apache/beam · error · java.lang.RuntimeException

Unable to encode constant members of ParamWindowedValueCoder

Error message

Unable to encode constant members of ParamWindowedValueCoder: 

What it means

ParamWindowedValueCoder encodes its constant members (timestamp, windows, pane info, value kind) to a byte payload during serialization; if FullWindowedValueCoder.encode() throws IOException it is rethrown as a RuntimeException with this message. It signals the sample windowed value could not be serialized while constructing the coder.

Source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/values/WindowedValues.java:1456

      // ParamWindowedValueCoder
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      WindowedValue<byte[]> windowedValue =
          WindowedValues.of(
              EMPTY_BYTES,
              from.getTimestamp(),
              from.getWindows(),
              from.getPaneInfo(),
              null,
              null,
              CausedByDrain.NORMAL,
              null,
              from.getValueKind());
      WindowedValues.FullWindowedValueCoder<byte[]> windowedValueCoder =
          WindowedValues.FullWindowedValueCoder.of(ByteArrayCoder.of(), from.getWindowCoder());
      try {
        windowedValueCoder.encode(windowedValue, baos);
      } catch (IOException e) {
        throw new RuntimeException(
            "Unable to encode constant members of ParamWindowedValueCoder: ", e);
      }
      return baos.toByteArray();
    }

    /** Create a {@link Coder} from its component {@link Coder coders}. */
    public static WindowedValues.ParamWindowedValueCoder<?> fromComponents(
        List<Coder<?>> components, byte[] payload) {
      Coder<? extends BoundedWindow> windowCoder =
          (Coder<? extends BoundedWindow>) components.get(1);
      WindowedValues.FullWindowedValueCoder<byte[]> windowedValueCoder =
          WindowedValues.FullWindowedValueCoder.of(ByteArrayCoder.of(), windowCoder);

      try {
        ByteArrayInputStream bais = new ByteArrayInputStream(payload);
        WindowedValue<byte[]> windowedValue = windowedValueCoder.decode(bais);
        return WindowedValues.ParamWindowedValueCoder.of(
            components.get(0),

View on GitHub (pinned to 12126d8942)

Solutions

1. Check the 'Caused by' IOException to see which component coder failed
2. Ensure the window Coder can serialize all custom windows used
3. Verify the value's coder is registered and stable across job submission and workers
4. Keep Beam SDK versions identical between submitter and expansion/worker environments

Example fix

// before: custom window without registered coder
MyCustomWindow w = new MyCustomWindow(...);
// after: provide a custom window coder
pipeline.getCoderRegistry().registerCoderForClass(MyCustomWindow.class, MyCustomWindowCoder.of());

Coder<T> coder = pipeline.getCoderRegistry().getCoder(valueType);
if (coder == null) throw new IllegalStateException("No registered coder for " + valueType);

try { ParamWindowedValueCoder.of(...); } catch (RuntimeException e) { if (e.getCause() instanceof IOException) { /* coder encoding failure: fix component coder */ } throw e; }

When it happens

Trigger: Constructing a ParamWindowedValueCoder (e.g., via its constructor from an existing WindowedValue) where windowedValueCoder.encode(windowedValue, baos) fails with IOException, usually because the value or window coder cannot serialize the given element.

Common situations: Custom window types without a proper Coder; values with non-serializable fields when Java serialization crosses worker boundaries; mismatched coder versions after a Beam upgrade.

Understand the failure class

Background: json.Marshal / "failed to marshal" errors in Go: why "unsupported type" happens and how to fix it — this error's family across 22 libraries.

Related errors

• cannot encode a null String
• cannot encode a null Integer
• cannot encode a null ValueKind
• {errorContext}: unable to encode value {value} using {coder}
• Unable to decode constant members from payload for ParamWind

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/55b9316c3150ccc3. Report an issue: GitHub.