @@
-<?xml version="1.0"?>
-<xsl:stylesheet
- xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
- version="1.0">
-<!--
-/**
- * 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.
- */
--->
- <xsl:import href="urn:docbkx:stylesheet"/>
- <xsl:import href="urn:docbkx:stylesheet/highlight.xsl"/>
- <xsl:output method="html" encoding="UTF-8" indent="no"/>
-
- <xsl:template name="user.header.content">
- <script type="text/javascript">
- var disqus_shortname = 'hbase'; // required: replace example with your forum shortname
- var disqus_url = 'http://hbase.apache.org/book/<xsl:value-of select="@xml:id" />.html';
- <!--var disqus_identifier = '<xsl:value-of select="@xml:id" />';--></script>
- </xsl:template>
-
- <xsl:template name="user.footer.content">
-<div id="disqus_thread"></div>
-<script type="text/javascript">
- /* * * DON'T EDIT BELOW THIS LINE * * */
- (function() {
- var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true;
- dsq.src = 'http://' + disqus_shortname + '.disqus.com/embed.js';
- (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
- })();
-</script>
-<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript>
-<a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a>
- </xsl:template>
-
-</xsl:stylesheet>
http://git-wip-us.apache.org/repos/asf/hbase/blob/e80b3092/src/main/docbkx/datamodel.xml
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+++ /dev/null
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-<?xml version="1.0" encoding="UTF-8"?>
-<chapter
- xml:id="datamodel"
- version="5.0"
- xmlns="http://docbook.org/ns/docbook"
- xmlns:xlink="http://www.w3.org/1999/xlink"
- xmlns:xi="http://www.w3.org/2001/XInclude"
- xmlns:svg="http://www.w3.org/2000/svg"
- xmlns:m="http://www.w3.org/1998/Math/MathML"
- xmlns:html="http://www.w3.org/1999/xhtml"
- xmlns:db="http://docbook.org/ns/docbook">
- <!--/**
- * 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.
- */
--->
-
- <title>Data Model</title>
- <para>In HBase, data is stored in tables, which have rows and columns. This is a terminology
- overlap with relational databases (RDBMSs), but this is not a helpful analogy. Instead, it can
- be helpful to think of an HBase table as a multi-dimensional map.</para>
- <variablelist>
- <title>HBase Data Model Terminology</title>
- <varlistentry>
- <term>Table</term>
- <listitem>
- <para>An HBase table consists of multiple rows.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Row</term>
- <listitem>
- <para>A row in HBase consists of a row key and one or more columns with values associated
- with them. Rows are sorted alphabetically by the row key as they are stored. For this
- reason, the design of the row key is very important. The goal is to store data in such a
- way that related rows are near each other. A common row key pattern is a website domain.
- If your row keys are domains, you should probably store them in reverse (org.apache.www,
- org.apache.mail, org.apache.jira). This way, all of the Apache domains are near each
- other in the table, rather than being spread out based on the first letter of the
- subdomain.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Column</term>
- <listitem>
- <para>A column in HBase consists of a column family and a column qualifier, which are
- delimited by a <literal>:</literal> (colon) character.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Column Family</term>
- <listitem>
- <para>Column families physically colocate a set of columns and their values, often for
- performance reasons. Each column family has a set of storage properties, such as whether
- its values should be cached in memory, how its data is compressed or its row keys are
- encoded, and others. Each row in a table has the same column
- families, though a given row might not store anything in a given column family.</para>
- <para>Column families are specified when you create your table, and influence the way your
- data is stored in the underlying filesystem. Therefore, the column families should be
- considered carefully during schema design.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Column Qualifier</term>
- <listitem>
- <para>A column qualifier is added to a column family to provide the index for a given
- piece of data. Given a column family <literal>content</literal>, a column qualifier
- might be <literal>content:html</literal>, and another might be
- <literal>content:pdf</literal>. Though column families are fixed at table creation,
- column qualifiers are mutable and may differ greatly between rows.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Cell</term>
- <listitem>
- <para>A cell is a combination of row, column family, and column qualifier, and contains a
- value and a timestamp, which represents the value's version.</para>
- <para>A cell's value is an uninterpreted array of bytes.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Timestamp</term>
- <listitem>
- <para>A timestamp is written alongside each value, and is the identifier for a given
- version of a value. By default, the timestamp represents the time on the RegionServer
- when the data was written, but you can specify a different timestamp value when you put
- data into the cell.</para>
- <caution>
- <para>Direct manipulation of timestamps is an advanced feature which is only exposed for
- special cases that are deeply integrated with HBase, and is discouraged in general.
