[api-draft][connector] Add simplified connector api

[ashulin]

Purpose of this pull request

Add simplified connector api

Check list

• Code changed are covered with tests, or it does not need tests for reason:
• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/incubator-seatunnel/tree/dev/docs

[EricJoy2048]

+1
CC @Hisoka-X @CalvinKirs

[Hisoka-X]

Hi, please resolve conflict

[lhyundeadsoul]

Why doesn't org.apache.seatunnel.connectors.seatunnel.common.source.SingleSplitReaderContext extend SourceReader.Context rather than composition?

[ashulin]

Using composition prevents developers from using the SourceReader.Context#sendSplitRequest and SourceReader.Context#sendSourceEventToCoordinator methods. If we use implements, we need to throw exceptions in this methods.

[lhyundeadsoul]

It's a little unintelligible that SingleSplitReaderContext is NOT a subclass of SourceReader.Context , How about following alternatives:

1. SingleSplitReaderContext implements SourceReader.Context, and remove sendSplitRequest sendSourceEventToCoordinator from SourceReader.Context if possible. I think these method may not belong Context Object.
2. Use implements, and throw exceptions. That also makes sense.

[ashulin]

The instantiation of SourceReader.Context is hidden, so although SingleSplitReaderContext can implements SourceReader.Context, it must be combined with the internal SourceReader.Context.

[lhyundeadsoul]

Sorry, I still don't understand why ParallelReaderContext can implement SourceReader.Context but SingleSplitReaderContext can not?

[ashulin]

ParallelReaderContext belongs to the translation module, and the translation module can define how to translate the api to the source of the engine (that is, how the reader and the enumerator interact is determined by the engine, etc.);
SingleSplitReaderContext belongs to the connector module, and it cannot define how the Context actually works.

[lhyundeadsoul]

ParallelReaderContext belongs to the translation module, and the translation module can define how to translate the api to the source of the engine (that is, how the reader and the enumerator interact is determined by the engine, etc.);

SingleSplitReaderContext belongs to the connector module, and it cannot define how the Context actually works.

Thx for your explanation. I can better understand translation module

[lhyundeadsoul]

xxxAware is a better class name when there is only onesetXXXmethod.
Refer to spring framewok org.springframework.web.context.ServletConfigAware ``org.springframework.context.ApplicationContextAware

[ashulin]

This's good idea! Can you contribute to it?

[lhyundeadsoul]

No problem