聊聊flink的FsCheckpointStorage

序

本文主要研究一下flink的FsCheckpointStorage

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介面主要定義了持久化checkpoint data及metadata streams的基本方法；supportsHighlyAvailableStorage方法返回該backend是否支援highly available storage；hasDefaultSavepointLocation方法是否有預設的savepoint location；resolveCheckpoint方法用於解析checkpoint location返回CompletedCheckpointStorageLocation；initializeLocationForCheckpoint方法根據checkpointId來初始化storage location；initializeLocationForSavepoint方法用於根據checkpointId來初始化savepoint的storage location；resolveCheckpointStorageLocation方法解析CheckpointStorageLocationReference返回CheckpointStreamFactory；createTaskOwnedStateStream方法用於開啟一個stream來持久化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主要是實現了CheckpointStorage介面的hasDefaultSavepointLocation、resolveCheckpoint、initializeLocationForSavepoint方法
• resolveCheckpoint方法主要做兩件事情，一個是解析checkpoint/savepoint path，一個是解析checkpoint/savepoint的metadata path，獲取他們的FileStatus，然後建立FsCompletedCheckpointStorageLocation
• initializeLocationForSavepoint方法主要是給savepoint建立一個CheckpointStorageLocation，它可以根據externalLocationPointer來建立，該值為null的話則使用defaultSavepointDirectory，該方法裡頭呼叫了createSavepointLocation抽象方法，由子類去實現

FsCheckpointStorage

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

/**
 * An implementation of durable checkpoint storage to file systems.
 */
public class FsCheckpointStorage extends AbstractFsCheckpointStorage {

    private final FileSystem fileSystem;

    private final Path checkpointsDirectory;

    private final Path sharedStateDirectory;

    private final Path taskOwnedStateDirectory;

    private final int fileSizeThreshold;

    public FsCheckpointStorage(
            Path checkpointBaseDirectory,
            @Nullable Path defaultSavepointDirectory,
            JobID jobId,
            int fileSizeThreshold) throws IOException {

        this(checkpointBaseDirectory.getFileSystem(),
                checkpointBaseDirectory,
                defaultSavepointDirectory,
                jobId,
                fileSizeThreshold);
    }

    public FsCheckpointStorage(
            FileSystem fs,
            Path checkpointBaseDirectory,
            @Nullable Path defaultSavepointDirectory,
            JobID jobId,
            int fileSizeThreshold) throws IOException {

        super(jobId, defaultSavepointDirectory);

        checkArgument(fileSizeThreshold >= 0);

        this.fileSystem = checkNotNull(fs);
        this.checkpointsDirectory = getCheckpointDirectoryForJob(checkpointBaseDirectory, jobId);
        this.sharedStateDirectory = new Path(checkpointsDirectory, CHECKPOINT_SHARED_STATE_DIR);
        this.taskOwnedStateDirectory = new Path(checkpointsDirectory, CHECKPOINT_TASK_OWNED_STATE_DIR);
        this.fileSizeThreshold = fileSizeThreshold;

        // initialize the dedicated directories
        fileSystem.mkdirs(checkpointsDirectory);
        fileSystem.mkdirs(sharedStateDirectory);
        fileSystem.mkdirs(taskOwnedStateDirectory);
    }

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

    public Path getCheckpointsDirectory() {
        return checkpointsDirectory;
    }

    // ------------------------------------------------------------------------
    //  CheckpointStorage implementation
    // ------------------------------------------------------------------------

    @Override
    public boolean supportsHighlyAvailableStorage() {
        return true;
    }

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

        // prepare all the paths needed for the checkpoints
        final Path checkpointDir = createCheckpointDirectory(checkpointsDirectory, checkpointId);

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

        return new FsCheckpointStorageLocation(
                fileSystem,
                checkpointDir,
                sharedStateDirectory,
                taskOwnedStateDirectory,
                CheckpointStorageLocationReference.getDefault(),
                fileSizeThreshold);
    }

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

        if (reference.isDefaultReference()) {
            // default reference, construct the default location for that particular checkpoint
            final Path checkpointDir = createCheckpointDirectory(checkpointsDirectory, checkpointId);

            return new FsCheckpointStorageLocation(
                    fileSystem,
                    checkpointDir,
                    sharedStateDirectory,
                    taskOwnedStateDirectory,
                    reference,
                    fileSizeThreshold);
        }
        else {
            // location encoded in the reference
            final Path path = decodePathFromReference(reference);

            return new FsCheckpointStorageLocation(
                    path.getFileSystem(),
                    path,
                    path,
                    path,
                    reference,
                    fileSizeThreshold);
        }
    }

    @Override
    public CheckpointStateOutputStream createTaskOwnedStateStream() throws IOException {
        return new FsCheckpointStateOutputStream(
                taskOwnedStateDirectory,
                fileSystem,
                FsCheckpointStreamFactory.DEFAULT_WRITE_BUFFER_SIZE,
                fileSizeThreshold);
    }

    @Override
    protected CheckpointStorageLocation createSavepointLocation(FileSystem fs, Path location) throws IOException {
        final CheckpointStorageLocationReference reference = encodePathAsReference(location);
        return new FsCheckpointStorageLocation(fs, location, location, location, reference, fileSizeThreshold);
    }
}

• FsCheckpointStorage繼承了AbstractFsCheckpointStorage，實現了它定義的createSavepointLocation方法，這裡返回的是FsCheckpointStorageLocation
• FsCheckpointStorage還實現了CheckpointStorage介面定義的AbstractFsCheckpointStorage未實現的幾個方法：supportsHighlyAvailableStorage、initializeLocationForCheckpoint、resolveCheckpointStorageLocation、createTaskOwnedStateStream
• supportsHighlyAvailableStorage這裡直接返回true；initializeLocationForCheckpoint這裡建立的是FsCheckpointStorageLocation；resolveCheckpointStorageLocation這裡建立的是FsCheckpointStorageLocation；而createTaskOwnedStateStream建立的是FsCheckpointStateOutputStream

小結

• CheckpointStorage介面主要定義了持久化checkpoint data及metadata streams的基本方法；AbstractFsCheckpointStorage主要是實現了CheckpointStorage介面的hasDefaultSavepointLocation、resolveCheckpoint、initializeLocationForSavepoint方法，同時定義了一個抽象方法createSavepointLocation
• FsCheckpointStorage繼承了AbstractFsCheckpointStorage，實現了它定義的createSavepointLocation方法，同時還實現了CheckpointStorage介面定義的AbstractFsCheckpointStorage未實現的幾個方法：supportsHighlyAvailableStorage、initializeLocationForCheckpoint、resolveCheckpointStorageLocation、createTaskOwnedStateStream
• FsCheckpointStorage的supportsHighlyAvailableStorage方法直接返回true；initializeLocationForCheckpoint方法建立的是FsCheckpointStorageLocation；resolveCheckpointStorageLocation方法建立的是FsCheckpointStorageLocation；而createTaskOwnedStateStream方法建立的是FsCheckpointStateOutputStream

doc

• State Backends
• 聊聊flink的MemoryBackendCheckpointStorage