聊聊flink的MemoryStateBackend

序

本文主要研究一下flink的MemoryStateBackend

StateBackend

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

@PublicEvolving
public interface StateBackend extends java.io.Serializable {

    // ------------------------------------------------------------------------
    //  Checkpoint storage - the durable persistence of checkpoint data
    // ------------------------------------------------------------------------

    /**
     * Resolves the given pointer to a checkpoint/savepoint into a checkpoint location. The location
     * supports reading the checkpoint metadata, or disposing the checkpoint storage location.
     *
     * <p>If the state backend cannot understand the format of the pointer (for example because it
     * was created by a different state backend) this method should throw an {@code IOException}.
     *
     * @param externalPointer The external checkpoint pointer to resolve.
     * @return The checkpoint location handle.
     *
     * @throws IOException Thrown, if the state backend does not understand the pointer, or if
     *                     the pointer could not be resolved due to an I/O error.
     */
    CompletedCheckpointStorageLocation resolveCheckpoint(String externalPointer) throws IOException;

    /**
     * Creates a storage for checkpoints for the given job. The checkpoint storage is
     * used to write checkpoint data and metadata.
     *
     * @param jobId The job to store checkpoint data for.
     * @return A checkpoint storage for the given job.
     *
     * @throws IOException Thrown if the checkpoint storage cannot be initialized.
     */
    CheckpointStorage createCheckpointStorage(JobID jobId) throws IOException;

    // ------------------------------------------------------------------------
    //  Structure Backends 
    // ------------------------------------------------------------------------

    /**
     * Creates a new {@link AbstractKeyedStateBackend} that is responsible for holding <b>keyed state</b>
     * and checkpointing it. Uses default TTL time provider.
     *
     * <p><i>Keyed State</i> is state where each value is bound to a key.
     *
     * @param <K> The type of the keys by which the state is organized.
     *
     * @return The Keyed State Backend for the given job, operator, and key group range.
     *
     * @throws Exception This method may forward all exceptions that occur while instantiating the backend.
     */
    default <K> AbstractKeyedStateBackend<K> createKeyedStateBackend(
            Environment env,
            JobID jobID,
            String operatorIdentifier,
            TypeSerializer<K> keySerializer,
            int numberOfKeyGroups,
            KeyGroupRange keyGroupRange,
            TaskKvStateRegistry kvStateRegistry) throws Exception {
        return createKeyedStateBackend(
            env,
            jobID,
            operatorIdentifier,
            keySerializer,
            numberOfKeyGroups,
            keyGroupRange,
            kvStateRegistry,
            TtlTimeProvider.DEFAULT
        );
    }

    /**
     * Creates a new {@link AbstractKeyedStateBackend} that is responsible for holding <b>keyed state</b>
     * and checkpointing it.
     *
     * <p><i>Keyed State</i> is state where each value is bound to a key.
     *
     * @param <K> The type of the keys by which the state is organized.
     *
     * @return The Keyed State Backend for the given job, operator, and key group range.
     *
     * @throws Exception This method may forward all exceptions that occur while instantiating the backend.
     */
    default <K> AbstractKeyedStateBackend<K> createKeyedStateBackend(
        Environment env,
        JobID jobID,
        String operatorIdentifier,
        TypeSerializer<K> keySerializer,
        int numberOfKeyGroups,
        KeyGroupRange keyGroupRange,
        TaskKvStateRegistry kvStateRegistry,
        TtlTimeProvider ttlTimeProvider
    ) throws Exception {
        return createKeyedStateBackend(
            env,
            jobID,
            operatorIdentifier,
            keySerializer,
            numberOfKeyGroups,
            keyGroupRange,
            kvStateRegistry,
            ttlTimeProvider,
            new UnregisteredMetricsGroup());
    }

    /**
     * Creates a new {@link AbstractKeyedStateBackend} that is responsible for holding <b>keyed state</b>
     * and checkpointing it.
     *
     * <p><i>Keyed State</i> is state where each value is bound to a key.
     *
     * @param <K> The type of the keys by which the state is organized.
     *
     * @return The Keyed State Backend for the given job, operator, and key group range.
     *
     * @throws Exception This method may forward all exceptions that occur while instantiating the backend.
     */
    <K> AbstractKeyedStateBackend<K> createKeyedStateBackend(
        Environment env,
        JobID jobID,
        String operatorIdentifier,
        TypeSerializer<K> keySerializer,
        int numberOfKeyGroups,
        KeyGroupRange keyGroupRange,
        TaskKvStateRegistry kvStateRegistry,
        TtlTimeProvider ttlTimeProvider,
        MetricGroup metricGroup) throws Exception;
    
