聊聊flink的MemCheckpointStreamFactory

序

本文主要研究一下flink的MemCheckpointStreamFactory

CheckpointStreamFactory

flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/CheckpointStreamFactory.java

/**
 * A factory for checkpoint output streams, which are used to persist data for checkpoints.
 *
 * <p>Stream factories can be created from the {@link CheckpointStorage} through
 * {@link CheckpointStorage#resolveCheckpointStorageLocation(long, CheckpointStorageLocationReference)}.
 */
public interface CheckpointStreamFactory {

    CheckpointStateOutputStream createCheckpointStateOutputStream(CheckpointedStateScope scope) throws IOException;

    abstract class CheckpointStateOutputStream extends FSDataOutputStream {

        @Nullable
        public abstract StreamStateHandle closeAndGetHandle() throws IOException;

        @Override
        public abstract void close() throws IOException;
    }
}

• CheckpointStreamFactory为checkpoint output streams(用于持久化checkpoint的数据)的工厂，它定义了createCheckpointStateOutputStream方法，这里返回的是CheckpointStateOutputStream；CheckpointStateOutputStream继承了FSDataOutputStream，它定义了closeAndGetHandle及close两个抽象方法
• CheckpointStreamFactory有两个以factory命名的实现类，分别是MemCheckpointStreamFactory(它有两个子类分别为NonPersistentMetadataCheckpointStorageLocation、PersistentMetadataCheckpointStorageLocation)、FsCheckpointStreamFactory(它有一个子类为FsCheckpointStorageLocation)
• CheckpointStorageLocation接口继承了CheckpointStreamFactory接口，它有三个实现类，分别是NonPersistentMetadataCheckpointStorageLocation、PersistentMetadataCheckpointStorageLocation、FsCheckpointStorageLocation

FSDataOutputStream

flink-core-1.7.0-sources.jar!/org/apache/flink/core/fs/FSDataOutputStream.java

@Public
public abstract class FSDataOutputStream extends OutputStream {

    public abstract long getPos() throws IOException;

    public abstract void flush() throws IOException;

    public abstract void sync() throws IOException;

    public abstract void close() throws IOException;
}

• FSDataOutputStream继承了java的OutputStream，它多定义了getPos、flush、sync、close几个抽象方法

CheckpointStorageLocation

flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/CheckpointStorageLocation.java

/**
 * A storage location for one particular checkpoint, offering data persistent, metadata persistence,
 * and lifecycle/cleanup methods.
 *
 * <p>CheckpointStorageLocations are typically created and initialized via
 * {@link CheckpointStorage#initializeLocationForCheckpoint(long)} or
 * {@link CheckpointStorage#initializeLocationForSavepoint(long, String)}.
 */
public interface CheckpointStorageLocation extends CheckpointStreamFactory {

    CheckpointMetadataOutputStream createMetadataOutputStream() throws IOException;

    void disposeOnFailure() throws IOException;

    CheckpointStorageLocationReference getLocationReference();
}

• CheckpointStorageLocation继承了CheckpointStreamFactory接口，它通常是由CheckpointStorage来创建及初始化，提供数据持久化、metadata存储及lifecycle/cleanup相关方法；这里定义了createMetadataOutputStream方法用来创建CheckpointMetadataOutputStream；disposeOnFailure方法用于在checkpoint失败的时候dispose checkpoint location；getLocationReference用于返回CheckpointStorageLocationReference

MemCheckpointStreamFactory

flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/memory/MemCheckpointStreamFactory.java

/**
 * {@link CheckpointStreamFactory} that produces streams that write to in-memory byte arrays.
 */
public class MemCheckpointStreamFactory implements CheckpointStreamFactory {

    /** The maximal size that the snapshotted memory state may have */
    private final int maxStateSize;

    /**
     * Creates a new in-memory stream factory that accepts states whose serialized forms are
     * up to the given number of bytes.
     *
     * @param maxStateSize The maximal size of the serialized state
     */
    public MemCheckpointStreamFactory(int maxStateSize) {
        this.maxStateSize = maxStateSize;
    }

    @Override
    public CheckpointStateOutputStream createCheckpointStateOutputStream(
            CheckpointedStateScope scope) throws IOException
    {
        return new MemoryCheckpointOutputStream(maxStateSize);
    }

    @Override
    public String toString() {
        return "In-Memory Stream Factory";
    }

    static void checkSize(int size, int maxSize) throws IOException {
        if (size > maxSize) {
            throw new IOException(
                    "Size of the state is larger than the maximum permitted memory-backed state. Size="
                            + size + " , maxSize=" + maxSize
                            + " . Consider using a different state backend, like the File System State backend.");
        }
    }



