Data-dependent sinks

[damccorm]

Current sink API writes all data to a single destination, but there are many use cases where different pieces of data need to be routed to different destinations where the set of destinations is data-dependent (so can't be implemented with a Partition transform).

One internally discussed proposal was an API of the form:

PCollection<Void> PCollection<T>.apply(
    Write.using(DoFn<T, SinkT> where,
                MapFn<SinkT,
WriteOperation<WriteResultT, T>> how)

so an item T gets written to a destination (or multiple destinations) determined by "where"; and the writing strategy is determined by "how" that produces a WriteOperation (current API - global init/write/global finalize hooks) for any given destination.

This API also has other benefits:

• allows the SinkT to be computed dynamically (in "where"), rather than specified at pipeline construction time
• removes the necessity for a Sink class entirely
• is sequenceable w.r.t. downstream transforms (you can stick transforms onto the returned PCollection<Void>, while the current Write.to() returns a PDone)

Imported from Jira BEAM-92. Original Jira may contain additional context.
Reported by: jkff.