Guardrails around fromTopic (pubsub) #20753
Description
Fundamentally, we should strongly discourage the use of ephemeral topics (fromTopic() in PubsubIO) in favor of user-created subscriptions. Currently, the documentation for fromTopic states this matter-of-factly, without drawing attention to the fact that this is almost certainly not what the wants:
The Beam runner will start reading data published on this topic from the time the pipeline is started. Any data published on the topic before the pipeline is started will not be read by the runner.
Suggestion: We should include an execution warning whenever fromTopic is used, as well as update the documentation.
Imported from Jira BEAM-11632. Original Jira may contain additional context.
Reported by: altay.