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Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
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Motivation
As discussed in FLIP-131, Flink will deprecate the DataSet API in favor of DataStream API and Table API. Users should be able to use DataStream API to write jobs that support both bounded and unbounded execution modes. However Flink does not provide a sink API to guarantee the exactly once semantics in both bounded and unbounded scenarios, which blocks the unification.
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The document includes three parts: The first part describes the semantics the unified sink API should support. According to the first part the second part proposes a transactional new unified sink API. In the last part we introduce two open questions related to the API.
Semantics
This section tries to figure out the sink developer would be responsible for which part and Flink would be responsible for which part in terms of ensuring consistency. After this we could know what semantics the sink API should have to support the exact once semantics.
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In summary the sink API should be responsible for producing What to commit & providing How to commit. Flink should be responsible for guaranteeing the exact semantics. Flink could “optimize” When & Where to commit according to the execution mode and these optimizations should be transparent to the sink developer.
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Sink API
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/** * This interface lets the sink developer to build a simple transactional sink topology pattern, which satisfiescould guarantee the HDFS/S3/Iceberg sink.exactly once * This sink topology includes one {@link Writer} + onesemantics in both batch and stream execution mode if there is a {@link Committer} + oneor {@link GlobalCommitter}. * 1. The {@link WriterSinkWriter} is responsible for producing the committable. * 2. The {@link Committer} is responsible for committing a single committable. * 3. The {@link GlobalCommitter} is responsible for committing an aggregated committable, which we calledcall the global committable. * And the parallelism ofcommittable. theThe {@link GlobalCommitter} is always executed with a parallelism of 1. * BothNote: the Developers need to ensure the idempotence of {@link Committer} and the {@link GlobalCommitter} are optional.. * * @param <InputT> The type of the sink's input * @param <CommT> The type of the committable datainformation needed to commit data staged by the sink * @param <GlobalCommT><WriterStateT> The type of the sink aggregatedwriter's committablestate * @param <GlobalCommT> <WriterStateT> The type of the writer'saggregated statecommittable */ public interface TransactionalSink<InputTSink<InputT, CommT, GlobalCommTWriterStateT, WriterStateT> {GlobalCommT> extends Serializable { /** * Create a {@link WriterSinkWriter}. * * @param context the runtime context. * @param states the previous writerswriter's state. * * @return A sink writer. * * @throws IOException if fail to create a writer. */ Writer<InputTSinkWriter<InputT, CommT, WriterStateT> createWriter( InitContext context, List<WriterStateT> states) throws IOException; /** * @returnCreates a {@link Committer}. */ Optional<Committer<CommT>> createCommitter() * @return A committer. * * @throws IOException if fail to create a committer. */ Optional<Committer<CommT>> createCommitter() throws IOException; /** * @returnCreates a {@link GlobalCommitter}. */ Optional<GlobalCommitter<CommT, GlobalCommT>> createGlobalCommitter(); Optional<SimpleVersionedSerializer<CommT>> getCommittableSerializer(); Optional<SimpleVersionedSerializer<GlobalCommT>> getGlobalCommittableSerializer(); Optional<SimpleVersionedSerializer<WriterStateT>> getWriterStateSerializer(); interface InitContext { int getSubtaskId(); MetricGroup metricGroup(); } } | ||||||||||
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/** * The interface is responsible for writing data and handling any potential tmp area used to write yet un-staged data, e.g. in-progress files. * As soon as some data is ready to commit, they (or metadata pointing to where the actual data is staged) are shipped to an operator who knows when to commit them. * * @param <InputT> The type of the writer's input * @param <CommT> The type of the committable data * @param <WriterStateT> The type of the writer's state */ interface Writer<InputT, CommT, WriterStateT> { * @return A global committer. * * @throws IOException if fail to create a global committer. */ Optional<GlobalCommitter<CommT, GlobalCommT>> createGlobalCommitter() throws IOException; /** * Returns the serializer of the committable type. */ Optional<SimpleVersionedSerializer<CommT>> getCommittableSerializer(); /** * Returns the serializer of the aggregated committable type. */ Optional<SimpleVersionedSerializer<GlobalCommT>> getGlobalCommittableSerializer(); /** * Return the serializer of the writer's state type. */ Optional<SimpleVersionedSerializer<WriterStateT>> getWriterStateSerializer(); /** * The interface exposes some runtime info for creating a {@link SinkWriter}. */ interface InitContext { /** * Returns a {@link ProcessingTimeService} that can be used to * get the current time and register timers. */ ProcessingTimeService getProcessingTimeService(); /** * @return The id of task where the writer is. */ int getSubtaskId(); /** * @return The metric group this writer belongs to. */ MetricGroup metricGroup(); } /** * AddA service anthat elementallows to get the writer. * @param element The input recordcurrent processing time and register timers that * @paramwill ctxexecute Thethe additionalgiven information{@link aboutProcessingTimeCallback} the input recordwhen firing. */ interface @param output The committable data could be shipped to the committer by this ProcessingTimeService { /** Returns the current processing time. */ voidlong write(InputT element, Context ctx, WriterOutput<CommT> outputgetCurrentProcessingTime(); /** * Prepare for a commit. * @param flush whether flushing the un-staged committable or not Invokes the given callback at the given timestamp. * * @param time Time when the callback is invoked at * @param outputprocessingTimerCallback The committablecallback data couldto be shipped to the committer by thisinvoked. */ void prepareCommitregisterProcessingTimer(booleanlong flushtime, WriterOutput<CommT>ProcessingTimeCallback outputprocessingTimerCallback); /** * A @returncallback thethat writer's statecan be registered via {@link #registerProcessingTimer(long, * ProcessingTimeCallback)}. */ List<WriterStateT> snapshotState(); interface ContextProcessingTimeCallback { long currentProcessingTime(); long currentWatermark(); Long timestamp(); } } | ||||||||||
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/** * This interface knows how to commit the data to the external system. * * @param <CommT> The type of the committable data. */ public interface Committer<CommT> { void commit(CommT committable) throws Exception; /** * This method is invoked with the time which the callback register for. * * @param time The time this callback was registered for. */ void onProcessingTime(long time) throws IOException; } } } |
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/** * The {@link@code GlobalCommitterSinkWriter} is responsible for committingwriting andata aggregatedand committable,handling whichany wepotential calledtmp globalarea committables. *used to write yet un-staged * @param <CommT> data, e.g. in-progress files. The typedata of(or themetadata committablepointing data to *where @paramthe <GlobalCommT>actual data Theis typestaged) ofready theto aggregatedcommit committableis */ interface GlobalCommitter<CommT, GlobalCommT> { /** * This method is called when restoring from a failover. * @param globalCommittables the global committable that are not committed in the previous session. * @return the global committables that should be committed again in the current session. */ List<GlobalCommT> filterRecoveredCommittables(List<GlobalCommT> globalCommittables); /** * Compute an aggregated committable from a collection of committables. * @param committables a collection of committables that are needed to combine * @return an aggregated committable */ GlobalCommT combine(List<CommT> committables); CommitResult commit(GlobalCommT globalCommittable); /** * There is no committable any more returned to the system by the {@link #prepareCommit(boolean)}. * * @param <InputT> The type of the sink writer's input * @param <CommT> The type of information needed to commit data staged by the sink * @param <WriterStateT> The type of the writer's state */ public interface SinkWriter<InputT, CommT, WriterStateT> extends AutoCloseable { /** * Add an element to the writer. * * @param element The input record * @param context The additional information about the input record * * @throws IOException if fail to add an element. */ void endOfInput()write(InputT element, Context context) throws IOException; enum CommitResult {/** SUCCESS, FAILURE, RETRY } } * Prepare for a commit. * * <p>This will be called before we checkpoint the Writer's state in Streaming execution mode. * * @param flush Whether flushing the un-staged data or not * @return The data is ready to commit. * * @throws IOException if fail to prepare for a commit. */ List<CommT> prepareCommit(boolean flush) throws IOException; /** * @return The writer's state. * * @throws IOException if fail to snapshot writer's state. */ List<WriterStateT> snapshotState() throws IOException; /** * Context that {@link #write} can use for getting additional data about an input record. */ interface Context { /** * Returns the current event-time watermark. */ long currentWatermark(); /** * Returns the timestamp of the current input record or {@code null} if the element does not * have an assigned timestamp. */ Long timestamp(); } } |
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/**
* The {@code Committer} is responsible for committing the data staged by the sink.
