[GitHub] flink pull request #5362: [FLINK-8421] [DataStream] Make timer serializers r...

[tzulitai <gi...@git.apache.org>]

Github user tzulitai commented on a diff in the pull request:

    https://github.com/apache/flink/pull/5362#discussion_r165573730
  
    --- Diff: flink-core/src/main/java/org/apache/flink/core/io/PostVersionedIOReadableWritable.java ---
    @@ -0,0 +1,78 @@
    +/*
    + * 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.flink.core.io;
    +
    +import org.apache.flink.annotation.Internal;
    +import org.apache.flink.core.memory.DataInputView;
    +import org.apache.flink.core.memory.DataInputViewStreamWrapper;
    +import org.apache.flink.core.memory.DataOutputView;
    +import org.apache.flink.core.memory.InputStreamViewWrapper;
    +
    +import java.io.IOException;
    +import java.io.PushbackInputStream;
    +import java.util.Arrays;
    +
    +/**
    + * A {@link VersionedIOReadableWritable} which allows to differentiate whether the previous
    + * data was versioned with a {@link VersionedIOReadableWritable}. This can be used if previously
    + * written data was not versioned, and is to be migrated to a versioned format.
    + */
    +@Internal
    +public abstract class PostVersionedIOReadableWritable extends VersionedIOReadableWritable {
    +
    +	/** NOTE: CANNOT CHANGE! */
    +	private static final byte[] VERSIONED_IDENTIFIER = new byte[] {-15, -51, -123, -97};
    +
    +	/**
    +	 * Read from the provided {@link DataInputView in}. A flag {@code wasVersioned} can be
    +	 * used to determine whether or not the data to read was previously written
    +	 * by a {@link VersionedIOReadableWritable}.
    +	 */
    +	protected abstract void read(DataInputView in, boolean wasVersioned) throws IOException;
    +
    +	@Override
    +	public void write(DataOutputView out) throws IOException {
    +		out.write(VERSIONED_IDENTIFIER);
    +		super.write(out);
    +	}
    +
    +	/**
    +	 * This read attempts to first identify if the input view contains the special
    +	 * {@link #VERSIONED_IDENTIFIER} by reading and buffering the first few bytes.
    +	 * If identified to be versioned, the usual version resolution read path
    +	 * in {@link VersionedIOReadableWritable#read(DataInputView)} is invoked.
    +	 * Otherwise, we "reset" the input view by pushing back the read buffered bytes
    +	 * into the stream.
    +	 */
    +	@Override
    +	public final void read(DataInputView in) throws IOException {
    +		PushbackInputStream stream = new PushbackInputStream(new InputStreamViewWrapper(in), VERSIONED_IDENTIFIER.length);
    +
    +		byte[] tmp = new byte[VERSIONED_IDENTIFIER.length];
    +		stream.read(tmp);
    +
    +		if (Arrays.equals(tmp, VERSIONED_IDENTIFIER)) {
    +			super.read(in);
    +			read(in, true);
    +		} else {
    +			stream.unread(tmp);
    +			read(new DataInputViewStreamWrapper(stream), false);
    +		}
    --- End diff --
    
    Not-so-nice things about this current implementation is:
    1) it requires several layers of transforming back and forth between a `DataInputView` and `InputStream`, and 
    2) it uses a separate `read(DataInputView, boolean)` method in order to wrap a "reset" `DataInputView` for the remaining reads.
    
    I think the implementation would have been much more elegant if `DataInputView` has an `unread(byte[])` method, though I'm not sure how non-trivial it is to support this across all subclasses.
    Maybe a food for thought for the future ..


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