- Encoding a timestamp at the application level is the preferred pattern.</para>
- </caution>
- <para>You can specify the maximum number of versions of a value that HBase retains, per column
- family. When the maximum number of versions is reached, the oldest versions are
- eventually deleted. By default, only the newest version is kept.</para>
- </listitem>
- </varlistentry>
- </variablelist>
-
- <section
- xml:id="conceptual.view">
- <title>Conceptual View</title>
- <para>You can read a very understandable explanation of the HBase data model in the blog post <link
- xlink:href="http://jimbojw.com/wiki/index.php?title=Understanding_Hbase_and_BigTable">Understanding
- HBase and BigTable</link> by Jim R. Wilson. Another good explanation is available in the
- PDF <link
- xlink:href="http://0b4af6cdc2f0c5998459-c0245c5c937c5dedcca3f1764ecc9b2f.r43.cf2.rackcdn.com/9353-login1210_khurana.pdf">Introduction
- to Basic Schema Design</link> by Amandeep Khurana. It may help to read different
- perspectives to get a solid understanding of HBase schema design. The linked articles cover
- the same ground as the information in this section.</para>
- <para> The following example is a slightly modified form of the one on page 2 of the <link
- xlink:href="http://research.google.com/archive/bigtable.html">BigTable</link> paper. There
- is a table called <varname>webtable</varname> that contains two rows
- (<literal>com.cnn.www</literal>
- and <literal>com.example.www</literal>), three column families named
- <varname>contents</varname>, <varname>anchor</varname>, and <varname>people</varname>. In
- this example, for the first row (<literal>com.cnn.www</literal>),
- <varname>anchor</varname> contains two columns (<varname>anchor:cssnsi.com</varname>,
- <varname>anchor:my.look.ca</varname>) and <varname>contents</varname> contains one column
- (<varname>contents:html</varname>). This example contains 5 versions of the row with the
- row key <literal>com.cnn.www</literal>, and one version of the row with the row key
- <literal>com.example.www</literal>. The <varname>contents:html</varname> column qualifier contains the entire
- HTML of a given website. Qualifiers of the <varname>anchor</varname> column family each
- contain the external site which links to the site represented by the row, along with the
- text it used in the anchor of its link. The <varname>people</varname> column family represents
- people associated with the site.
- </para>
- <note>
- <title>Column Names</title>
- <para> By convention, a column name is made of its column family prefix and a
- <emphasis>qualifier</emphasis>. For example, the column
- <emphasis>contents:html</emphasis> is made up of the column family
- <varname>contents</varname> and the <varname>html</varname> qualifier. The colon
- character (<literal>:</literal>) delimits the column family from the column family
- <emphasis>qualifier</emphasis>. </para>
- </note>
- <table
- frame="all">
- <title>Table <varname>webtable</varname></title>
- <tgroup
- cols="5"
- align="left"
- colsep="1"
- rowsep="1">
- <colspec
- colname="c1" />
- <colspec
- colname="c2" />
- <colspec
- colname="c3" />
- <colspec
- colname="c4" />
- <colspec
- colname="c5" />
- <thead>
- <row>
- <entry>Row Key</entry>
- <entry>Time Stamp</entry>
- <entry>ColumnFamily <varname>contents</varname></entry>
- <entry>ColumnFamily <varname>anchor</varname></entry>
- <entry>ColumnFamily <varname>people</varname></entry>
- </row>
- </thead>
- <tbody>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t9</entry>
- <entry />
- <entry><varname>anchor:cnnsi.com</varname> = "CNN"</entry>
- <entry />
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t8</entry>
- <entry />
- <entry><varname>anchor:my.look.ca</varname> = "CNN.com"</entry>
- <entry />
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t6</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- <entry />
- <entry />
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t5</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- <entry />
- <entry />
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t3</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- <entry />
- <entry />
- </row>
- <row>
- <entry>"com.example.www"</entry>
- <entry>t5</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- <entry></entry>
- <entry>people:author = "John Doe"</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>Cells in this table that appear to be empty do not take space, or in fact exist, in
- HBase. This is what makes HBase "sparse." A tabular view is not the only possible way to
- look at data in HBase, or even the most accurate. The following represents the same
- information as a multi-dimensional map. This is only a mock-up for illustrative
- purposes and may not be strictly accurate.</para>
- <programlisting><![CDATA[
-{
- "com.cnn.www": {
- contents: {
- t6: contents:html: "<html>..."
