[BEAM-9917] Document BigQuery file load behavior with dynamic destinations

[Vaishnavi-Pandey12]

This PR adds a small documentation note clarifying the current behavior of
file load-based BigQuery writes when used with dynamic destinations and a
non-zero triggering_frequency.

Issue: BEAM-9917

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[gemini-code-assist]

Summary of Changes

Hello @Vaishnavi-Pandey12, 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 enhances the Apache Beam documentation by adding a crucial clarification regarding the behavior of BigQuery file load-based writes. It highlights a known limitation where using dynamic destinations in conjunction with a non-zero triggering_frequency can result in temporary tables being repeatedly created and loads not being finalized into their intended destination tables. This update aims to provide users with better guidance and awareness of this specific operational caveat within the BigQuery IO connector.

Highlights

• Documentation Update: Added a critical note to the BigQuery IO documentation clarifying a known limitation related to file load-based BigQuery writes.
• BigQuery Write Behavior: The note specifically addresses the scenario where dynamic destinations are used with a non-zero triggering_frequency, leading to repeated temporary table creation and unfinalized loads (BEAM-9917).

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[mohamedawnallah]

It looks like the goal of this issue (#20242) is to resolve the root cause not documenting the issue/bug?

[Vaishnavi-Pandey12]

Thanks for the clarification — that makes sense.

I added the documentation note to clarify the current behavior while
investigating the issue, but I understand that the primary goal is to
address the root cause.

As a next step, I can add a failing unit test that captures the current
behavior with dynamic destinations and triggering_frequency, or help
with investigation into the finalization logic. Please let me know which
direction would be preferred.

[mohamedawnallah]

If we can reproduce this issue locally, we are halfway towards the resolution. A reproducibility experiment can be something along those lines:

• We can see where BigQueryBatchFileLoads is located in the codebase (using keyword-based search in the IDE)
• Once we know where it is located, we can see how it has been tested e.g with single table/multiple tables
• If there are tests for multiple tables, we can see if there are ongoing residual temporary tables (as mentioned in the issue)
• If there are no tests for multiple tables or not feasible to be integration-tested, we can test it with a free tier version of our real GCP

Once we can reproduce that issue, we can see the relevant codepaths and start to tweak them intentionally with relevant tests so that issue doesn't happen again as a regression

[mohamedawnallah]

As a next step, I can add a failing unit test that captures the current
behavior with dynamic destinations and triggering_frequency, or help
with investigation into the finalization logic. Please let me know which
direction would be preferred.

It would be great if we could have a reproducible test that captures the bug first, then we can iterate on the solution