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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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.
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.
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.
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.
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).
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.
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.
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.
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.
Checklist for Adding New Metadata
Define the metadata in beam_fn_api.proto (if applicable).
Add getters to the WindowedValue interface.
Update ValueInGlobalWindow, ValueInSingleWindow, ValueInEmptyWindows to store the metadata.
Update OutputBuilder to accept the metadata.
Update WindmillSink to serialize the metadata to the backend.
Update UngroupedWindmillReader and WindowingWindmillReader to deserialize the metadata.
Update WindmillKeyedWorkItem.
Update SimpleDoFnRunner, StatefulDoFnRunner, and FnApiDoFnRunner to propagate the metadata from input to output.
Update ReduceFnRunner and OutputAndTimeBoundedSplittableProcessElementInvoker for complex transform propagation.
If required by timers, update TimerData and TimerInternals.
If exposed to the user, update DoFnSignatures and ByteBuddyDoFnInvokerFactory.
Update other runners (Flink, Spark) to ensure they propagate the new WindowedValue fields correctly in their specific operators/runners.
