[FLINK-18428][API/DataStream] Rename StreamExecutionEnvironment#continuousSource() to StreamExecutionEnvironment#source().

[becketqin]

What is the purpose of the change

Rename StreamExecutionEnvironment#continuousSource() to StreamExecutionEnvironment#source() according to the original FLIP-27 design.

Brief change log

Rename StreamExecutionEnvironment#continuousSource() to StreamExecutionEnvironment#source()

Verifying this change

This is a method rename.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (docs / JavaDocs)

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[twalthr]

Why don't we call this method fromSource? We have fromElements, fromCollection, fromParallelCollection. It is confusing that we also have addSource, readFile, and createInput. Now we add another flavor that is just called source.

[dawidwys]

+1 from my side

Personally I am fine with either fromSource/addSource/source.

[becketqin]

@twalthr I think the clearest naming is either StreamExecutionEnvironment#dataStreamFromSource() or DataStream#fromSource(). But given the current status, I agree that StreamExecutionEnvironment#fromSource() is better than just source(). At least this would give a consistent style after we deprecate readFile(), addSource() and createInput(). I'll update the patch.

[becketqin]

@twalthr @dawidwys Thanks for the review. I'll merge the patch after the CI tests pass.

[StephanEwen]

+1 from my side

[StephanEwen]

Merging this as part of the current batch of fixes...