- t5: contents:html: "<html>..."
- t3: contents:html: "<html>..."
- }
- anchor: {
- t9: anchor:cnnsi.com = "CNN"
- t8: anchor:my.look.ca = "CNN.com"
- }
- people: {}
- }
- "com.example.www": {
- contents: {
- t5: contents:html: "<html>..."
- }
- anchor: {}
- people: {
- t5: people:author: "John Doe"
- }
- }
-}
- ]]></programlisting>
-
- </section>
- <section
- xml:id="physical.view">
- <title>Physical View</title>
- <para> Although at a conceptual level tables may be viewed as a sparse set of rows, they are
- physically stored by column family. A new column qualifier (column_family:column_qualifier)
- can be added to an existing column family at any time.</para>
- <table
- frame="all">
- <title>ColumnFamily <varname>anchor</varname></title>
- <tgroup
- cols="3"
- align="left"
- colsep="1"
- rowsep="1">
- <colspec
- colname="c1" />
- <colspec
- colname="c2" />
- <colspec
- colname="c3" />
- <thead>
- <row>
- <entry>Row Key</entry>
- <entry>Time Stamp</entry>
- <entry>Column Family <varname>anchor</varname></entry>
- </row>
- </thead>
- <tbody>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t9</entry>
- <entry><varname>anchor:cnnsi.com</varname> = "CNN"</entry>
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t8</entry>
- <entry><varname>anchor:my.look.ca</varname> = "CNN.com"</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <table
- frame="all">
- <title>ColumnFamily <varname>contents</varname></title>
- <tgroup
- cols="3"
- align="left"
- colsep="1"
- rowsep="1">
- <colspec
- colname="c1" />
- <colspec
- colname="c2" />
- <colspec
- colname="c3" />
- <thead>
- <row>
- <entry>Row Key</entry>
- <entry>Time Stamp</entry>
- <entry>ColumnFamily "contents:"</entry>
- </row>
- </thead>
- <tbody>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t6</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t5</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- </row>
- <row>
- <entry>"com.cnn.www"</entry>
- <entry>t3</entry>
- <entry><varname>contents:html</varname> = "<html>..."</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>The empty cells shown in the
- conceptual view are not stored at all.
- Thus a request for the value of the <varname>contents:html</varname> column at time stamp
- <literal>t8</literal> would return no value. Similarly, a request for an
- <varname>anchor:my.look.ca</varname> value at time stamp <literal>t9</literal> would
- return no value. However, if no timestamp is supplied, the most recent value for a
- particular column would be returned. Given multiple versions, the most recent is also the
- first one found, since timestamps
- are stored in descending order. Thus a request for the values of all columns in the row
- <varname>com.cnn.www</varname> if no timestamp is specified would be: the value of
- <varname>contents:html</varname> from timestamp <literal>t6</literal>, the value of
- <varname>anchor:cnnsi.com</varname> from timestamp <literal>t9</literal>, the value of
- <varname>anchor:my.look.ca</varname> from timestamp <literal>t8</literal>. </para>
- <para>For more information about the internals of how Apache HBase stores data, see <xref
- linkend="regions.arch" />. </para>
- </section>
-
- <section
- xml:id="namespace">
- <title>Namespace</title>
- <para> A namespace is a logical grouping of tables analogous to a database in relation
- database systems. This abstraction lays the groundwork for upcoming multi-tenancy related
- features: <itemizedlist>
- <listitem>
- <para>Quota Management (HBASE-8410) - Restrict the amount of resources (ie regions,
- tables) a namespace can consume.</para>
- </listitem>
- <listitem>
- <para>Namespace Security Administration (HBASE-9206) - provide another level of security
- administration for tenants.</para>
- </listitem>
- <listitem>
- <para>Region server groups (HBASE-6721) - A namespace/table can be pinned onto a subset
- of regionservers thus guaranteeing a course level of isolation.</para>
- </listitem>
