[incubator-seatunnel] branch api-draft updated: add new api doc English version (#2050)

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The following commit(s) were added to refs/heads/api-draft by this push:
     new f35fdc8d5 add new api doc English version (#2050)
f35fdc8d5 is described below

commit f35fdc8d52dcf795ff9256920ef6cdcbd87b29f1
Author: Hisoka <fa...@qq.com>
AuthorDate: Thu Jun 23 10:25:51 2022 +0800

    add new api doc English version (#2050)
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+# 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 with quick overview regarding API, translation layer improvement, and development of new Connector.
+
+## **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. All the relevant code at this stage is kept on the ``api-draft`` branch.
+
+### **Example**
+We have prepared a new version of the locally executable example program in ``seatunnel-examples``, which can be directly called using ``seatunnel-flink-new-connector-example`` or ``seatunnel-spark-new-connector-example`` in ``SeaTunnelApiExample``. This is also the debugging method that is often used in the local development of Connector. The corresponding configuration files are saved in the same module ``resources/examples`` folder as before.
+
+
+### **Startup Class**
+Aside from the old startup class, we have created two new startup class projects, 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, th [...]
+
+        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 pro [...]
+
+#### **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/seatunnel-connectors-seatuunelmodule``, and the examples that can be referred to at this stage are under this module.
+
+