Purpose

This article introduces the new interface and the new code structure on account of the newly designed API for Connectors in Apache SeaTunnel. This helps developers quickly understand API and transformation layer improvements. On the other hand, it can guide contributors how to use the new API to develop new connectors.See this issue for details.

Code Structure

In order to separate from the old code, we have defined new modules for execution flow. This facilitates parallel development at the current stage, and reduces the difficulty of merging.

Example

We have prepared two new version of the locally executable example program in seatunnel-examples,one is seatunnel-examples/seatunnel-flink-connector-v2-example/src/main/java/org/apache/seatunnel/example/flink/v2/SeaTunnelApiExample.java , it runs in the Flink engine. Another one is seatunnel-examples/seatunnel-spark-connector-v2-example/src/main/java/org/apache/seatunnel/example/spark/v2/SeaTunnelApiExample.java , it runs in the Spark engine. This is also the debugging method that is often used in the local development of Connector. You can debug these examples, which will help you better understand the running logic of the program. The configuration files used in example are saved in the "resources/examples" folder. If you want to add examples for your own connectors, you need to follow the steps below.

1. Add the groupId, artifactId and version of the connector to be tested to seatunnel-examples/seatunnel-flink-connector-v2-example/pom.xml(or add it to seatunnel-examples/seatunnel-spark-connector-v2-example/pom.xml when you want to runs it in Spark engine) as a dependency.
2. Find the dependency in your connector pom file which scope is test or provided and then add them to seatunnel-examples/seatunnel-flink-connector-v2-example/pom.xml(or add it to seatunnel-examples/seatunnel-spark-connector-v2-example/pom.xml) file and modify the scope to compile.
3. Refer to the SeaTunnelApiExample class to develop your sample code.

Startup Class

Aside from the old startup class, we have created two new startup modules, namely seatunnel-core/seatunnel-flink-starter and seatunnel-core/seatunnel-spark-starter. You can find out how to parse the configuration file into an executable Flink/Spark process here.

SeaTunnel API

A new seatunnel-api (not seatunnel-apis) module has been created to store the new interfaces defined by the SeaTunnel API. By implementing these interfaces, developers can complete the SeaTunnel Connector that supports multiple engines.

Translation Layer

We realize the conversion between SeaTunnel API and Engine API by adapting the interfaces of different engines, so as to achieve the effect of translation, and let our SeaTunnel Connector support the operation of multiple different engines. The corresponding code address, seatunnel-translation, this module has the corresponding translation layer implementation. If you are interested, you can view the code and help us improve the current code.

API introduction

The API design of the current version of SeaTunnel draws on the design concept of Flink.

Source

SeaTunnelSource.java

• The Source of SeaTunnel adopts the design of stream-batch integration, getBoundedness which determines whether the current Source is a stream Source or a batch Source, so you can specify a Source by dynamic configuration (refer to the default method), which can be either a stream or a batch.
• getRowTypeInfo To get the schema of the data, the connector can choose to hard-code to implement a fixed schema, or run the user to customize the schema through config configuration. The latter is recommended.
• SeaTunnelSource is a class executed on the driver side, through which objects such as SourceReader, SplitEnumerator and serializers are obtained.
• Currently, the data type supported by SeaTunnelSource must be SeaTunnelRow.

SourceSplitEnumerator.java

Use this enumerator to get the data read shard (SourceSplit) situation, different shards may be assigned to different SourceReaders to read data. Contains several key methods:

• run: Used to perform a spawn SourceSplit and call SourceSplitEnumerator.Context.assignSplit: to distribute the shards to the SourceReader.
• addSplitsBackSourceSplitEnumerator: is required to redistribute these Splits when SourceSplit cannot be processed normally or restarted due to the exception of SourceReader.
• registerReaderProcess: some SourceReaders that are registered after the run is run. If there is no SourceSplit distributed at this time, it can be distributed to these new readers (yes, you need to maintain your SourceSplit distribution in SourceSplitEnumerator most of the time).
• handleSplitRequest: If some Readers actively request SourceSplit from SourceSplitEnumerator, this method can be called SourceSplitEnumerator.Context.assignSplit to sends shards to the corresponding Reader.
• snapshotState: It is used for stream processing to periodically return the current state that needs to be saved. If there is a state restoration, it will be called SeaTunnelSource.restoreEnumerator to constructs a SourceSplitEnumerator and restore the saved state to the SourceSplitEnumerator.
• notifyCheckpointComplete: It is used for subsequent processing after the state is successfully saved, and can be used to store the state or mark in third-party storage.