    /**
     * A {@code CheckpointStateOutputStream} that writes into a byte array.
     */
    public static class MemoryCheckpointOutputStream extends CheckpointStateOutputStream {

        private final ByteArrayOutputStreamWithPos os = new ByteArrayOutputStreamWithPos();

        private final int maxSize;

        private AtomicBoolean closed;

        boolean isEmpty = true;

        public MemoryCheckpointOutputStream(int maxSize) {
            this.maxSize = maxSize;
            this.closed = new AtomicBoolean(false);
        }

        @Override
        public void write(int b) throws IOException {
            os.write(b);
            isEmpty = false;
        }

        @Override
        public void write(byte[] b, int off, int len) throws IOException {
            os.write(b, off, len);
            isEmpty = false;
        }

        @Override
        public void flush() throws IOException {
            os.flush();
        }

        @Override
        public void sync() throws IOException { }

        // --------------------------------------------------------------------

        @Override
        public void close() {
            if (closed.compareAndSet(false, true)) {
                closeInternal();
            }
        }

        @Nullable
        @Override
        public StreamStateHandle closeAndGetHandle() throws IOException {
            if (isEmpty) {
                return null;
            }
            return new ByteStreamStateHandle(String.valueOf(UUID.randomUUID()), closeAndGetBytes());
        }

        @Override
        public long getPos() throws IOException {
            return os.getPosition();
        }

        public boolean isClosed() {
            return closed.get();
        }

        /**
         * Closes the stream and returns the byte array containing the stream's data.
         * @return The byte array containing the stream's data.
         * @throws IOException Thrown if the size of the data exceeds the maximal
         */
        public byte[] closeAndGetBytes() throws IOException {
            if (closed.compareAndSet(false, true)) {
                checkSize(os.size(), maxSize);
                byte[] bytes = os.toByteArray();
                closeInternal();
                return bytes;
            } else {
                throw new IOException("stream has already been closed");
            }
        }

        private void closeInternal() {
            os.reset();
        }
    }
}

• MemCheckpointStreamFactory实现了CheckpointStreamFactory接口，这里createCheckpointStateOutputStream方法返回MemoryCheckpointOutputStream
• MemoryCheckpointOutputStream继承了CheckpointStateOutputStream，里头使用了ByteArrayOutputStreamWithPos，它在closeAndGetHandle的时候会校验大小是否超过maxSize的限制，超出则抛出IOException异常
• MemCheckpointStreamFactory有两个子类分别为NonPersistentMetadataCheckpointStorageLocation、PersistentMetadataCheckpointStorageLocation，它们都实现了CheckpointStorageLocation接口

NonPersistentMetadataCheckpointStorageLocation

flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/memory/NonPersistentMetadataCheckpointStorageLocation.java

/**
 * A checkpoint storage location for the {@link MemoryStateBackend} in case no durable persistence
 * for metadata has been configured.
 */
public class NonPersistentMetadataCheckpointStorageLocation
        extends MemCheckpointStreamFactory
        implements CheckpointStorageLocation {

    /** The external pointer returned for checkpoints that are not externally addressable. */
    public static final String EXTERNAL_POINTER = "<checkpoint-not-externally-addressable>";

    public NonPersistentMetadataCheckpointStorageLocation(int maxStateSize) {
        super(maxStateSize);
    }

    @Override
    public CheckpointMetadataOutputStream createMetadataOutputStream() throws IOException {
        return new MetadataOutputStream();
    }

    @Override
    public void disposeOnFailure() {}

    @Override
    public CheckpointStorageLocationReference getLocationReference() {
        return CheckpointStorageLocationReference.getDefault();
    }

    // ------------------------------------------------------------------------
    //  CompletedCheckpointStorageLocation
    // ------------------------------------------------------------------------

    /**
     * A {@link CompletedCheckpointStorageLocation} that is not persistent and only holds the
     * metadata in an internal byte array.
     */
    private static class NonPersistentCompletedCheckpointStorageLocation implements CompletedCheckpointStorageLocation {

        private static final long serialVersionUID = 1L;

        private final ByteStreamStateHandle metaDataHandle;

        NonPersistentCompletedCheckpointStorageLocation(ByteStreamStateHandle metaDataHandle) {
            this.metaDataHandle = metaDataHandle;
        }

        @Override
        public String getExternalPointer() {
            return EXTERNAL_POINTER;
        }

        @Override
        public StreamStateHandle getMetadataHandle() {
            return metaDataHandle;
        }

        @Override
        public void disposeStorageLocation() {}
    }

    // ------------------------------------------------------------------------
    //  CheckpointMetadataOutputStream
    // ------------------------------------------------------------------------

    private static class MetadataOutputStream extends CheckpointMetadataOutputStream {

        private final ByteArrayOutputStreamWithPos os = new ByteArrayOutputStreamWithPos();

        private boolean closed;

        @Override
        public void write(int b) throws IOException {
            os.write(b);
        }

        @Override
        public void write(byte[] b, int off, int len) throws IOException {
            os.write(b, off, len);
        }