- </itemizedlist>
- </para>
- <section
- xml:id="namespace_creation">
- <title>Namespace management</title>
- <para> A namespace can be created, removed or altered. Namespace membership is determined
- during table creation by specifying a fully-qualified table name of the form:</para>
-
- <programlisting language="xml"><![CDATA[<table namespace>:<table qualifier>]]></programlisting>
-
-
- <example>
- <title>Examples</title>
-
- <programlisting language="bourne">
-#Create a namespace
-create_namespace 'my_ns'
- </programlisting>
- <programlisting language="bourne">
-#create my_table in my_ns namespace
-create 'my_ns:my_table', 'fam'
- </programlisting>
- <programlisting language="bourne">
-#drop namespace
-drop_namespace 'my_ns'
- </programlisting>
- <programlisting language="bourne">
-#alter namespace
-alter_namespace 'my_ns', {METHOD => 'set', 'PROPERTY_NAME' => 'PROPERTY_VALUE'}
- </programlisting>
- </example>
- </section>
- <section
- xml:id="namespace_special">
- <title>Predefined namespaces</title>
- <para> There are two predefined special namespaces: </para>
- <itemizedlist>
- <listitem>
- <para>hbase - system namespace, used to contain hbase internal tables</para>
- </listitem>
- <listitem>
- <para>default - tables with no explicit specified namespace will automatically fall into
- this namespace.</para>
- </listitem>
- </itemizedlist>
- <example>
- <title>Examples</title>
-
- <programlisting language="bourne">
-#namespace=foo and table qualifier=bar
-create 'foo:bar', 'fam'
-
-#namespace=default and table qualifier=bar
-create 'bar', 'fam'
-</programlisting>
- </example>
- </section>
- </section>
-
- <section
- xml:id="table">
- <title>Table</title>
- <para> Tables are declared up front at schema definition time. </para>
- </section>
-
- <section
- xml:id="row">
- <title>Row</title>
- <para>Row keys are uninterrpreted bytes. Rows are lexicographically sorted with the lowest
- order appearing first in a table. The empty byte array is used to denote both the start and
- end of a tables' namespace.</para>
- </section>
-
- <section
- xml:id="columnfamily">
- <title>Column Family<indexterm><primary>Column Family</primary></indexterm></title>
- <para> Columns in Apache HBase are grouped into <emphasis>column families</emphasis>. All
- column members of a column family have the same prefix. For example, the columns
- <emphasis>courses:history</emphasis> and <emphasis>courses:math</emphasis> are both
- members of the <emphasis>courses</emphasis> column family. The colon character
- (<literal>:</literal>) delimits the column family from the <indexterm><primary>column
- family qualifier</primary><secondary>Column Family Qualifier</secondary></indexterm>.
- The column family prefix must be composed of <emphasis>printable</emphasis> characters. The
- qualifying tail, the column family <emphasis>qualifier</emphasis>, can be made of any
- arbitrary bytes. Column families must be declared up front at schema definition time whereas
- columns do not need to be defined at schema time but can be conjured on the fly while the
- table is up an running.</para>
- <para>Physically, all column family members are stored together on the filesystem. Because
- tunings and storage specifications are done at the column family level, it is advised that
- all column family members have the same general access pattern and size
- characteristics.</para>
-
- </section>
- <section
- xml:id="cells">
- <title>Cells<indexterm><primary>Cells</primary></indexterm></title>
- <para>A <emphasis>{row, column, version} </emphasis>tuple exactly specifies a
- <literal>cell</literal> in HBase. Cell content is uninterrpreted bytes</para>
- </section>
- <section
- xml:id="data_model_operations">
- <title>Data Model Operations</title>
- <para>The four primary data model operations are Get, Put, Scan, and Delete. Operations are
- applied via <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html">Table</link>
- instances.
