[Drain] Expose drain to dofn processElement and onTimer by stankiewicz · Pull Request #37825 · apache/beam

stankiewicz · 2026-03-11T13:09:14Z

exposes

@ProcessElement
      public void process(ProcessContext c, CausedByDrain causedByDrain) {}
@OnTimer(timerId)
             public void onTimer(CausedByDrain causedByDrain) {}

added tests

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gemini-code-assist · 2026-03-11T13:39:15Z

Summary of Changes

Hello, 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 Apache Beam's DoFn capabilities by exposing the CausedByDrain parameter to both @ProcessElement and @OnTimer methods. This change allows developers to write more robust data processing pipelines that can gracefully handle job draining scenarios, enabling custom logic to be executed when a pipeline is being shut down or reconfigured.

Highlights

CausedByDrain Parameter for ProcessElement: Introduced the CausedByDrain parameter to @ProcessElement methods, allowing DoFns to react to job draining events.
CausedByDrain Parameter for OnTimer: Enabled the CausedByDrain parameter for @OnTimer methods, providing timer-based DoFns with draining context.
Enhanced DoFn Signature Reflection: Updated the DoFn signature reflection mechanism to correctly identify and handle the new CausedByDrain parameter type.
Comprehensive Testing: Added comprehensive unit tests to validate the proper recognition and handling of the CausedByDrain parameter in DoFn signatures.

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Changelog

runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java
- Implemented the causedByDrain method in DoFnProcessContext, DoFnTimerContext, and DoFnContext to provide access to the CausedByDrain object.
sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/DoFnInvoker.java
- Added CausedByDrain import and introduced causedByDrain to the ArgumentProvider interface and its default/delegating implementations.
sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/DoFnSignature.java
- Extended the Parameter hierarchy with CausedByDrainParameter and updated the visitor pattern to include it.
sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/DoFnSignatures.java
- Updated allowed parameter lists for various DoFn lifecycle methods to include CausedByDrainParameter and added logic to recognize CausedByDrain as a valid parameter type.
sdks/java/core/src/main/java/org/apache/beam/sdk/util/construction/SplittableParDoNaiveBounded.java
- Updated the ProcessContext implementation to expose the causedByDrain method.
sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/reflect/DoFnSignaturesTest.java
- Added new test cases and updated existing ones to verify the correct handling and reflection of the CausedByDrain parameter in DoFn signatures for both @ProcessElement and @OnTimer methods.

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kennknowles

LGTM - of course the thing missing is tests of actually using it :-)

stankiewicz · 2026-03-11T15:04:18Z

Run Java_Examples_Dataflow PreCommit

stankiewicz · 2026-03-11T15:04:36Z

Run Java_PVR_Prism_Loopback PreCommit

stankiewicz · 2026-03-11T15:08:18Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces support for a CausedByDrain parameter in Apache Beam's DoFn methods. This involved adding causedByDrain methods to various DoFnRunner implementations (SimpleDoFnRunner, SplittableParDoNaiveBounded, FnApiDoFnRunner), extending the DoFnInvoker interface and its ByteBuddy factory, and updating DoFnSignature and DoFnSignatures to recognize and process this new parameter type. A new test case was also added for CausedByDrain on onTimer methods. The code review suggests adding Javadoc comments to the newly introduced causedByDrain methods across several files for better documentation, and an inline comment to clarify the use of isAssignableFrom in DoFnSignatures.

gemini-code-assist · 2026-03-11T15:24:27Z

+    public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) {
+      return elem.causedByDrain();
+    }


medium: This method should include a Javadoc comment explaining its purpose and behavior, similar to other methods in this class.

/** * Provide a reference to the caused by drain. */ @Override public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) { return elem.causedByDrain(); }

gemini-code-assist · 2026-03-11T15:24:27Z

+    public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) {
+      return causedByDrain;
+    }


medium: This method should include a Javadoc comment explaining its purpose and behavior, similar to other methods in this class.

/** * Provide a reference to the caused by drain. */ @Override public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) { return causedByDrain; }

gemini-code-assist · 2026-03-11T15:24:27Z

+    public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) {
+      throw new UnsupportedOperationException("CausedByDrain parameters are not supported.");
+    }


medium: This method should include a Javadoc comment explaining its purpose and behavior, similar to other methods in this class.

/** * Provide a reference to the caused by drain. */ @Override public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) { throw new UnsupportedOperationException("CausedByDrain parameters are not supported."); }

github-actions · 2026-03-11T16:23:28Z

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 · 2026-03-11T22:54:34Z

Run Java_GCP_IO_Direct PreCommit

stankiewicz · 2026-03-12T09:40:40Z

Run Java_GCP_IO_Direct PreCommit

github-actions · 2026-03-12T11:42:54Z

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R: @chamikaramj for label java.

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expose drain in dofn

2542f18

github-actions Bot added the java label Mar 11, 2026

stankiewicz force-pushed the expose_drain_to_dofn branch from 7ca2362 to 62e7ee9 Compare March 11, 2026 13:26

github-actions Bot added runners core labels Mar 11, 2026

kennknowles approved these changes Mar 11, 2026

View reviewed changes

tests, remove default implementations.

c581911

stankiewicz force-pushed the expose_drain_to_dofn branch from 62e7ee9 to c581911 Compare March 11, 2026 13:59

gemini-code-assist Bot reviewed Mar 11, 2026

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stankiewicz mentioned this pull request Mar 12, 2026

SDF draining in dataflow runner v1 #37831

Merged

3 tasks

github-actions Bot added the Next Action: Reviewers label Mar 12, 2026

stankiewicz requested a review from kennknowles March 12, 2026 12:27

kennknowles merged commit 630f32a into apache:master Mar 12, 2026
29 of 33 checks passed

stankiewicz mentioned this pull request Apr 8, 2026

[Drain] Expose drain to dofn processElement and onTimer - add missing implementation in ProcessContext #38119

Merged

3 tasks

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stankiewicz commented Mar 11, 2026

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