[Task]: Spark runner flatMap output should not be required to fit in the memory

[JozoVilcek]

What needs to happen?

Currently on Spark runner, if single processElement call produces multiple output elements, they all needs to fit in the memory [1]. This is problematic e.g. for ParquetIO, which instead of Source<> based reads uses DoFn and let reader from inside DoFn push all elements to the output. Similar happens with JdbcIO and was discussed here [2].

The goal is to overcome this constraint and allow to produce large output from DoFn on Spark runner.

[1] https://github.com/apache/beam/blob/v2.39.0/runners/spark/src/main/java/org/apache/beam/runners/spark/translation/SparkProcessContext.java#L125

[2] https://www.mail-archive.com/dev@beam.apache.org/msg16806.html

Issue Priority

Priority: 2

Issue Component

Component: runner-spark

[JozoVilcek]

.take-issue

[JozoVilcek]

e-mail thread for collecting feedback on initial WIP implementation
https://www.mail-archive.com/dev@beam.apache.org/msg27521.html

[JozoVilcek]

This was fixed for SparkRunner by adding and option to enable it via experiment. @mosche I wonder if it make sense to make necessary changes also for structured streaming or portable runner. What do you think?

[mosche]

@JozoVilcek It looks like SDFs on portable pipelines are expanded using a different mechanisms. Though, I haven't ever looked deeply into it to be honest.
In any case it makes sense to open a similar issue for the structured streaming runner 👍