聊聊flink的MemoryBackendCheckpointStorage

序

本文主要研究一下flink的MemoryBackendCheckpointStorage

CheckpointStorage

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

/**
 * CheckpointStorage implements the durable storage of checkpoint data and metadata streams.
 * An individual checkpoint or savepoint is stored to a {@link CheckpointStorageLocation},
 * created by this class.
 */
public interface CheckpointStorage {


    boolean supportsHighlyAvailableStorage();

    boolean hasDefaultSavepointLocation();

    CompletedCheckpointStorageLocation resolveCheckpoint(String externalPointer) throws IOException;

    CheckpointStorageLocation initializeLocationForCheckpoint(long checkpointId) throws IOException;

    CheckpointStorageLocation initializeLocationForSavepoint(
            long checkpointId,
            @Nullable String externalLocationPointer) throws IOException;

    CheckpointStreamFactory resolveCheckpointStorageLocation(
            long checkpointId,
            CheckpointStorageLocationReference reference) throws IOException;

    CheckpointStateOutputStream createTaskOwnedStateStream() throws IOException;
}

• CheckpointStorage接口主要定义了持久化checkpoint data及metadata streams的基本方法；supportsHighlyAvailableStorage方法返回该backend是否支持highly available storage；hasDefaultSavepointLocation方法是否有默认的savepoint location；resolveCheckpoint方法用于解析checkpoint location返回CompletedCheckpointStorageLocation；initializeLocationForCheckpoint方法根据checkpointId来初始化storage location；initializeLocationForSavepoint方法用于根据checkpointId来初始化savepoint的storage location；resolveCheckpointStorageLocation方法解析CheckpointStorageLocationReference返回CheckpointStreamFactory；createTaskOwnedStateStream方法用于打开一个stream来持久化checkpoint state

AbstractFsCheckpointStorage

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

/**
 * An implementation of durable checkpoint storage to file systems.
 */
public abstract class AbstractFsCheckpointStorage implements CheckpointStorage {

    // ------------------------------------------------------------------------
    //  Constants
    // ------------------------------------------------------------------------

    /** The prefix of the directory containing the data exclusive to a checkpoint. */
    public static final String CHECKPOINT_DIR_PREFIX = "chk-";

    /** The name of the directory for shared checkpoint state. */
    public static final String CHECKPOINT_SHARED_STATE_DIR = "shared";

    /** The name of the directory for state not owned/released by the master, but by the TaskManagers. */
    public static final String CHECKPOINT_TASK_OWNED_STATE_DIR = "taskowned";

    /** The name of the metadata files in checkpoints / savepoints. */
    public static final String METADATA_FILE_NAME = "_metadata";

    /** The magic number that is put in front of any reference. */
    private static final byte[] REFERENCE_MAGIC_NUMBER = new byte[] { 0x05, 0x5F, 0x3F, 0x18 };

    // ------------------------------------------------------------------------
    //  Fields and properties
    // ------------------------------------------------------------------------

    /** The jobId, written into the generated savepoint directories. */
    private final JobID jobId;

    /** The default location for savepoints. Null, if none is configured. */
    @Nullable
    private final Path defaultSavepointDirectory;

    @Override
    public boolean hasDefaultSavepointLocation() {
        return defaultSavepointDirectory != null;
    }

    @Override
    public CompletedCheckpointStorageLocation resolveCheckpoint(String checkpointPointer) throws IOException {
        return resolveCheckpointPointer(checkpointPointer);
    }

    /**
     * Creates a file system based storage location for a savepoint.
     *
     * <p>This methods implements the logic that decides which location to use (given optional
     * parameters for a configured location and a location passed for this specific savepoint)
     * and how to name and initialize the savepoint directory.
     *
     * @param externalLocationPointer    The target location pointer for the savepoint.
     *                                   Must be a valid URI. Null, if not supplied.
     * @param checkpointId               The checkpoint ID of the savepoint.
     *
     * @return The checkpoint storage location for the savepoint.
     *
     * @throws IOException Thrown if the target directory could not be created.
     */
    @Override
    public CheckpointStorageLocation initializeLocationForSavepoint(
            @SuppressWarnings("unused") long checkpointId,
            @Nullable String externalLocationPointer) throws IOException {