- </para>
- <section
- xml:id="get">
- <title>Get</title>
- <para><link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Get.html">Get</link>
- returns attributes for a specified row. Gets are executed via <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#get(org.apache.hadoop.hbase.client.Get)">
- Table.get</link>. </para>
- </section>
- <section
- xml:id="put">
- <title>Put</title>
- <para><link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Put.html">Put</link>
- either adds new rows to a table (if the key is new) or can update existing rows (if the
- key already exists). Puts are executed via <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#put(org.apache.hadoop.hbase.client.Put)">
- Table.put</link> (writeBuffer) or <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#batch(java.util.List, java.lang.Object[])">
- Table.batch</link> (non-writeBuffer). </para>
- </section>
- <section
- xml:id="scan">
- <title>Scans</title>
- <para><link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Scan.html">Scan</link>
- allow iteration over multiple rows for specified attributes. </para>
- <para>The following is an example of a Scan on a Table instance. Assume that a table is
- populated with rows with keys "row1", "row2", "row3", and then another set of rows with
- the keys "abc1", "abc2", and "abc3". The following example shows how to set a Scan
- instance to return the rows beginning with "row".</para>
-<programlisting language="java">
-public static final byte[] CF = "cf".getBytes();
-public static final byte[] ATTR = "attr".getBytes();
-...
-
-Table table = ... // instantiate a Table instance
-
-Scan scan = new Scan();
-scan.addColumn(CF, ATTR);
-scan.setRowPrefixFilter(Bytes.toBytes("row"));
-ResultScanner rs = table.getScanner(scan);
-try {
- for (Result r = rs.next(); r != null; r = rs.next()) {
- // process result...
-} finally {
- rs.close(); // always close the ResultScanner!
-</programlisting>
- <para>Note that generally the easiest way to specify a specific stop point for a scan is by
- using the <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/filter/InclusiveStopFilter.html">InclusiveStopFilter</link>
- class. </para>
- </section>
- <section
- xml:id="delete">
- <title>Delete</title>
- <para><link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Delete.html">Delete</link>
- removes a row from a table. Deletes are executed via <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#delete(org.apache.hadoop.hbase.client.Delete)">
- HTable.delete</link>. </para>
- <para>HBase does not modify data in place, and so deletes are handled by creating new
- markers called <emphasis>tombstones</emphasis>. These tombstones, along with the dead
- values, are cleaned up on major compactions. </para>
- <para>See <xref
- linkend="version.delete" /> for more information on deleting versions of columns, and
- see <xref
- linkend="compaction" /> for more information on compactions. </para>
-
- </section>
-
- </section>
-
-
- <section
- xml:id="versions">
- <title>Versions<indexterm><primary>Versions</primary></indexterm></title>
-
- <para>A <emphasis>{row, column, version} </emphasis>tuple exactly specifies a
- <literal>cell</literal> in HBase. It's possible to have an unbounded number of cells where
- the row and column are the same but the cell address differs only in its version
- dimension.</para>
-
- <para>While rows and column keys are expressed as bytes, the version is specified using a long
- integer. Typically this long contains time instances such as those returned by
- <code>java.util.Date.getTime()</code> or <code>System.currentTimeMillis()</code>, that is:
- <quote>the difference, measured in milliseconds, between the current time and midnight,
- January 1, 1970 UTC</quote>.</para>
-
- <para>The HBase version dimension is stored in decreasing order, so that when reading from a
- store file, the most recent values are found first.</para>
-
- <para>There is a lot of confusion over the semantics of <literal>cell</literal> versions, in
- HBase. In particular:</para>
- <itemizedlist>
- <listitem>
- <para>If multiple writes to a cell have the same version, only the last written is
- fetchable.</para>
- </listitem>
-
- <listitem>
- <para>It is OK to write cells in a non-increasing version order.</para>
- </listitem>
- </itemizedlist>
-
- <para>Below we describe how the version dimension in HBase currently works. See <link
- xlink:href="https://issues.apache.org/jira/browse/HBASE-2406">HBASE-2406</link> for
- discussion of HBase versions. <link
- xlink:href="http://outerthought.org/blog/417-ot.html">Bending time in HBase</link>
- makes for a good read on the version, or time, dimension in HBase. It has more detail on
- versioning than is provided here. As of this writing, the limiitation
