Drain - model part + windowedValue changes

[stankiewicz]

Stacked on top of #36523

1. Adds proto definition for drain
2. Extends WindowedValue interface, plumbs default implementation where needed

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[gemini-code-assist]

Summary of Changes

Hello @stankiewicz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces foundational changes to support a 'drain' mechanism within Apache Beam's data processing model. It extends the WindowedValue interface and updates the protocol buffer definitions to include a draining state for elements. These modifications are crucial for enabling more robust and controlled shutdown procedures in streaming pipelines, allowing elements to be gracefully processed even as a job is winding down. The changes involve updating core data structures and simplifying DoFn output contexts by consolidating methods and leveraging a builder pattern for element construction.

Highlights

• Drain Protocol Buffer Definition: A new DrainMode enum and an optional drain field have been added to the ElementMetadata within the beam_fn_api.proto definition. This introduces a standardized way to signal the draining state of elements in the Beam Fn API.
• WindowedValue Interface Extension: The WindowedValue interface has been extended to include an isDraining() method, and the OutputBuilder now supports a setDraining() method. This change propagates the draining state through the core data model.
• Simplified DoFn Output Contexts: Multiple output and outputWindowedValue overloads in DoFn.FinishBundleContext and DoFn.ProcessContext that previously accepted currentRecordId and currentRecordOffset parameters have been removed. This streamlines the API by consolidating output methods and leveraging the OutputBuilder for detailed element construction.
• Draining State Plumbing: The new draining state has been plumbed through various core Java runners (Dataflow, Spark), SDK components (WindowedValues, DoFnTester), and the Fn Harness. This ensures that the draining information is consistently available and propagated across the Beam ecosystem.

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[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[stankiewicz]

:sdks:java:io:mqtt:test is flaky
python tests are flaky

[github-actions]

Assigning reviewers:

R: @ahmedabu98 for label java.

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[kennknowles]

R: @kennknowles

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

[kennknowles]

Nit: this might be named causedByDrain or firedDueToDrain or some way of saying "element exists because of drain" or "element impacted by drain". I don't have a good idea, really. But I couldn't help commenting, because technically this is a piece of metadata about the element ("might be incomplete because fired while draining"), not about the state of the world ("is draining").

But I'm going to approve and merge anyhow, because it isn't worth kicking all our flaky tests just to rename, unless we have a much better name.

[stankiewicz]

I can rename it.. :)