apache/beam · error

bad windowed value

Error message

bad windowed value: %v

What it means

EncodeCoderRef encodes coder.WindowedValue coders, which must have exactly one element component and a non-nil Window coder describing the windowing scheme. Anything else cannot be serialized into the runner form, so encoding fails with the coder printed in the message.

Solutions

1. Ensure the WindowedValue coder has exactly one element component and a valid Window coder attached.
2. Construct windowed coders via the SDK helpers that pair the element coder with the appropriate window coder.
3. Check custom transforms that replace windowed coders and restore the Window field.
4. Verify the windowing strategy (fixed/sliding/session/global) is initialized before the graph is serialized.

Example fix

// before
c := &coder.Coder{Kind: coder.WindowedValue, T: t, Components: []*coder.Coder{elm}} // Window nil
ref, err := graphx.EncodeCoderRef(c)

// after
c := &coder.Coder{Kind: coder.WindowedValue, T: t, Components: []*coder.Coder{elm}, Window: coder.NewIntervalWindowCoder()}
ref, err := graphx.EncodeCoderRef(c)

func validWindowed(c *coder.Coder) bool {
	return c != nil && c.Kind == coder.WindowedValue && len(c.Components) == 1 && c.Window != nil
}

func hasWindow(c *coder.Coder) bool { return c != nil && c.Window != nil }

ref, err := graphx.EncodeCoderRef(c)
if err != nil {
	if strings.Contains(err.Error(), "bad windowed value") {
		return fmt.Errorf("windowed coder needs 1 element component and a window coder: %w", err)
	}
	return err
}

When it happens

Trigger: Encoding a coder.Coder of Kind coder.WindowedValue where len(c.Components) != 1 or c.Window == nil — e.g. a windowed coder assembled without attaching a window coder.

Common situations: Custom windowing code building WindowedValue coders without calling the window-coder constructor (e.g. coder.NewIntervalWindowCoder); global-window pipelines where the window field was cleared; copying coder structs and dropping the unexported/optional Window field.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• bad CoGBK
• bad I
• bad N
• bad window kind
• bad window

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/68caee433b387aeb. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/runtime/graphx/dataflow.go:166

		refs, err := EncodeCoderRefs(c.Components)
		if err != nil {
			return nil, err
		}

		value := refs[1]
		if len(c.Components) > 2 {
			// TODO(https://github.com/apache/beam/issues/18032): don't inject union coder for CoGBK.

			union := &CoderRef{Type: cogbklistType, Components: refs[1:]}
			value = &CoderRef{Type: lengthPrefixType, Components: []*CoderRef{union}}
		}

		stream := &CoderRef{Type: streamType, Components: []*CoderRef{value}, IsStreamLike: true}
		return &CoderRef{Type: pairType, Components: []*CoderRef{refs[0], stream}, IsPairLike: true}, nil

	case coder.WindowedValue:
		if len(c.Components) != 1 || c.Window == nil {
			return nil, errors.Errorf("bad windowed value: %v", c)
		}

		elm, err := EncodeCoderRef(c.Components[0])
		if err != nil {
			return nil, err
		}
		w, err := encodeWindowCoder(c.Window)
		if err != nil {
			return nil, err
		}
		return &CoderRef{Type: windowedValueType, Components: []*CoderRef{elm, w}, IsWrapper: true}, nil

	case coder.IW:
		return &CoderRef{Type: intervalWindowType}, nil

	case coder.Bytes:
		return &CoderRef{Type: bytesType}, nil

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