- <emphasis>Overwriting values at existing timestamps</emphasis> mentioned in the
- article no longer holds in HBase. This section is basically a synopsis of this article
- by Bruno Dumon.</para>
-
- <section xml:id="specify.number.of.versions">
- <title>Specifying the Number of Versions to Store</title>
- <para>The maximum number of versions to store for a given column is part of the column
- schema and is specified at table creation, or via an <command>alter</command> command, via
- <code>HColumnDescriptor.DEFAULT_VERSIONS</code>. Prior to HBase 0.96, the default number
- of versions kept was <literal>3</literal>, but in 0.96 and newer has been changed to
- <literal>1</literal>.</para>
- <example>
- <title>Modify the Maximum Number of Versions for a Column</title>
- <para>This example uses HBase Shell to keep a maximum of 5 versions of column
- <code>f1</code>. You could also use <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HColumnDescriptor.html"
- >HColumnDescriptor</link>.</para>
- <screen><![CDATA[hbase> alter ‘t1′, NAME => ‘f1′, VERSIONS => 5]]></screen>
- </example>
- <example>
- <title>Modify the Minimum Number of Versions for a Column</title>
- <para>You can also specify the minimum number of versions to store. By default, this is
- set to 0, which means the feature is disabled. The following example sets the minimum
- number of versions on field <code>f1</code> to <literal>2</literal>, via HBase Shell.
- You could also use <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HColumnDescriptor.html"
- >HColumnDescriptor</link>.</para>
- <screen><![CDATA[hbase> alter ‘t1′, NAME => ‘f1′, MIN_VERSIONS => 2]]></screen>
- </example>
- <para>Starting with HBase 0.98.2, you can specify a global default for the maximum number of
- versions kept for all newly-created columns, by setting
- <option>hbase.column.max.version</option> in <filename>hbase-site.xml</filename>. See
- <xref linkend="hbase.column.max.version"/>.</para>
- </section>
-
- <section
- xml:id="versions.ops">
- <title>Versions and HBase Operations</title>
-
- <para>In this section we look at the behavior of the version dimension for each of the core
- HBase operations.</para>
-
- <section>
- <title>Get/Scan</title>
-
- <para>Gets are implemented on top of Scans. The below discussion of <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Get.html">Get</link>
- applies equally to <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Scan.html">Scans</link>.</para>
-
- <para>By default, i.e. if you specify no explicit version, when doing a
- <literal>get</literal>, the cell whose version has the largest value is returned
- (which may or may not be the latest one written, see later). The default behavior can be
- modified in the following ways:</para>
-
- <itemizedlist>
- <listitem>
- <para>to return more than one version, see <link
- xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Get.html#setMaxVersions()">Get.setMaxVersions()</link></para>
- </listitem>
-
- <listitem>
- <para>to return versions other than the latest, see <link
- xlink:href="???">Get.setTimeRange()</link></para>
-
- <para>To retrieve the latest version that is less than or equal to a given value, thus
- giving the 'latest' state of the record at a certain point in time, just use a range
- from 0 to the desired version and set the max versions to 1.</para>
- </listitem>
- </itemizedlist>
-
- </section>
- <section
- xml:id="default_get_example">
- <title>Default Get Example</title>
- <para>The following Get will only retrieve the current version of the row</para>
- <programlisting language="java">
-public static final byte[] CF = "cf".getBytes();
-public static final byte[] ATTR = "attr".getBytes();
-...
-Get get = new Get(Bytes.toBytes("row1"));
-Result r = table.get(get);
-byte[] b = r.getValue(CF, ATTR); // returns current version of value
-</programlisting>
- </section>
- <section
- xml:id="versioned_get_example">
- <title>Versioned Get Example</title>
- <para>The following Get will return the last 3 versions of the row.</para>
- <programlisting language="java">
-public static final byte[] CF = "cf".getBytes();
-public static final byte[] ATTR = "attr".getBytes();
-...
-Get get = new Get(Bytes.toBytes("row1"));
-get.setMaxVersions(3); // will return last 3 versions of row
-Result r = table.get(get);
-byte[] b = r.getValue(CF, ATTR); // returns current version of value
-List<KeyValue> kv = r.getColumn(CF, ATTR); // returns all versions of this column
-</programlisting>
- </section>
-
- <section>
- <title>Put</title>
-
- <para>Doing a put always creates a new version of a <literal>cell</literal>, at a certain
- timestamp. By default the system uses the server's <literal>currentTimeMillis</literal>,
- but you can specify the version (= the long integer) yourself, on a per-column level.
