Details
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New Feature
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Status: Closed
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Major
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Resolution: Fixed
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None
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None
Description
Some source systems require to be notified prior to starting a checkpoint, in order to do preparatory work for the checkpoint.
I propose to add an interface to allow sources to register hooks that are called by the checkpoint coordinator when triggering / restoring a checkpoint.
These hooks may produce state that is stores with the checkpoint metadata.
Envisioned interface for the hooks
/** * The interface for hooks that can be called by the checkpoint coordinator when triggering or * restoring a checkpoint. Such a hook is useful for example when preparing external systems for * taking or restoring checkpoints. * * <p>The {@link #triggerCheckpoint(long, long, Executor)} method (called when triggering a checkpoint) * can return a result (via a future) that will be stored as part of the checkpoint metadata. * When restoring a checkpoint, that stored result will be given to the {@link #restoreCheckpoint(long, Object)} * method. The hook's {@link #getIdentifier() identifier} is used to map data to hook in the presence * of multiple hooks, and when resuming a savepoint that was potentially created by a different job. * The identifier has a similar role as for example the operator UID in the streaming API. * * <p>The MasterTriggerRestoreHook is defined when creating the streaming dataflow graph. It is attached * to the job graph, which gets sent to the cluster for execution. To avoid having to make the hook * itself serializable, these hooks are attached to the job graph via a {@link MasterTriggerRestoreHook.Factory}. * * @param <T> The type of the data produced by the hook and stored as part of the checkpoint metadata. * If the hook never stores any data, this can be typed to {@code Void}. */ public interface MasterTriggerRestoreHook<T> { /** * Gets the identifier of this hook. The identifier is used to identify a specific hook in the * presence of multiple hooks and to give it the correct checkpointed data upon checkpoint restoration. * * <p>The identifier should be unique between different hooks of a job, but deterministic/constant * so that upon resuming a savepoint, the hook will get the correct data. * For example, if the hook calls into another storage system and persists namespace/schema specific * information, then the name of the storage system, together with the namespace/schema name could * be an appropriate identifier. * * <p>When multiple hooks of the same name are created and attached to a job graph, only the first * one is actually used. This can be exploited to deduplicate hooks that would do the same thing. * * @return The identifier of the hook. */ String getIdentifier(); /** * This method is called by the checkpoint coordinator prior when triggering a checkpoint, prior * to sending the "trigger checkpoint" messages to the source tasks. * * <p>If the hook implementation wants to store data as part of the checkpoint, it may return * that data via a future, otherwise it should return null. The data is stored as part of * the checkpoint metadata under the hooks identifier (see {@link #getIdentifier()}). * * <p>If the action by this hook needs to be executed synchronously, then this method should * directly execute the action synchronously and block until it is complete. The returned future * (if any) would typically be a completed future. * * <p>If the action should be executed asynchronously and only needs to complete before the * checkpoint is considered completed, then the method may use the given executor to execute the * actual action and would signal its completion by completing the future. For hooks that do not * need to store data, the future would be completed with null. * * @param checkpointId The ID (logical timestamp, monotonously increasing) of the checkpoint * @param timestamp The wall clock timestamp when the checkpoint was triggered, for * info/logging purposes. * @param executor The executor for asynchronous actions * * @return Optionally, a future that signals when the hook has completed and that contains * data to be stored with the checkpoint. * * @throws Exception Exceptions encountered when calling the hook will cause the checkpoint to abort. */ @Nullable Future<T> triggerCheckpoint(long checkpointId, long timestamp, Executor executor) throws Exception; /** * This method is called by the checkpoint coordinator prior to restoring the state of a checkpoint. * If the checkpoint did store data from this hook, that data will be passed to this method. * * @param checkpointId The The ID (logical timestamp) of the restored checkpoint * @param checkpointData The data originally stored in the checkpoint by this hook, possibly null. * * @throws Exception Exceptions thrown while restoring the checkpoint will cause the restore * operation to fail and to possibly fall back to another checkpoint. */ void restoreCheckpoint(long checkpointId, @Nullable T checkpointData) throws Exception; /** * Creates a the serializer to (de)serializes the data stored by this hook. The serializer * serializes the result of the Future returned by the {@link #triggerCheckpoint(long, long, Executor)} * method, and deserializes the data stored in the checkpoint into the object passed to the * {@link #restoreCheckpoint(long, Object)} method. * * <p>If the hook never returns any data to be stored, then this method may return null as the * serializer. * * @return The serializer to (de)serializes the data stored by this hook */ @Nullable SimpleVersionedSerializer<T> createCheckpointDataSerializer();
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