org.apache.flink.runtime.operators.coordination

Interface OperatorCoordinator

All Superinterfaces:

AutoCloseable, CheckpointListener

All Known Implementing Classes:

CollectSinkOperatorCoordinator, DynamicFilteringDataCollectorOperatorCoordinator, RecreateOnResetOperatorCoordinator, SourceCoordinator

@Internal
public interface OperatorCoordinator
extends CheckpointListener, AutoCloseable

A coordinator for runtime operators. The OperatorCoordinator runs on the master, associated with the job vertex of the operator. It communicates with operators via sending operator events.

Operator coordinators are for example source and sink coordinators that discover and assign work, or aggregate and commit metadata.

Thread Model

All coordinator methods are called by the Job Manager's main thread (mailbox thread). That means that these methods must not, under any circumstances, perform blocking operations (like I/O or waiting on locks or futures). That would run a high risk of bringing down the entire JobManager.

Coordinators that involve more complex operations should hence spawn threads to handle the I/O work. The methods on the OperatorCoordinator.Context are safe to be called from another thread than the thread that calls the Coordinator's methods.

Consistency

The coordinator's view of the task execution is highly simplified, compared to the Scheduler's view, but allows for consistent interaction with the operators running on the parallel subtasks. In particular, the following methods are guaranteed to be called strictly in order:

1. executionAttemptReady(int, int, SubtaskGateway): Called once you can send events to the subtask execution attempt. The provided gateway is bound to that specific execution attempt. This is the start of interaction with the operator subtask attempt.
2. executionAttemptFailed(int, int, Throwable): Called for each subtask execution attempt as soon as the attempt failed or was cancelled. At this point, interaction with the subtask attempt should stop.
3. subtaskReset(int, long) or resetToCheckpoint(long, byte[]): Once the scheduler determined which checkpoint to restore, these methods notify the coordinator of that. The former method is called in case of a regional failure/recovery (affecting possible a subset of subtasks), the later method in case of a global failure/recovery. This method should be used to determine which actions to recover, because it tells you which checkpoint to fall back to. The coordinator implementation needs to recover the interactions with the relevant tasks since the checkpoint that is restored. It will be called only after executionAttemptFailed(int, int, Throwable) has been called on all the attempts of the subtask.
4. executionAttemptReady(int, int, SubtaskGateway): Called again, once the recovered tasks (new attempts) are ready to go. This is later than subtaskReset(int, long), because between those methods, the new attempts are scheduled and deployed.

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static interface	OperatorCoordinator.Context The context gives the OperatorCoordinator access to contextual information and provides a gateway to interact with other components, such as sending operator events.
static interface	OperatorCoordinator.Provider The provider creates an OperatorCoordinator and takes a OperatorCoordinator.Context to pass to the OperatorCoordinator.
static interface	OperatorCoordinator.SubtaskGateway The SubtaskGateway is the way to interact with a specific parallel instance of the Operator (an Operator subtask), like sending events to the operator.

Field Summary

Fields

Modifier and Type	Field and Description
static long	BATCH_CHECKPOINT_ID The checkpoint ID passed to the restore methods when batch scenarios.
static long	NO_CHECKPOINT The checkpoint ID passed to the restore methods when no completed checkpoint exists, yet.

Method Summary

Modifier and Type	Method and Description
void	checkpointCoordinator(long checkpointId, CompletableFuture<byte[]> resultFuture) Takes a checkpoint of the coordinator.
void	close() This method is called when the coordinator is disposed.
void	executionAttemptFailed(int subtask, int attemptNumber, Throwable reason) Called when any subtask execution attempt of the task running the coordinated operator is failed/canceled.
void	executionAttemptReady(int subtask, int attemptNumber, OperatorCoordinator.SubtaskGateway gateway) This is called when a subtask execution attempt of the Operator becomes ready to receive events.
void	handleEventFromOperator(int subtask, int attemptNumber, OperatorEvent event) Hands an OperatorEvent coming from a parallel Operator instance (one attempt of the parallel subtasks).
default void	notifyCheckpointAborted(long checkpointId) We override the method here to remove the checked exception.
void	notifyCheckpointComplete(long checkpointId) We override the method here to remove the checked exception.
void	resetToCheckpoint(long checkpointId, byte[] checkpointData) Resets the coordinator to the given checkpoint.
void	start() Starts the coordinator.
void	subtaskReset(int subtask, long checkpointId) Called if a subtask is recovered as part of a partial failover, meaning a failover handled by the scheduler's failover strategy (by default recovering a pipelined region).
default boolean	supportsBatchSnapshot() Whether the operator coordinator supports taking snapshot in no-checkpoint/batch scenarios.

Field Detail

NO_CHECKPOINT

static final long NO_CHECKPOINT

The checkpoint ID passed to the restore methods when no completed checkpoint exists, yet. It indicates that the restore is to the "initial state" of the coordinator or the failed subtask.

See Also:

Constant Field Values

BATCH_CHECKPOINT_ID

static final long BATCH_CHECKPOINT_ID

The checkpoint ID passed to the restore methods when batch scenarios.

See Also:

Constant Field Values

Method Detail

start

void start()
    throws Exception

Starts the coordinator. This method is called once at the beginning, before any other methods.

Throws:

Exception - Any exception thrown from this method causes a full job failure.

close

void close()
    throws Exception

This method is called when the coordinator is disposed. This method should release currently held resources. Exceptions in this method do not cause the job to fail.