        // determine where to write the savepoint to

        final Path savepointBasePath;
        if (externalLocationPointer != null) {
            savepointBasePath = new Path(externalLocationPointer);
        }
        else if (defaultSavepointDirectory != null) {
            savepointBasePath = defaultSavepointDirectory;
        }
        else {
            throw new IllegalArgumentException("No savepoint location given and no default location configured.");
        }

        // generate the savepoint directory

        final FileSystem fs = savepointBasePath.getFileSystem();
        final String prefix = "savepoint-" + jobId.toString().substring(0, 6) + '-';

        Exception latestException = null;
        for (int attempt = 0; attempt < 10; attempt++) {
            final Path path = new Path(savepointBasePath, FileUtils.getRandomFilename(prefix));

            try {
                if (fs.mkdirs(path)) {
                    // we make the path qualified, to make it independent of default schemes and authorities
                    final Path qp = path.makeQualified(fs);

                    return createSavepointLocation(fs, qp);
                }
            } catch (Exception e) {
                latestException = e;
            }
        }

        throw new IOException("Failed to create savepoint directory at " + savepointBasePath, latestException);
    }

    protected abstract CheckpointStorageLocation createSavepointLocation(FileSystem fs, Path location) throws IOException;

    //......
}

• AbstractFsCheckpointStorage主要是实现了CheckpointStorage接口的hasDefaultSavepointLocation、resolveCheckpoint、initializeLocationForSavepoint方法
• resolveCheckpoint方法主要做两件事情，一个是解析checkpoint/savepoint path，一个是解析checkpoint/savepoint的metadata path，获取他们的FileStatus，然后创建FsCompletedCheckpointStorageLocation
• initializeLocationForSavepoint方法主要是给savepoint创建一个CheckpointStorageLocation，它可以根据externalLocationPointer来创建，该值为null的话则使用defaultSavepointDirectory，该方法里头调用了createSavepointLocation抽象方法，由子类去实现

MemoryBackendCheckpointStorage

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

/**
 * An implementation of a checkpoint storage for the {@link MemoryStateBackend}.
 * Depending on whether this is created with a checkpoint location, the setup supports
 * durable checkpoints (durable metadata) or not.
 */
public class MemoryBackendCheckpointStorage extends AbstractFsCheckpointStorage {

    /** The target directory for checkpoints (here metadata files only). Null, if not configured. */
    @Nullable
    private final Path checkpointsDirectory;

    /** The file system to persist the checkpoints to. Null if this does not durably persist checkpoints. */
    @Nullable
    private final FileSystem fileSystem;

    /** The maximum size of state stored in a state handle. */
    private final int maxStateSize;

    /**
     * Creates a new MemoryBackendCheckpointStorage.
     *
     * @param jobId The ID of the job writing the checkpoints.
     * @param checkpointsBaseDirectory The directory to write checkpoints to. May be null,
     *                                 in which case this storage does not support durable persistence.
     * @param defaultSavepointLocation The default savepoint directory, or null, if none is set.
     * @param maxStateSize The maximum size of each individual piece of state.
     *
     * @throws IOException Thrown if a checkpoint base directory is given configured and the
     *                     checkpoint directory cannot be created within that directory.
     */
    public MemoryBackendCheckpointStorage(
            JobID jobId,
            @Nullable Path checkpointsBaseDirectory,
            @Nullable Path defaultSavepointLocation,
            int maxStateSize) throws IOException {

        super(jobId, defaultSavepointLocation);

        checkArgument(maxStateSize > 0);
        this.maxStateSize = maxStateSize;

        if (checkpointsBaseDirectory == null) {
            checkpointsDirectory = null;
            fileSystem = null;
        }
        else {
            this.fileSystem = checkpointsBaseDirectory.getFileSystem();
            this.checkpointsDirectory = getCheckpointDirectoryForJob(checkpointsBaseDirectory, jobId);

            fileSystem.mkdirs(checkpointsDirectory);
        }
    }

    // ------------------------------------------------------------------------
    //  Properties
    // ------------------------------------------------------------------------

    /**
     * Gets the size (in bytes) that a individual chunk of state may have at most.
     */
    public int getMaxStateSize() {
        return maxStateSize;
    }