        @Override
        public void flush() throws IOException {
            os.flush();
        }

        @Override
        public long getPos() throws IOException {
            return os.getPosition();
        }

        @Override
        public void sync() throws IOException { }

        @Override
        public CompletedCheckpointStorageLocation closeAndFinalizeCheckpoint() throws IOException {
            synchronized (this) {
                if (!closed) {
                    closed = true;

                    byte[] bytes = os.toByteArray();
                    ByteStreamStateHandle handle = new ByteStreamStateHandle(UUID.randomUUID().toString(), bytes);
                    return new NonPersistentCompletedCheckpointStorageLocation(handle);
                } else {
                    throw new IOException("Already closed");
                }
            }
        }

        @Override
        public void close() {
            if (!closed) {
                closed = true;
                os.reset();
            }
        }
    }
}

• MemoryBackendCheckpointStorage在没有配置checkpointsDirectory的时候创建的是NonPersistentMetadataCheckpointStorageLocation；其createMetadataOutputStream方法创建的是MetadataOutputStream
• MetadataOutputStream继承了CheckpointMetadataOutputStream，里头使用的是ByteArrayOutputStreamWithPos，而closeAndFinalizeCheckpoint返回的是NonPersistentCompletedCheckpointStorageLocation
• NonPersistentCompletedCheckpointStorageLocation实现了CompletedCheckpointStorageLocation接口，其getMetadataHandle方法返回的是ByteStreamStateHandle

PersistentMetadataCheckpointStorageLocation

flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/memory/PersistentMetadataCheckpointStorageLocation.java

/**
 * A checkpoint storage location for the {@link MemoryStateBackend} when it durably
 * persists the metadata in a file system.
 */
public class PersistentMetadataCheckpointStorageLocation
        extends MemCheckpointStreamFactory
        implements CheckpointStorageLocation {

    private final FileSystem fileSystem;

    private final Path checkpointDirectory;

    private final Path metadataFilePath;

    /**
     * Creates a checkpoint storage persists metadata to a file system and stores state
     * in line in state handles with the metadata.
     *
     * @param fileSystem The file system to which the metadata will be written.
     * @param checkpointDir The directory where the checkpoint metadata will be written.
     */
    public PersistentMetadataCheckpointStorageLocation(
            FileSystem fileSystem,
            Path checkpointDir,
            int maxStateSize) {

        super(maxStateSize);

        this.fileSystem = checkNotNull(fileSystem);
        this.checkpointDirectory = checkNotNull(checkpointDir);
        this.metadataFilePath = new Path(checkpointDir, AbstractFsCheckpointStorage.METADATA_FILE_NAME);
    }

    // ------------------------------------------------------------------------

    @Override
    public CheckpointMetadataOutputStream createMetadataOutputStream() throws IOException {
        return new FsCheckpointMetadataOutputStream(fileSystem, metadataFilePath, checkpointDirectory);
    }

    @Override
    public void disposeOnFailure() throws IOException {
        // on a failure, no chunk in the checkpoint directory needs to be saved, so
        // we can drop it as a whole
        fileSystem.delete(checkpointDirectory, true);
    }

    @Override
    public CheckpointStorageLocationReference getLocationReference() {
        return CheckpointStorageLocationReference.getDefault();
    }
}

• MemoryBackendCheckpointStorage在配置了checkpointsDirectory的时候创建的是PersistentMetadataCheckpointStorageLocation；其createMetadataOutputStream方法创建的是FsCheckpointMetadataOutputStream；FsCheckpointMetadataOutputStream的构造器接收三个参数，分别是fileSystem、metadataFilePath、exclusiveCheckpointDir；其中fileSystem用于根据metadataFilePath来创建FSDataOutputStream，而exclusiveCheckpointDir则在返回FsCompletedCheckpointStorageLocation的时候用到

小结

• MemoryBackendCheckpointStorage在没有配置checkpointsDirectory的时候创建的是NonPersistentMetadataCheckpointStorageLocation；在配置了checkpointsDirectory的时候创建的是PersistentMetadataCheckpointStorageLocation
• NonPersistentMetadataCheckpointStorageLocation及PersistentMetadataCheckpointStorageLocation都继承了MemCheckpointStreamFactory类，同时实现了CheckpointStorageLocation接口(其createMetadataOutputStream方法返回的CheckpointMetadataOutputStream类型分别为MetadataOutputStream、FsCheckpointMetadataOutputStream)
• MemCheckpointStreamFactory实现了CheckpointStreamFactory接口，它的createCheckpointStateOutputStream方法返回MemoryCheckpointOutputStream；CheckpointStorageLocation继承了CheckpointStreamFactory接口，它通常是由CheckpointStorage来创建及初始化，提供数据持久化、metadata存储及lifecycle/cleanup相关方法

doc

• The MemoryStateBackend