Pubsub on directrunner: direct_runner.py and transform_evaluator.py

[Udi Meiri]

Hi,
I'm having trouble understanding why there's an extra level of indirection
when doing pubsub reads via directrunner vs writes.

For reads, we have these translations:
beam_pubsub.ReadFromPubSub ->
direct_runner._DirectReadFromPubSub ->
transform_evaluator._PubSubReadEvaluator

For writes, this is abbreviated:
beam_pubsub.WriteStringsToPubSub ->
_DirectWriteToPubSub

What is the role of transform_evaluator._TransformEvaluator?
Why do we need it for reads and not for writes?

smime.p7s
Description: S/MIME Cryptographic Signature