*
* @param <CommT> The type of information needed to commit the staged data
*/
public interface Committer<CommT> extends AutoCloseable {
/**
* Commit the given list of {@link CommT}.
* @param committables A list of information needed to commit data staged by the sink.
* @return A list of {@link CommT} needed to re-commit, which is needed in case we implement a "commit-with-retry" pattern.
* @throws IOException if the commit operation fail and do not want to retry any more.
*/
List<CommT> commit(List<CommT> committables) throws IOException;
} |
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/**
* The {@code GlobalCommitter} is responsible for creating and committing an aggregated committable,
* which we call global committable (see {@link #combine}).
*
* <p>The {@code GlobalCommitter} runs with parallelism equal to 1.
*
* @param <CommT> The type of information needed to commit data staged by the sink
* @param <GlobalCommT> The type of the aggregated committable
*/
public interface GlobalCommitter<CommT, GlobalCommT> extends AutoCloseable {
/**
* Find out which global committables need to be retried when recovering from the failure.
* @param globalCommittables A list of {@link GlobalCommT} for which we want to verify
* which ones were successfully committed and which ones did not.
*
* @return A list of {@link GlobalCommT} that should be committed again.
*
* @throws IOException if fail to filter the recovered committables.
*/
List<GlobalCommT> filterRecoveredCommittables(List<GlobalCommT> globalCommittables) throws IOException;
/**
* Compute an aggregated committable from a list of committables.
* @param committables A list of {@link CommT} to be combined into a {@link GlobalCommT}.
*
* @return an aggregated committable
*
* @throws IOException if fail to combine the given committables.
*/
GlobalCommT combine(List<CommT> committables) throws IOException;
/**
* Commit the given list of {@link GlobalCommT}.
*
* @param globalCommittables a list of {@link GlobalCommT}.
*
* @return A list of {@link GlobalCommT} needed to re-commit, which is needed in case we implement a "commit-with-retry" pattern.
*
* @throws IOException if the commit operation fail and do not want to retry any more.
*/
List<GlobalCommT> commit(List<GlobalCommT> globalCommittables) throws IOException; | ||||||||||
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/** * The {@link Writer} uses this interface to send the committable data to the operator who knows when to commit to the external system. * * @param <CommT> The type of the committable data. */ public interface WriterOutput<CommT> { /** * SendSignals thethat committablethere datais tono thecommittable operator who knows when to commitany more. * * @param@throws committableIOException Theif datafail thatto ishandle readythis for committingnotification. */ void sendToCommitendOfInput(CommT committable) throws IOException; } |
Open Questions
There are still two open questions related to the unified sink API
How does the sink API support to write to the Hive?
In general HiveSink needs three steps before committing the data to the Hive:
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From the discussion we will not support the HiveSink in the first version.
Is the sink an operator or a topology?
The scenario could be more complicated. For example, some users want to merge the files in one bucket before committing to the HMS. Where to place this logic? Do we want to put all these logic into one operator or a sink topology?
From the discussion in the long run we should give the sink developer the ability of building “arbitrary” topologies. But for Flink-1.12 we should be more focused on only satisfying the S3/HDFS/Iceberg sink.
Compatibility, Deprecation, and Migration Plan
- We does not change the current streaming and batch style sink API. So there is no compatibility issue to the existing sink implementation.
- In the long run we might need to deprecate the old streaming and batch style sink API.
- At first we plan to migrate the StreamingFileSink to this new api.
Rejected Alternatives
The difference between the rejected version and accepted version is how to expose the state to the user. The accepted version could give the framework a greater opportunity to optimize state handling.
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