apache/beam · error

makeWindowCoders, unknown urn

Error message

makeWindowCoders, unknown urn: %v

What it means

prism's makeWindowCoders maps the pipeline's window coder URN to an engine window coder. It supports fixed, global, and interval windows; any other URN (e.g. custom windowing) hits the default case, logs an error, and panics because custom window functions are not yet supported by the runner.

Solutions

1. Use a supported built-in windowing (Fixed, Sliding, Global, Sessions where mapped) instead of a custom WindowFn
2. Check the logged urn in the error to identify which window coder is unsupported
3. Rewrite the pipeline to avoid custom windowing when targeting prism
4. Implement/extend support in prism's coders.go if you control the runner build (see TODO for issue 31921)

Example fix

// before
pcf := beam.WindowInto(s, window.NewCustomWindowFn(myFn), col)
// after
pcf := beam.WindowInto(s, window.NewFixedWindows(time.Minute), col)

if !isBuiltInWindowFn(pipeline) {
    return errors.New("prism does not support custom window fns; use Fixed/Sliding/Global windows")
}

defer func() {
    if r := recover(); r != nil && strings.Contains(fmt.Sprint(r), "makeWindowCoders, unknown urn") {
        err = errors.New("pipeline uses unsupported windowing for prism")
    }
}()

When it happens

Trigger: Submitting a pipeline to the prism runner whose GBK/window value coder uses a windowing URN other than the built-in ones, e.g. beam:coder:window:v1 payload specifying a custom WindowFn.

Common situations: Using custom WindowFn implementations (user code) with prism, or pipeline graphs produced by other SDKs with non-standard window coders.

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

• buildDescriptor: failed to handle coder on stage
• buildDescriptor: failed to handle coder on stage
• error decoding append bag user state window key
• Iterable coder must have only one component
• Nullable coder must have only one component

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/eb5f8a49abf42847. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/runners/prism/internal/coders.go:126

	return wvcID, nil
}

// makeWindowCoders categorizes and provides the encoder, decoder pair for the type of window.
func makeWindowCoders(wc *pipepb.Coder) (engine.WinCoderType, exec.WindowDecoder, exec.WindowEncoder) {
	var cwc *coder.WindowCoder
	var winCoder engine.WinCoderType
	switch wc.GetSpec().GetUrn() {
	case urns.CoderGlobalWindow:
		winCoder = engine.WinGlobal
		cwc = coder.NewGlobalWindow()
	case urns.CoderIntervalWindow:
		winCoder = engine.WinInterval
		cwc = coder.NewIntervalWindow()
	default:
		// TODO(https://github.com/apache/beam/issues/31921): Support custom windowfns instead of panicking here.
		winCoder = engine.WinCustom
		slog.LogAttrs(context.TODO(), slog.LevelError, "makeWindowCoders: unknown urn", slog.String("urn", wc.GetSpec().GetUrn()))
		panic(fmt.Sprintf("makeWindowCoders, unknown urn: %v", prototext.Format(wc)))
	}
	return winCoder, exec.MakeWindowDecoder(cwc), exec.MakeWindowEncoder(cwc)
}

// lpUnknownCoders takes a coder, and populates coders with any new coders
// coders that the runner needs to be safe, and speedy.
// It returns either the passed in coder id, or the new safe coder id.
func lpUnknownCoders(cID string, bundle, base map[string]*pipepb.Coder) (string, error) {
	// First check if we've already added the LP version of this coder to coders already.
	lpcID := cID + "_lp"
	// Check if we've done this one before.
	if _, ok := bundle[lpcID]; ok {
		return lpcID, nil
	}
	// All coders in the coders map have been processed.
	if _, ok := bundle[cID]; ok {
		return cID, nil
	}

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