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https://issues.apache.org/jira/browse/FLINK-6390?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Stephan Ewen closed FLINK-6390.
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> Add Trigger Hooks to the Checkpoint Coordinator
> -----------------------------------------------
>
> Key: FLINK-6390
> URL: https://issues.apache.org/jira/browse/FLINK-6390
> Project: Flink
> Issue Type: New Feature
> Components: State Backends, Checkpointing
> Reporter: Stephan Ewen
> Assignee: Stephan Ewen
> Fix For: 1.3.0
>
>
> 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
> {code}
> /**
> * 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();
> {code}
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