Interface OperatorCoordinator
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- 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:
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.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.subtaskReset(int, long)
orresetToCheckpoint(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 afterexecutionAttemptFailed(int, int, Throwable)
has been called on all the attempts of the subtask.executionAttemptReady(int, int, SubtaskGateway)
: Called again, once the recovered tasks (new attempts) are ready to go. This is later thansubtaskReset(int, long)
, because between those methods, the new attempts are scheduled and deployed.
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Nested Class Summary
Nested Classes Modifier and Type Interface 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 aOperatorCoordinator.Context
to pass to the OperatorCoordinator.static interface
OperatorCoordinator.SubtaskGateway
TheSubtaskGateway
is the way to interact with a specific parallel instance of the Operator (an Operator subtask), like sending events to the operator.
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Field Summary
Fields Modifier and Type Field 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.
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Method Summary
All Methods Instance Methods Abstract Methods Default Methods Modifier and Type Method 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.
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Field Detail
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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
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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
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Method Detail
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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.
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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 interfaceAutoCloseable
- Throws:
Exception
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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.
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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 givenCompletableFuture
must be completed exceptionally. In any case, the givenCompletableFuture
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.
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notifyCheckpointComplete
void notifyCheckpointComplete(long checkpointId)
We override the method here to remove the checked exception. Please check the Java docs ofCheckpointListener.notifyCheckpointComplete(long)
for more detail semantic of the method.- Specified by:
notifyCheckpointComplete
in interfaceCheckpointListener
- Parameters:
checkpointId
- The ID of the checkpoint that has been completed.
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notifyCheckpointAborted
default void notifyCheckpointAborted(long checkpointId)
We override the method here to remove the checked exception. Please check the Java docs ofCheckpointListener.notifyCheckpointAborted(long)
for more detail semantic of the method.- Specified by:
notifyCheckpointAborted
in interfaceCheckpointListener
- Parameters:
checkpointId
- The ID of the checkpoint that has been aborted.
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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 thenotifyCheckpointComplete(long)
method).- Throws:
Exception
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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.
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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.
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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 givenSubtaskGateway
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.
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supportsBatchSnapshot
default boolean supportsBatchSnapshot()
Whether the operator coordinator supports taking snapshot in no-checkpoint/batch scenarios. If it returns true, thecheckpointCoordinator(long, java.util.concurrent.CompletableFuture<byte[]>)
andresetToCheckpoint(long, byte[])
methods supports taking snapshot and restoring from a snapshot in batch processing scenarios. In such scenarios, the checkpointId will always be -1.
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