    // ------------------------------------------------------------------------
    //  Checkpoint Storage
    // ------------------------------------------------------------------------

    @Override
    public boolean supportsHighlyAvailableStorage() {
        return checkpointsDirectory != null;
    }

    @Override
    public CheckpointStorageLocation initializeLocationForCheckpoint(long checkpointId) throws IOException {
        checkArgument(checkpointId >= 0);

        if (checkpointsDirectory != null) {
            // configured for durable metadata
            // prepare all the paths needed for the checkpoints
            checkState(fileSystem != null);

            final Path checkpointDir = createCheckpointDirectory(checkpointsDirectory, checkpointId);

            // create the checkpoint exclusive directory
            fileSystem.mkdirs(checkpointDir);

            return new PersistentMetadataCheckpointStorageLocation(fileSystem, checkpointDir, maxStateSize);
        }
        else {
            // no durable metadata - typical in IDE or test setup case
            return new NonPersistentMetadataCheckpointStorageLocation(maxStateSize);
        }
    }

    @Override
    public CheckpointStreamFactory resolveCheckpointStorageLocation(
            long checkpointId,
            CheckpointStorageLocationReference reference) throws IOException {

        // no matter where the checkpoint goes, we always return the storage location that stores
        // state inline with the state handles.
        return new MemCheckpointStreamFactory(maxStateSize);
    }

    @Override
    public CheckpointStateOutputStream createTaskOwnedStateStream() throws IOException {
        return new MemoryCheckpointOutputStream(maxStateSize);
    }

    @Override
    protected CheckpointStorageLocation createSavepointLocation(FileSystem fs, Path location) throws IOException {
        return new PersistentMetadataCheckpointStorageLocation(fs, location, maxStateSize);
    }

    // ------------------------------------------------------------------------
    //  Utilities
    // ------------------------------------------------------------------------

    @Override
    public String toString() {
        return "MemoryBackendCheckpointStorage {" +
                "checkpointsDirectory=" + checkpointsDirectory +
                ", fileSystem=" + fileSystem +
                ", maxStateSize=" + maxStateSize +
                '}';
    }
}

• MemoryBackendCheckpointStorage继承了AbstractFsCheckpointStorage，实现了它定义的createSavepointLocation方法，这里返回的是PersistentMetadataCheckpointStorageLocation
• MemoryBackendCheckpointStorage还实现了CheckpointStorage接口定义的AbstractFsCheckpointStorage未实现的几个方法：supportsHighlyAvailableStorage、initializeLocationForCheckpoint、resolveCheckpointStorageLocation、createTaskOwnedStateStream
• supportsHighlyAvailableStorage是根据是否有配置checkpointsDirectory来判断；initializeLocationForCheckpoint这个根据checkpointsDirectory是否有设置来创建，为null的话，创建的是NonPersistentMetadataCheckpointStorageLocation，不为null创建的是PersistentMetadataCheckpointStorageLocation；resolveCheckpointStorageLocation这里创建的是MemCheckpointStreamFactory；而createTaskOwnedStateStream创建的是MemoryCheckpointOutputStream

小结

• CheckpointStorage接口主要定义了持久化checkpoint data及metadata streams的基本方法；AbstractFsCheckpointStorage主要是实现了CheckpointStorage接口的hasDefaultSavepointLocation、resolveCheckpoint、initializeLocationForSavepoint方法，同时定义了一个抽象方法createSavepointLocation
• MemoryBackendCheckpointStorage继承了AbstractFsCheckpointStorage，实现了它定义的createSavepointLocation方法，同时还实现了CheckpointStorage接口定义的AbstractFsCheckpointStorage未实现的几个方法：supportsHighlyAvailableStorage、initializeLocationForCheckpoint、resolveCheckpointStorageLocation、createTaskOwnedStateStream
• 这里可以看到MemoryBackendCheckpointStorage虽然是memory的，但是如果有配置checkpointsDirectory(highly available storage)，checkpoint location使用的是PersistentMetadataCheckpointStorageLocation，否则使用NonPersistentMetadataCheckpointStorageLocation；而savepoint location使用的是PersistentMetadataCheckpointStorageLocation(checkpiont可以选择是否使用文件存储，而savepoint只能使用文件存储)

doc

• The MemoryStateBackend