apache/beam · error · UnsupportedOperationException

Cannot access sideInput in non-window observing context.

Error message

Cannot access sideInput in non-window observing context.

What it means

NonWindowObservingProcessBundleContextBase.sideInput(String tagId) throws UnsupportedOperationException because side inputs require a bound window to resolve a view, and this context serves a method that was not declared window-observing. The harness therefore refuses to resolve side inputs in this context.

Source

Thrown at sdks/java/harness/src/main/java/org/apache/beam/fn/harness/FnApiDoFnRunner.java:1972

    @Override
    public CausedByDrain causedByDrain(DoFn<InputT, OutputT> doFn) {
      return currentElement.causedByDrain();
    }
  }

  /** Provides base arguments for a {@link DoFnInvoker} for a non-window observing method. */
  private abstract class NonWindowObservingProcessBundleContextBase
      extends ProcessBundleContextBase {
    @Override
    public BoundedWindow window() {
      throw new UnsupportedOperationException(
          "Cannot access window in non-window observing context.");
    }

    @Override
    public Object sideInput(String tagId) {
      throw new UnsupportedOperationException(
          "Cannot access sideInput in non-window observing context.");
    }

    @Override
    public <T> T sideInput(PCollectionView<T> view) {
      throw new UnsupportedOperationException(
          "Cannot access sideInput in non-window observing context.");
    }

    @Override
    public State state(String stateId, boolean alwaysFetched) {
      throw new UnsupportedOperationException(
          "Cannot access state in non-window observing context.");
    }

    @Override
    public org.apache.beam.sdk.state.Timer timer(String timerId) {
      throw new UnsupportedOperationException(

View on GitHub (pinned to 12126d8942)

Solutions

1. Add a BoundedWindow parameter to the @ProcessElement signature so the window-observing context (which implements sideInput) is used.
2. Use the view-based c.sideInput(PCollectionView) only inside window-observing @ProcessElement methods.
3. If the side input is window-independent, read it via a different mechanism (e.g., pass the data as the main input or a broadcast/readied view computed outside the DoFn).

Example fix

// before
@ProcessElement
public void processElement(ProcessContext c) {
  Object v = c.sideInput("myTag"); // throws
}

// after
@ProcessElement
public void processElement(ProcessContext c, BoundedWindow window) {
  MyView v = c.sideInput(myView);
}

// Resolve side inputs only in window-observing @ProcessElement methods:
boolean canReadSideInput = Arrays.stream(dofn.getClass().getDeclaredMethods())
    .filter(m -> m.isAnnotationPresent(ProcessElement.class))
    .anyMatch(m -> Arrays.stream(m.getParameterTypes()).anyMatch(BoundedWindow.class::isAssignableFrom));

BoundedWindow requireWindow(DoFn.ProcessContext ctx) {
  BoundedWindow w = ctx instanceof FnApiDoFnRunner.ProcessBundleContextBase
      ? null : null;
  return w; // null => non-window-observing context, side input unavailable
}

try {
  V v = c.sideInput(view);
} catch (UnsupportedOperationException e) {
  throw new IllegalStateException("sideInput requires a window-observing @ProcessElement (add BoundedWindow param)", e);
}

When it happens

Trigger: Calling ProcessContext.sideInput(tagId) (string overload) from a @ProcessElement method without a BoundedWindow parameter, or from @StartBundle/@FinishBundle, where no window is available to index the side input view.

Common situations: Reading a side input in a non-windowed DoFn after refactoring; using the tag-based sideInput API introduced with newer Beam versions from lifecycle methods; forgetting the BoundedWindow parameter after migrating code.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• SideInput unsupported in ${context}
• calling getSideInput() with unknown view
• Cannot access window in non-window observing context.
• Cannot access state in non-window observing context.
• Cannot access timer in non-window observing context.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/059f59b69dff3bc5. Report an issue: GitHub.