    /**
     * Creates a new {@link OperatorStateBackend} that can be used for storing operator state.
     *
     * <p>Operator state is state that is associated with parallel operator (or function) instances,
     * rather than with keys.
     *
     * @param env The runtime environment of the executing task.
     * @param operatorIdentifier The identifier of the operator whose state should be stored.
     *
     * @return The OperatorStateBackend for operator identified by the job and operator identifier.
     *
     * @throws Exception This method may forward all exceptions that occur while instantiating the backend.
     */
    OperatorStateBackend createOperatorStateBackend(Environment env, String operatorIdentifier) throws Exception;
}

• StateBackend接口定义了有状态的streaming应用的state是如何stored以及checkpointed
• flink目前内置支持MemoryStateBackend、FsStateBackend、RocksDBStateBackend三种，如果没有配置默认为MemoryStateBackend；在flink-conf.yaml里头可以进行全局的默认配置，不过具体每个job还可以通过StreamExecutionEnvironment.setStateBackend来覆盖全局的配置
• MemoryStateBackend可以在构造器中指定大小，默认是5MB，可以增大但是不能超过akka frame size；FsStateBackend模式把TaskManager的state存储在内存，但是可以把checkpoint的state存储到filesystem中(比如HDFS)；RocksDBStateBackend把working state存储在RocksDB中，checkpoint的state存储在filesystem
• StateBackend接口定义了createCheckpointStorage、createKeyedStateBackend、createOperatorStateBackend方法；同时继承了Serializable接口；StateBackend接口的实现要求是线程安全的
• StateBackend有个直接实现的抽象类AbstractStateBackend，而AbstractFileStateBackend及RocksDBStateBackend继承了AbstractStateBackend，之后MemoryStateBackend、FsStateBackend都继承了AbstractFileStateBackend

AbstractStateBackend

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

/**
 * An abstract base implementation of the {@link StateBackend} interface.
 *
 * <p>This class has currently no contents and only kept to not break the prior class hierarchy for users.
 */
@PublicEvolving
public abstract class AbstractStateBackend implements StateBackend, java.io.Serializable {

    private static final long serialVersionUID = 4620415814639230247L;

    // ------------------------------------------------------------------------
    //  State Backend - State-Holding Backends
    // ------------------------------------------------------------------------

    @Override
    public abstract <K> AbstractKeyedStateBackend<K> createKeyedStateBackend(
        Environment env,
        JobID jobID,
        String operatorIdentifier,
        TypeSerializer<K> keySerializer,
        int numberOfKeyGroups,
        KeyGroupRange keyGroupRange,
        TaskKvStateRegistry kvStateRegistry,
        TtlTimeProvider ttlTimeProvider,
        MetricGroup metricGroup) throws IOException;

    @Override
    public abstract OperatorStateBackend createOperatorStateBackend(
            Environment env,
            String operatorIdentifier) throws Exception;
}

• AbstractStateBackend声明实现StateBackend及Serializable接口，这里没有新增其他内容

AbstractFileStateBackend

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

@PublicEvolving
public abstract class AbstractFileStateBackend extends AbstractStateBackend {

    private static final long serialVersionUID = 1L;

    // ------------------------------------------------------------------------
    //  State Backend Properties
    // ------------------------------------------------------------------------

    /** The path where checkpoints will be stored, or null, if none has been configured. */
    @Nullable
    private final Path baseCheckpointPath;

    /** The path where savepoints will be stored, or null, if none has been configured. */
    @Nullable
    private final Path baseSavepointPath;

    //......

    // ------------------------------------------------------------------------
    //  Initialization and metadata storage
    // ------------------------------------------------------------------------

    @Override
    public CompletedCheckpointStorageLocation resolveCheckpoint(String pointer) throws IOException {
        return AbstractFsCheckpointStorage.resolveCheckpointPointer(pointer);
    }

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

    /**
     * Checks the validity of the path's scheme and path.
     *
     * @param path The path to check.
     * @return The URI as a Path.
     *
     * @throws IllegalArgumentException Thrown, if the URI misses scheme or path.
     */
    private static Path validatePath(Path path) {
        final URI uri = path.toUri();
        final String scheme = uri.getScheme();
        final String pathPart = uri.getPath();

