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;
}
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• 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;
}
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• 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.");
			}
		}
	}
}
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• 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 + ")";
	}
}
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• 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