- This means you could assign a time in the past or the future, or use the long value for
- non-time purposes.</para>
-
- <para>To overwrite an existing value, do a put at exactly the same row, column, and
- version as that of the cell you would overshadow.</para>
- <section
- xml:id="implicit_version_example">
- <title>Implicit Version Example</title>
- <para>The following Put will be implicitly versioned by HBase with the current
- time.</para>
- <programlisting language="java">
-public static final byte[] CF = "cf".getBytes();
-public static final byte[] ATTR = "attr".getBytes();
-...
-Put put = new Put(Bytes.toBytes(row));
-put.add(CF, ATTR, Bytes.toBytes( data));
-table.put(put);
-</programlisting>
- </section>
- <section
- xml:id="explicit_version_example">
- <title>Explicit Version Example</title>
- <para>The following Put has the version timestamp explicitly set.</para>
- <programlisting language="java">
-public static final byte[] CF = "cf".getBytes();
-public static final byte[] ATTR = "attr".getBytes();
-...
-Put put = new Put( Bytes.toBytes(row));
-long explicitTimeInMs = 555; // just an example
-put.add(CF, ATTR, explicitTimeInMs, Bytes.toBytes(data));
-table.put(put);
-</programlisting>
- <para>Caution: the version timestamp is internally by HBase for things like time-to-live
- calculations. It's usually best to avoid setting this timestamp yourself. Prefer using
- a separate timestamp attribute of the row, or have the timestamp a part of the rowkey,
- or both. </para>
- </section>
-
- </section>
-
- <section
- xml:id="version.delete">
- <title>Delete</title>
-
- <para>There are three different types of internal delete markers. See Lars Hofhansl's blog
- for discussion of his attempt adding another, <link
- xlink:href="http://hadoop-hbase.blogspot.com/2012/01/scanning-in-hbase.html">Scanning
- in HBase: Prefix Delete Marker</link>. </para>
- <itemizedlist>
- <listitem>
- <para>Delete: for a specific version of a column.</para>
- </listitem>
- <listitem>
- <para>Delete column: for all versions of a column.</para>
- </listitem>
- <listitem>
- <para>Delete family: for all columns of a particular ColumnFamily</para>
- </listitem>
- </itemizedlist>
- <para>When deleting an entire row, HBase will internally create a tombstone for each
- ColumnFamily (i.e., not each individual column). </para>
- <para>Deletes work by creating <emphasis>tombstone</emphasis> markers. For example, let's
- suppose we want to delete a row. For this you can specify a version, or else by default
- the <literal>currentTimeMillis</literal> is used. What this means is <quote>delete all
- cells where the version is less than or equal to this version</quote>. HBase never
- modifies data in place, so for example a delete will not immediately delete (or mark as
- deleted) the entries in the storage file that correspond to the delete condition.
- Rather, a so-called <emphasis>tombstone</emphasis> is written, which will mask the
- deleted values. When HBase does a major compaction, the tombstones are processed to
- actually remove the dead values, together with the tombstones themselves. If the version
- you specified when deleting a row is larger than the version of any value in the row,
- then you can consider the complete row to be deleted.</para>
- <para>For an informative discussion on how deletes and versioning interact, see the thread <link
- xlink:href="http://comments.gmane.org/gmane.comp.java.hadoop.hbase.user/28421">Put w/
- timestamp -> Deleteall -> Put w/ timestamp fails</link> up on the user mailing
- list.</para>
- <para>Also see <xref
- linkend="keyvalue" /> for more information on the internal KeyValue format. </para>
- <para>Delete markers are purged during the next major compaction of the store, unless the
- <option>KEEP_DELETED_CELLS</option> option is set in the column family. To keep the
- deletes for a configurable amount of time, you can set the delete TTL via the
- <option>hbase.hstore.time.to.purge.deletes</option> property in
- <filename>hbase-site.xml</filename>. If
- <option>hbase.hstore.time.to.purge.deletes</option> is not set, or set to 0, all
- delete markers, including those with timestamps in the future, are purged during the
- next major compaction. Otherwise, a delete marker with a timestamp in the future is kept
- until the major compaction which occurs after the time represented by the marker's
- timestamp plus the value of <option>hbase.hstore.time.to.purge.deletes</option>, in
- milliseconds. </para>
- <note>
- <para>This behavior represents a fix for an unexpected change that was introduced in
- HBase 0.94, and was fixed in <link
- xlink:href="https://issues.apache.org/jira/browse/HBASE-10118">HBASE-10118</link>.