SourceSplit.java

The interface used to save shards. Different shards need to define different splitIds. You can implement this interface to save the data that shards need to save, such as kafka's partition and topic, hbase's columnfamily and other information, which are used by SourceReader to determine Which part of the total data should be read.

SourceReader.java

The interface that directly interacts with the data source, and the action of reading data from the data source is completed by implementing this interface.

• pollNext: It is the core of Reader. Through this interface, the process of reading the data of the data source and returning it to SeaTunnel is realized. Whenever you are ready to pass data to SeaTunnel, you can call the Collector.collect method in the parameter, which can be called an infinite number of times to complete a large amount of data reading. But the data format supported at this stage can only be SeaTunnelRow. Because our Source is a stream-batch integration, the Connector has to decide when to end data reading in batch mode. For example, a batch reads 100 pieces of data at a time. After the reading is completed, it needs pollNext to call in to SourceReader.Context.signalNoMoreElementnotify SeaTunnel that there is no data to read . , then you can use these 100 pieces of data for batch processing. Stream processing does not have this requirement, so most SourceReaders with integrated stream batches will have the following code:

if(Boundedness.BOUNDED.equals(context.getBoundedness())){
    // signal to the source that we have reached the end of the data.
    context.signalNoMoreElement();
    break;
    }

It means that SeaTunnel will be notified only in batch mode.

• addSplits: Used by the framework to assign SourceSplit to different SourceReaders, SourceReader should save the obtained shards, and then pollNextread the corresponding shard data in it, but there may be times when the Reader does not read shards (maybe SourceSplit has not been generated or The current Reader is indeed not allocated), at this time, pollNextcorresponding processing should be made, such as continuing to wait.
• handleNoMoreSplits: When triggered, it indicates that there are no more shards, and the Connector Source is required to optionally make corresponding feedback
• snapshotStateIt: is used for stream processing to periodically return the current state that needs to be saved, that is, the fragmentation information (SeaTunnel saves the fragmentation information and state together to achieve dynamic allocation).
• notifyCheckpointComplete: Like notifyCheckpointAborted the name, it is a callback for different states of checkpoint.

Sink

SeaTunnelSink.java

It is used to define the way to write data to the destination, and obtain instances such as SinkWriter and SinkCommitter through this interface. An important feature of the sink side is the processing of distributed transactions. SeaTunnel defines two different Committers: SinkCommitter used to process transactions for different subTasks SinkAggregatedCommitter. Process transaction results for all nodes. Different Connector Sinks can be selected according to component properties, whether to implement only SinkCommitter or SinkAggregatedCommitter, or both.

SinkWriter.java

It is used to directly interact with the output source, and provide the data obtained by SeaTunnel through the data source to the Writer for data writing.

• write: Responsible for transferring data to SinkWriter, you can choose to write it directly, or write it after buffering a certain amount of data. Currently, only the data type is supported SeaTunnelRow.
• prepareCommit: Executed before commit, you can write data directly here, or you can implement phase one in 2pc, and then implement phase two in SinkCommitter or SinkAggregatedCommitter. What this method returns is the commit information, which will be provided SinkCommitter and SinkAggregatedCommitter used for the next stage of transaction processing.

SinkCommitter.java

It is used to process SinkWriter.prepareCommit the returned data information, including transaction information that needs to be submitted.

SinkAggregatedCommitter.java

It is used to process SinkWriter.prepareCommit the returned data information, including transaction information that needs to be submitted, etc., but it will be processed together on a single node, which can avoid the problem of inconsistency of the state caused by the failure of the second part of the stage.

• combine: Used SinkWriter.prepareCommit to aggregate the returned transaction information, and then generate aggregated transaction information.

Implement SinkCommitter or SinkAggregatedCommitter?

In the current version, it is recommended to implement SinkAggregatedCommitter as the first choice, which can provide strong consistency guarantee in Flink/Spark. At the same time, commit should be idempotent, and save engine retry can work normally.

Result

All Connector implementations should be under the seatunnel-connectors-v2, and the examples that can be referred to at this stage are under this module.