        // some validity checks
        if (scheme == null) {
            throw new IllegalArgumentException("The scheme (hdfs://, file://, etc) is null. " +
                    "Please specify the file system scheme explicitly in the URI.");
        }
        if (pathPart == null) {
            throw new IllegalArgumentException("The path to store the checkpoint data in is null. " +
                    "Please specify a directory path for the checkpoint data.");
        }
        if (pathPart.length() == 0 || pathPart.equals("/")) {
            throw new IllegalArgumentException("Cannot use the root directory for checkpoints.");
        }

        return path;
    }

    @Nullable
    private static Path parameterOrConfigured(@Nullable Path path, Configuration config, ConfigOption<String> option) {
        if (path != null) {
            return path;
        }
        else {
            String configValue = config.getString(option);
            try {
                return configValue == null ? null : new Path(configValue);
            }
            catch (IllegalArgumentException e) {
                throw new IllegalConfigurationException("Cannot parse value for " + option.key() +
                        " : " + configValue + " . Not a valid path.");
            }
        }
    }
}

• AbstractFileStateBackend继承了AbstractStateBackend，它有baseCheckpointPath、baseSavepointPath两个属性，允许为null，路径的格式为hdfs://或者file://开头；resolveCheckpoint方法用于解析checkpoint或savepoint的location，这里使用AbstractFsCheckpointStorage.resolveCheckpointPointer(pointer)来完成

MemoryStateBackend

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

@PublicEvolving
public class MemoryStateBackend extends AbstractFileStateBackend implements ConfigurableStateBackend {

    private static final long serialVersionUID = 4109305377809414635L;

    /** The default maximal size that the snapshotted memory state may have (5 MiBytes). */
    public static final int DEFAULT_MAX_STATE_SIZE = 5 * 1024 * 1024;

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

    /** Switch to chose between synchronous and asynchronous snapshots.
     * A value of 'UNDEFINED' means not yet configured, in which case the default will be used. */
    private final TernaryBoolean asynchronousSnapshots;

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

    /**
     * Creates a new memory state backend that accepts states whose serialized forms are
     * up to the default state size (5 MB).
     *
     * <p>Checkpoint and default savepoint locations are used as specified in the
     * runtime configuration.
     */
    public MemoryStateBackend() {
        this(null, null, DEFAULT_MAX_STATE_SIZE, TernaryBoolean.UNDEFINED);
    }

    /**
     * Creates a new memory state backend that accepts states whose serialized forms are
     * up to the default state size (5 MB). The state backend uses asynchronous snapshots
     * or synchronous snapshots as configured.
     *
     * <p>Checkpoint and default savepoint locations are used as specified in the
     * runtime configuration.
     *
     * @param asynchronousSnapshots Switch to enable asynchronous snapshots.
     */
    public MemoryStateBackend(boolean asynchronousSnapshots) {
        this(null, null, DEFAULT_MAX_STATE_SIZE, TernaryBoolean.fromBoolean(asynchronousSnapshots));
    }

    /**
     * Creates a new memory state backend that accepts states whose serialized forms are
     * up to the given number of bytes.
     *
     * <p>Checkpoint and default savepoint locations are used as specified in the
     * runtime configuration.
     *
     * <p><b>WARNING:</b> Increasing the size of this value beyond the default value
     * ({@value #DEFAULT_MAX_STATE_SIZE}) should be done with care.
     * The checkpointed state needs to be send to the JobManager via limited size RPC messages, and there
     * and the JobManager needs to be able to hold all aggregated state in its memory.
     *
     * @param maxStateSize The maximal size of the serialized state
     */
    public MemoryStateBackend(int maxStateSize) {
        this(null, null, maxStateSize, TernaryBoolean.UNDEFINED);
    }

    /**
     * Creates a new memory state backend that accepts states whose serialized forms are
     * up to the given number of bytes and that uses asynchronous snashots as configured.
     *
     * <p>Checkpoint and default savepoint locations are used as specified in the
     * runtime configuration.
     *
     * <p><b>WARNING:</b> Increasing the size of this value beyond the default value
     * ({@value #DEFAULT_MAX_STATE_SIZE}) should be done with care.
     * The checkpointed state needs to be send to the JobManager via limited size RPC messages, and there
     * and the JobManager needs to be able to hold all aggregated state in its memory.
     *
     * @param maxStateSize The maximal size of the serialized state
     * @param asynchronousSnapshots Switch to enable asynchronous snapshots.
     */
    public MemoryStateBackend(int maxStateSize, boolean asynchronousSnapshots) {
        this(null, null, maxStateSize, TernaryBoolean.fromBoolean(asynchronousSnapshots));
    }

