infoverload commented on code in PR #517: URL: https://github.com/apache/flink-web/pull/517#discussion_r861495533
########## _posts/2022-03-16-async-sink-base.md: ########## @@ -0,0 +1,160 @@ +--- +layout: post +title: "The Generic Asynchronous Base Sink" +date: 2022-04-05 16:00:00 +authors: +- CrynetLogistics: + name: "Zichen Liu" +excerpt: An overview of the new AsyncBaseSink and how to use it for building your own concrete sink +--- + +Flink sinks share a lot of similar behavior. All sinks batch records according to user defined buffering hints, sign requests, write them to the destination, retry unsuccessful or throttled requests, and participate in checkpointing. + +This is why for [Flink 1.15](https://cwiki.apache.org/confluence/display/FLINK/FLIP-171%3A+Async+Sink) we have decided to create the `AsyncSinkBase`, an abstract sink with a number of common functionalities extracted. Adding support for a new destination now only requires a lightweight shim that implements the specific interfaces of the destination using a client that supports async requests. + +This common abstraction will reduce the effort required to maintain individual sinks that extend from this abstract sink, with bugfixes and improvements to the sink core benefiting all implementations that extend it. The design of AsyncSyncBase focuses on extensibility and a broad support of destinations. The core of the sink is kept generic and free of any connector-specific dependencies. + +The sink base is designed to participate in checkpointing to provide at-least-once semantics and can work directly with destinations that provide a client that supports asynchronous requests. Alternatively, concrete sink implementers may manage their own thread pool with a synchronous client. + +{% toc %} + +# Adding the base sink as a dependency +In order to use the base sink, you will need to add the following dependency to your project. The example below follows the Maven syntax: + +```xml +<dependency> + <groupId>org.apache.flink</groupId> + <artifactId>flink-connector-base</artifactId> + <version>${flink.version}</version> +</dependency> +``` + + + +# The Public Interfaces of AsyncSinkBase + +## Generic Types + +`<InputT>` – type of elements in a DataStream that should be passed to the sink + +`<RequestEntryT>` – type of a payload containing the element and additional metadata that is required to submit a single element to the destination + + +## Element Converter Interface +[ElementConverter](https://github.com/apache/flink/blob/master/flink-connectors/flink-connector-base/src/main/java/org/apache/flink/connector/base/sink/writer/ElementConverter.java) + +```java +public interface ElementConverter<InputT, RequestEntryT> extends Serializable { + RequestEntryT apply(InputT element, SinkWriter.Context context); +} +``` +The concrete sink implementation should provide a way to convert from an element in the DataStream to the payload type that contains all the additional metadata required to submit that element to the destination by the sink. Ideally, this would be hidden from the end user since it allows concrete sink implementers to adapt to changes in the destination API without breaking end user code. + +## Sink Writer Interface +[AsyncSinkWriter](https://github.com/apache/flink/blob/master/flink-connectors/flink-connector-base/src/main/java/org/apache/flink/connector/base/sink/writer/AsyncSinkWriter.java) + +```java +public abstract class AsyncSinkWriter<InputT, RequestEntryT extends Serializable> + implements StatefulSink.StatefulSinkWriter<InputT, BufferedRequestState<RequestEntryT>> { + // ... + protected abstract void submitRequestEntries( + List<RequestEntryT> requestEntries, Consumer<List<RequestEntryT>> requestResult); + // ... +} +``` + +In this method, sink implementers should use the destination clients to submit `requestEntries` asynchronously to be written. + +Should any elements fail to be persisted, they should be requeued back in the buffer for retry using `requestResult.accept(...list of failed entries...)`. However, retrying any element that is known to be faulty and consistently failing, will result in that element being requeued forever, therefore a sensible strategy for determining what should be retried is highly recommended. + +If at any point, it is determined that a fatal error has occurred and that we should throw a runtime exception from the sink, we can call `getFatalExceptionCons().accept(...);` from anywhere in the concrete sink writer. + +```java +public abstract class AsyncSinkWriter<InputT, RequestEntryT extends Serializable> + implements StatefulSink.StatefulSinkWriter<InputT, BufferedRequestState<RequestEntryT>> { + // ... + protected abstract long getSizeInBytes(RequestEntryT requestEntry); + // ... +} +``` +The async sink has a concept of size of elements in the buffer. This allows users to specify a byte size threshold beyond which elements will be flushed. However the sink implementer is best positioned to determine what is the most sensible measure of size for each `RequestEntryT` is. + +```java +public abstract class AsyncSinkWriter<InputT, RequestEntryT extends Serializable> + implements StatefulSink.StatefulSinkWriter<InputT, BufferedRequestState<RequestEntryT>> { + // ... + public AsyncSinkWriter( + ElementConverter<InputT, RequestEntryT> elementConverter, + Sink.InitContext context, + int maxBatchSize, + int maxInFlightRequests, + int maxBufferedRequests, + long flushOnBufferSizeInBytes, + long maxTimeInBufferMS, + long maxRecordSizeInBytes) { /* ... */ } + // ... +} +``` + +By default, the method `snapshotState` returns all the elements in the buffer to be saved down for snapshots. Any elements that have been removed from the buffer for writing and are in flight have already been completed due to the sink operator calling `flush(true)` on the sink writer. +You may want to save down additional state from the concrete sink and you can achieve this by overriding `snapshotState`, and restoring in the constructor. + +```java +class MySinkWriter<InputT> extends AsyncSinkWriter<InputT, RequestEntryT> { + + MySinkWriter( + // ... + Collection<BufferedRequestState<Record>> initialStates) { + super( + // ... + initialStates); + // restore concrete sink state from initialStates + } + + @Override + public List<BufferedRequestState<RequestEntryT>> snapshotState(long checkpointId) { + super.snapshotState(checkpointId); + // ... + } + +} +``` + +## Sink Interface + +[AsyncSinkBase](https://github.com/apache/flink/blob/master/flink-connectors/flink-connector-base/src/main/java/org/apache/flink/connector/base/sink/AsyncSinkBase.java) + +```java +class MySink<InputT> extends AsyncSinkBase<InputT, RequestEntryT> { + // ... + @Override + public StatefulSinkWriter<InputT, BufferedRequestState<RequestEntryT>> createWriter(InitContext context) { + return new MySinkWriter(context); + } + // ... +} +``` +Sink implementers extending this will need to return their own extension of the `AsyncSinkWriter` from `createWriter()` inside their own implementation of `AsyncSinkBase`. + +At the time of writing, the [Kinesis Data Streams sink](https://github.com/apache/flink/tree/master/flink-connectors/flink-connector-aws-kinesis-streams) and [Kinesis Data Firehose sink](https://github.com/apache/flink/tree/master/flink-connectors/flink-connector-aws-kinesis-firehose) are using this base sink. + +# Metrics + +There are three metrics that automatically exist when you implement sinks (and, thus, should not be implemented by yourself). + +* CurrentSendTime Gauge - returns the amount of time in milliseconds it took for the most recent request to write records to return, whether successful or not. +* NumBytesOut Counter - counts the total number of bytes the sink has tried to write to the destination, using the method `getSizeInBytes` to determine the size of each record. This will double count failures that may need to be retried. +* NumRecordsOut Counter - similar to above, this counts the total number of records the sink has tried to write to the destination. This will double count failures that may need to be retried. + +# Sink Behaviour Review Comment: ```suggestion # Sink Behavior ``` -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: issues-unsubscr...@flink.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