- The change has been backported to HBase 0.94 and newer branches.</para>
- </note>
- </section>
- </section>
-
- <section>
- <title>Current Limitations</title>
-
- <section>
- <title>Deletes mask Puts</title>
-
- <para>Deletes mask puts, even puts that happened after the delete
- was entered. See <link xlink:href="https://issues.apache.org/jira/browse/HBASE-2256"
- >HBASE-2256</link>. Remember that a delete writes a tombstone, which only
- disappears after then next major compaction has run. Suppose you do
- a delete of everything <= T. After this you do a new put with a
- timestamp <= T. This put, even if it happened after the delete,
- will be masked by the delete tombstone. Performing the put will not
- fail, but when you do a get you will notice the put did have no
- effect. It will start working again after the major compaction has
- run. These issues should not be a problem if you use
- always-increasing versions for new puts to a row. But they can occur
- even if you do not care about time: just do delete and put
- immediately after each other, and there is some chance they happen
- within the same millisecond.</para>
- </section>
-
- <section
- xml:id="major.compactions.change.query.results">
- <title>Major compactions change query results</title>
-
- <para><quote>...create three cell versions at t1, t2 and t3, with a maximum-versions
- setting of 2. So when getting all versions, only the values at t2 and t3 will be
- returned. But if you delete the version at t2 or t3, the one at t1 will appear again.
- Obviously, once a major compaction has run, such behavior will not be the case
- anymore...</quote> (See <emphasis>Garbage Collection</emphasis> in <link
- xlink:href="http://outerthought.org/blog/417-ot.html">Bending time in
- HBase</link>.)</para>
- </section>
- </section>
- </section>
- <section xml:id="dm.sort">
- <title>Sort Order</title>
- <para>All data model operations HBase return data in sorted order. First by row,
- then by ColumnFamily, followed by column qualifier, and finally timestamp (sorted
- in reverse, so newest records are returned first).
- </para>
- </section>
- <section xml:id="dm.column.metadata">
- <title>Column Metadata</title>
- <para>There is no store of column metadata outside of the internal KeyValue instances for a ColumnFamily.
- Thus, while HBase can support not only a wide number of columns per row, but a heterogenous set of columns
- between rows as well, it is your responsibility to keep track of the column names.
- </para>
- <para>The only way to get a complete set of columns that exist for a ColumnFamily is to process all the rows.
- For more information about how HBase stores data internally, see <xref linkend="keyvalue" />.
- </para>
- </section>
- <section xml:id="joins"><title>Joins</title>
- <para>Whether HBase supports joins is a common question on the dist-list, and there is a simple answer: it doesn't,
- at not least in the way that RDBMS' support them (e.g., with equi-joins or outer-joins in SQL). As has been illustrated
- in this chapter, the read data model operations in HBase are Get and Scan.
- </para>
- <para>However, that doesn't mean that equivalent join functionality can't be supported in your application, but
- you have to do it yourself. The two primary strategies are either denormalizing the data upon writing to HBase,
- or to have lookup tables and do the join between HBase tables in your application or MapReduce code (and as RDBMS'
- demonstrate, there are several strategies for this depending on the size of the tables, e.g., nested loops vs.
- hash-joins). So which is the best approach? It depends on what you are trying to do, and as such there isn't a single
- answer that works for every use case.
- </para>
- </section>
- <section xml:id="acid"><title>ACID</title>
- <para>See <link xlink:href="http://hbase.apache.org/acid-semantics.html">ACID Semantics</link>.
- Lars Hofhansl has also written a note on
- <link xlink:href="http://hadoop-hbase.blogspot.com/2012/03/acid-in-hbase.html">ACID in HBase</link>.</para>
- </section>
- </chapter>