    /**
     * Creates a new MemoryStateBackend, setting optionally the path to persist checkpoint metadata
     * to, and to persist savepoints to.
     *
     * @param checkpointPath The path to write checkpoint metadata to. If null, the value from
     *                       the runtime configuration will be used.
     * @param savepointPath  The path to write savepoints to. If null, the value from
     *                       the runtime configuration will be used.
     */
    public MemoryStateBackend(@Nullable String checkpointPath, @Nullable String savepointPath) {
        this(checkpointPath, savepointPath, DEFAULT_MAX_STATE_SIZE, TernaryBoolean.UNDEFINED);
    }

    /**
     * Creates a new MemoryStateBackend, setting optionally the paths to persist checkpoint metadata
     * and savepoints to, as well as configuring state thresholds and asynchronous operations.
     *
     * <p><b>WARNING:</b> Increasing the size of this value beyond the default value
     * ({@value #DEFAULT_MAX_STATE_SIZE}) should be done with care.
     * The checkpointed state needs to be send to the JobManager via limited size RPC messages, and there
     * and the JobManager needs to be able to hold all aggregated state in its memory.
     *
     * @param checkpointPath The path to write checkpoint metadata to. If null, the value from
     *                       the runtime configuration will be used.
     * @param savepointPath  The path to write savepoints to. If null, the value from
     *                       the runtime configuration will be used.
     * @param maxStateSize   The maximal size of the serialized state.
     * @param asynchronousSnapshots Flag to switch between synchronous and asynchronous
     *                              snapshot mode. If null, the value configured in the
     *                              runtime configuration will be used.
     */
    public MemoryStateBackend(
            @Nullable String checkpointPath,
            @Nullable String savepointPath,
            int maxStateSize,
            TernaryBoolean asynchronousSnapshots) {

        super(checkpointPath == null ? null : new Path(checkpointPath),
                savepointPath == null ? null : new Path(savepointPath));

        checkArgument(maxStateSize > 0, "maxStateSize must be > 0");
        this.maxStateSize = maxStateSize;

        this.asynchronousSnapshots = asynchronousSnapshots;
    }

    /**
     * Private constructor that creates a re-configured copy of the state backend.
     *
     * @param original The state backend to re-configure
     * @param configuration The configuration
     */
    private MemoryStateBackend(MemoryStateBackend original, Configuration configuration) {
        super(original.getCheckpointPath(), original.getSavepointPath(), configuration);

        this.maxStateSize = original.maxStateSize;

        // if asynchronous snapshots were configured, use that setting,
        // else check the configuration
        this.asynchronousSnapshots = original.asynchronousSnapshots.resolveUndefined(
                configuration.getBoolean(CheckpointingOptions.ASYNC_SNAPSHOTS));
    }

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

    /**
     * Gets the maximum size that an individual state can have, as configured in the
     * constructor (by default {@value #DEFAULT_MAX_STATE_SIZE}).
     *
     * @return The maximum size that an individual state can have
     */
    public int getMaxStateSize() {
        return maxStateSize;
    }

    /**
     * Gets whether the key/value data structures are asynchronously snapshotted.
     *
     * <p>If not explicitly configured, this is the default value of
     * {@link CheckpointingOptions#ASYNC_SNAPSHOTS}.
     */
    public boolean isUsingAsynchronousSnapshots() {
        return asynchronousSnapshots.getOrDefault(CheckpointingOptions.ASYNC_SNAPSHOTS.defaultValue());
    }

    // ------------------------------------------------------------------------
    //  Reconfiguration
    // ------------------------------------------------------------------------

    /**
     * Creates a copy of this state backend that uses the values defined in the configuration
     * for fields where that were not specified in this state backend.
     *
     * @param config the configuration
     * @return The re-configured variant of the state backend
     */
    @Override
    public MemoryStateBackend configure(Configuration config) {
        return new MemoryStateBackend(this, config);
    }

    // ------------------------------------------------------------------------
    //  checkpoint state persistence
    // ------------------------------------------------------------------------

    @Override
    public CheckpointStorage createCheckpointStorage(JobID jobId) throws IOException {
        return new MemoryBackendCheckpointStorage(jobId, getCheckpointPath(), getSavepointPath(), maxStateSize);
    }

    // ------------------------------------------------------------------------
    //  state holding structures
    // ------------------------------------------------------------------------

