adding-new-metadata

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Guide on how to add and propagate new metadata fields in Apache Beam's WindowedValue, extending protos, windmill persistence, and runner interfaces to avoid metadata loss.

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Adding New Metadata to WindowedValue

This skill provides a comprehensive guide on adding new metadata (e.g., CDC metadata, drain mode flags, OpenTelemetry trace context) to Apache Beam's WindowedValue and ensuring it propagates correctly through the execution engine. Failing to propagate metadata in all necessary places will result in metadata loss during pipeline execution.

1. Extending the Proto Model

When adding new metadata that must cross worker boundaries or be serialized by the Fn API, the proto definitions must be updated.

Key Files: model/fn-execution/src/main/proto/org/apache/beam/model/fn_execution/v1/beam_fn_api.proto
Action: Add the new metadata field to the appropriate message (ElementMetadata).
Note: Add proper documentation in proto. Type of the field can be different from the type in WindowedValue, see OpenTelemetry trace context for example.

2. WindowedValue Interface and Implementations

The WindowedValue is the core container for elements flowing through a Beam pipeline. It holds the value, timestamp, windows, pane info, and any additional metadata.

Core Interface Updates

Key File: sdks/java/core/src/main/java/org/apache/beam/sdk/values/WindowedValue.java
Action: Add getter methods for your new metadata.

Concrete Implementations

You must update all concrete implementations of WindowedValue to store and return the new metadata. If you miss one, metadata will be silently dropped.

ValueInGlobalWindow
ValueInSingleWindow
ValueInEmptyWindows (often used inside runners, like Dataflow's worker package)
Action: Update constructors, factory methods (of()), fields in these classes and coders.

OutputBuilder vs. Context Output

IMPORTANT: Do not add new arguments to legacy methods like context.outputWindowedValue(...) or WindowedValue.of(value, timestamp, windows, pane). This causes brittleness and breaks the API for every new metadata field.
Action: Modify OutputBuilder (sdks/java/core/src/main/java/org/apache/beam/sdk/values/OutputBuilder.java) to accept the new metadata (e.g., .withDrainMode(...), .withTraceContext(...)). Use the builder pattern when constructing outputs to propagate offset and record IDs smoothly.

3. Windmill Persistence (Dataflow Streaming Engine) Runner v1

For the Dataflow streaming runner, metadata must survive serialization to and from the Windmill backend.

Serialization (Sink):
- File: runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/WindmillSink.java
- Action: Extract the metadata from the WindowedValue, and add it to already created ElementMetadata proto builder.
Deserialization (Reader):
- Files: runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/UngroupedWindmillReader.java and WindowingWindmillReader.java
- Action: Extract the metadata from ElementMetadata proto and reconstruct the WindowedValue using the updated factory methods/builders that include the metadata. This is incremental work, as plenty of metadata is already extracted from the proto.

4. Propagation Across Core Classes

Metadata must be explicitly copied or forwarded whenever a WindowedValue is transformed, buffered, or processed.

DoFn Runners (Java Core)

You must ensure that when a DoFn processes an element and outputs a new element, the appropriate metadata from the input is propagated to the output (unless explicitly changed by the logic).

runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java
runners/core-java/src/main/java/org/apache/beam/runners/core/StatefulDoFnRunner.java
runners/core-java/src/main/java/org/apache/beam/runners/core/LateDataDroppingDoFnRunner.java
runners/core-java/src/main/java/org/apache/beam/runners/core/ProcessFnRunner.java
sdks/java/harness/src/main/java/org/apache/beam/fn/harness/FnApiDoFnRunner.java

Action: When these runners call outputWindowedValue(), they should extract the metadata from the input or current context and attach it using the OutputBuilder or the new WindowedValue interfaces.

Grouping and Reducing

runners/core-java/src/main/java/org/apache/beam/runners/core/ReduceFnRunner.java
runners/core-java/src/main/java/org/apache/beam/runners/core/ReduceFnContextFactory.java
Action: Ensure that during GroupByKey/Combine operations, if metadata needs to be preserved (e.g., CausedByDrain), it is correctly passed into the ReduceFnContextFactory and propagated when outputting the grouped results.

Splittable DoFns (SDF)

runners/core-java/src/main/java/org/apache/beam/runners/core/OutputAndTimeBoundedSplittableProcessElementInvoker.java
sdks/java/core/src/main/java/org/apache/beam/sdk/util/construction/SplittableParDoNaiveBounded.java

Timers

If metadata needs to survive timer firings (e.g., knowing an @OnTimer fired because of a system drain), it must be added to Timer data structures. This is a bit of uncharted area which was only implemented for CausedByDrain metadata that comes from backend, not from persisted metadata. In order to persist all WindowedValue metadata across timer, more work has to be done, below are some pointers:

runners/core-java/src/main/java/org/apache/beam/runners/core/TimerInternals.java and implementations (e.g., WindmillTimerInternals.java in Dataflow).
Action: Add the field to TimerData, next to CausedByDrain. Propagate it when setting the timer and expose it when the timer fires so it bubbles up.
Eventually, metadata from Timer lands in WindowedValue, so it can be exposed to users. Keep field names, types, and getters similar to WindowedValue as much as possible, as common interface may be introduced eventually.

5. Exposing Metadata to the User (DoFn Signatures)

User needs to access the metadata in their DoFn (e.g., @ProcessElement public void process(ProcessContext c, CausedByDrain drain) { ... }), you must update the reflection and bytecode generation logic.

Files:
- sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/DoFnSignatures.java
- sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/DoFnSignature.java
- sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/DoFnInvoker.java
- sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/ByteBuddyDoFnInvokerFactory.java
Action: Add logic to detect the new parameter type in the DoFn method signature. Generate bytecode using ByteBuddy to extract the property from the WindowedValue or context and pass it as an argument during method invocation.