Specified by:

close in interface AutoCloseable

Throws:

Exception

handleEventFromOperator

void handleEventFromOperator(int subtask,
                             int attemptNumber,
                             OperatorEvent event)
                      throws Exception

Hands an OperatorEvent coming from a parallel Operator instance (one attempt of the parallel subtasks).

Throws:

Exception - Any exception thrown by this method results in a full job failure and recovery.

checkpointCoordinator

void checkpointCoordinator(long checkpointId,
                           CompletableFuture<byte[]> resultFuture)
                    throws Exception

Takes a checkpoint of the coordinator. The checkpoint is identified by the given ID.

To confirm the checkpoint and store state in it, the given CompletableFuture must be completed with the state. To abort or dis-confirm the checkpoint, the given CompletableFuture must be completed exceptionally. In any case, the given CompletableFuture must be completed in some way, otherwise the checkpoint will not progress.

Exactly-once Semantics

The semantics are defined as follows:

• The point in time when the checkpoint future is completed is considered the point in time when the coordinator's checkpoint takes place.
• The OperatorCoordinator implementation must have a way of strictly ordering the sending of events and the completion of the checkpoint future (for example the same thread does both actions, or both actions are guarded by a mutex).
• Every event sent before the checkpoint future is completed is considered before the checkpoint.
• Every event sent after the checkpoint future is completed is considered to be after the checkpoint.

Throws:

Exception - Any exception thrown by this method results in a full job failure and recovery.

notifyCheckpointComplete

void notifyCheckpointComplete(long checkpointId)

We override the method here to remove the checked exception. Please check the Java docs of CheckpointListener.notifyCheckpointComplete(long) for more detail semantic of the method.

Specified by:

notifyCheckpointComplete in interface CheckpointListener

Parameters:

checkpointId - The ID of the checkpoint that has been completed.

notifyCheckpointAborted

default void notifyCheckpointAborted(long checkpointId)

We override the method here to remove the checked exception. Please check the Java docs of CheckpointListener.notifyCheckpointAborted(long) for more detail semantic of the method.

Specified by:

notifyCheckpointAborted in interface CheckpointListener

Parameters:

checkpointId - The ID of the checkpoint that has been aborted.

resetToCheckpoint

void resetToCheckpoint(long checkpointId,
                       @Nullable
                       byte[] checkpointData)
                throws Exception

Resets the coordinator to the given checkpoint. When this method is called, the coordinator can discard all other in-flight working state. All subtasks will also have been reset to the same checkpoint.

This method is called in the case of a global failover of the system, which means a failover of the coordinator (JobManager). This method is not invoked on a partial failover; partial failovers call the subtaskReset(int, long) method for the involved subtasks.

This method is expected to behave synchronously with respect to other method calls and calls to Context methods. For example, Events being sent by the Coordinator after this method returns are assumed to take place after the checkpoint that was restored.

This method is called with a null state argument in the following situations:

• There is a recovery and there was no completed checkpoint yet.
• There is a recovery from a completed checkpoint/savepoint but it contained no state for the coordinator.

In both cases, the coordinator should reset to an empty (new) state.

Restoring implicitly notifies of Checkpoint Completion

Restoring to a checkpoint is a way of confirming that the checkpoint is complete. It is safe to commit side-effects that are predicated on checkpoint completion after this call.

Even if no call to notifyCheckpointComplete(long) happened, the checkpoint can still be complete (for example when a system failure happened directly after committing the checkpoint, before calling the notifyCheckpointComplete(long) method).

Throws:

Exception

subtaskReset

void subtaskReset(int subtask,
                  long checkpointId)

Called if a subtask is recovered as part of a partial failover, meaning a failover handled by the scheduler's failover strategy (by default recovering a pipelined region). The method is invoked for each subtask involved in that partial failover.

In contrast to this method, the resetToCheckpoint(long, byte[]) method is called in the case of a global failover, which is the case when the coordinator (JobManager) is recovered.

Note that this method will not be called if an execution attempt of a subtask failed, if the subtask is not entirely failed, i.e. if the subtask has other execution attempts that are not failed/canceled.

executionAttemptFailed

void executionAttemptFailed(int subtask,
                            int attemptNumber,
                            @Nullable
                            Throwable reason)

Called when any subtask execution attempt of the task running the coordinated operator is failed/canceled.

This method is called every time an execution attempt is failed/canceled, regardless of whether there it is caused by a partial failover or a global failover.

executionAttemptReady

void executionAttemptReady(int subtask,
                           int attemptNumber,
                           OperatorCoordinator.SubtaskGateway gateway)

This is called when a subtask execution attempt of the Operator becomes ready to receive events. The given SubtaskGateway can be used to send events to the execution attempt.

The given SubtaskGateway is bound to that specific execution attempt that became ready. All events sent through the gateway target that execution attempt; if the attempt is no longer running by the time the event is sent, then the events are failed.

supportsBatchSnapshot

default boolean supportsBatchSnapshot()

Whether the operator coordinator supports taking snapshot in no-checkpoint/batch scenarios. If it returns true, the checkpointCoordinator(long, java.util.concurrent.CompletableFuture<byte[]>) and resetToCheckpoint(long, byte[]) methods supports taking snapshot and restoring from a snapshot in batch processing scenarios. In such scenarios, the checkpointId will always be -1.