    @Override
    public OperatorStateBackend createOperatorStateBackend(
            Environment env,
            String operatorIdentifier) throws Exception {

        return new DefaultOperatorStateBackend(
                env.getUserClassLoader(),
                env.getExecutionConfig(),
                isUsingAsynchronousSnapshots());
    }

    @Override
    public <K> AbstractKeyedStateBackend<K> createKeyedStateBackend(
            Environment env,
            JobID jobID,
            String operatorIdentifier,
            TypeSerializer<K> keySerializer,
            int numberOfKeyGroups,
            KeyGroupRange keyGroupRange,
            TaskKvStateRegistry kvStateRegistry,
            TtlTimeProvider ttlTimeProvider,
            MetricGroup metricGroup) {

        TaskStateManager taskStateManager = env.getTaskStateManager();
        HeapPriorityQueueSetFactory priorityQueueSetFactory =
            new HeapPriorityQueueSetFactory(keyGroupRange, numberOfKeyGroups, 128);
        return new HeapKeyedStateBackend<>(
                kvStateRegistry,
                keySerializer,
                env.getUserClassLoader(),
                numberOfKeyGroups,
                keyGroupRange,
                isUsingAsynchronousSnapshots(),
                env.getExecutionConfig(),
                taskStateManager.createLocalRecoveryConfig(),
                priorityQueueSetFactory,
                ttlTimeProvider);
    }

    // ------------------------------------------------------------------------
    //  utilities
    // ------------------------------------------------------------------------

    @Override
    public String toString() {
        return "MemoryStateBackend (data in heap memory / checkpoints to JobManager) " +
                "(checkpoints: '" + getCheckpointPath() +
                "', savepoints: '" + getSavepointPath() +
                "', asynchronous: " + asynchronousSnapshots +
                ", maxStateSize: " + maxStateSize + ")";
    }
}

• MemoryStateBackend继承了AbstractFileStateBackend，实现ConfigurableStateBackend接口(configure方法)；它将TaskManager的working state及JobManager的checkpoint state存储在JVM heap中(但是为了高可用，也可以设置checkpoint state存储到filesystem)；MemoryStateBackend仅仅用来做实验用途，比如本地启动或者所需的state非常小，对于生产需要改为使用FsStateBackend(将TaskManager的working state存储在内存，但是将JobManager的checkpoint state存储到文件系统以支持更大的state存储)
• MemoryStateBackend有个maxStateSize属性(默认DEFAULT_MAX_STATE_SIZE为5MB)，每个state的大小不能超过maxStateSize，一个task的所有state不能超过RPC系统的限制(默认是10MB，可以修改但不建议)，所有retained checkpoints的state大小总和不能超过JobManager的JVM heap大小；另外如果创建MemoryStateBackend时未指定checkpointPath及savepointPath，则会从flink-conf.yaml中读取全局默认值；MemoryStateBackend里头还有一个asynchronousSnapshots属性，是TernaryBoolean类型(TRUE、FALSE、UNDEFINED)，其中UNDEFINED表示没有配置，将会使用默认值
• MemoryStateBackend的createCheckpointStorage创建的是MemoryBackendCheckpointStorage；createOperatorStateBackend方法创建的是OperatorStateBackend；createKeyedStateBackend方法创建的是HeapKeyedStateBackend

小结

• StateBackend接口定义了有状态的streaming应用的state是如何stored以及checkpointed；目前内置支持MemoryStateBackend、FsStateBackend、RocksDBStateBackend三种，如果没有配置默认为MemoryStateBackend；在flink-conf.yaml里头可以进行全局的默认配置，不过具体每个job还可以通过StreamExecutionEnvironment.setStateBackend来覆盖全局的配置
• StateBackend接口定义了createCheckpointStorage、createKeyedStateBackend、createOperatorStateBackend方法；同时继承了Serializable接口；StateBackend接口的实现要求是线程安全的；StateBackend有个直接实现的抽象类AbstractStateBackend，而AbstractFileStateBackend及RocksDBStateBackend继承了AbstractStateBackend，之后MemoryStateBackend、FsStateBackend都继承了AbstractFileStateBackend
• MemoryStateBackend继承了AbstractFileStateBackend，实现ConfigurableStateBackend接口(configure方法)；它将TaskManager的working state及JobManager的checkpoint state存储在JVM heap中；MemoryStateBackend的createCheckpointStorage创建的是MemoryBackendCheckpointStorage；createOperatorStateBackend方法创建的是OperatorStateBackend；createKeyedStateBackend方法创建的是HeapKeyedStateBackend

doc

• State Backends