ExecuteTransform[GBK] stage %v, transform %q %v: couldn't pr

Error message

ExecuteTransform[GBK] stage %v, transform %q %v: couldn't process value coder:
%w

What it means

ExecuteTransform for GBK resolves the KV's value component coder (component 1) last. If lpUnknownCoders fails on it, the runner panics with this message, identifying the stage, transform, and wrapped cause. It means the value side of the grouped KV could not be decoded/encoded as specified.

Source

Thrown at sdks/go/pkg/beam/runners/prism/internal/handlerunner.go:440

	case urns.TransformGBK:
		ws := windowingStrategy(comps, tid)
		kvc := onlyInputCoderForTransform(comps, tid)

		coders := map[string]*pipepb.Coder{}

		// TODO assert this is a KV. It's probably fine, but we should fail anyway.
		wcID, err := lpUnknownCoders(ws.GetWindowCoderId(), coders, comps.GetCoders())
		if err != nil {
			panic(fmt.Errorf("ExecuteTransform[GBK] stage %v, transform %q %v: couldn't process window coder:\n%w", stageID, tid, prototext.Format(t), err))
		}
		kcID, err := lpUnknownCoders(kvc.GetComponentCoderIds()[0], coders, comps.GetCoders())
		if err != nil {
			panic(fmt.Errorf("ExecuteTransform[GBK] stage %v, transform %q %v: couldn't process key coder:\n%w", stageID, tid, prototext.Format(t), err))
		}
		ecID, err := lpUnknownCoders(kvc.GetComponentCoderIds()[1], coders, comps.GetCoders())
		if err != nil {
			panic(fmt.Errorf("ExecuteTransform[GBK] stage %v, transform %q %v: couldn't process value coder:\n%w", stageID, tid, prototext.Format(t), err))
		}
		reconcileCoders(coders, comps.GetCoders())

		wc := coders[wcID]
		kc := coders[kcID]
		ec := coders[ecID]

		data = append(data, gbkBytes(ws, wc, kc, ec, inputData, coders))
		if len(data[0]) == 0 {
			panic("no data for GBK")
		}
	default:
		panic(fmt.Sprintf("unimplemented runner transform[%v]", urn))
	}

	// To avoid conflicts with these single transform
	// bundles, we suffix the transform IDs.
	var localID string

View on GitHub (pinned to 12126d8942)

Solutions

Check the wrapped error for the failing value coder reference.
Make sure value coders for all elements reaching the GBK are registered and emitted into the pipeline.
Regenerate the pipeline with a current SDK, or switch to built-in coders for the value type.

Defensive patterns

Strategy: validation

Validate before calling

// Check the GBK KV's value component coder resolves
kvc := codersByID[gbkInputCoderID]
if len(kvc.GetComponentCoderIds()) < 2 {
    log.Fatalf("GBK input coder missing value component")
}
valID := kvc.GetComponentCoderIds()[1]
if _, ok := codersByID[valID]; !ok {
    log.Fatalf("value coder %q not registered", valID)
}

Try / catch

defer func() {
    if r := recover(); r != nil && strings.Contains(fmt.Sprint(r), "couldn't process value coder") {
        log.Fatalf("GBK value coder unresolved: %v", r)
    }
}()

Prevention

Register custom value coders so they serialize into the pipeline
Test GBK pipelines with built-in types first to isolate custom-coder issues
Verify cross-language expansion emits value coders for grouped inputs

When it happens

Trigger: A GBK whose KV coder's componentCoderIds[1] points to an unresolvable or unsupported coder in comps.GetCoders().

Common situations: Custom value coders omitted from the pipeline proto; cross-language pipelines where the value coder isn't registered; malformed CoGroupByKey/GBK graphs.

Understand the failure class

Background: "cannot parse invalid wire-format data", "cannot unmarshal", "failed unmarshalling": protobuf unmarshal errors explained — this error's family across 10 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/6bed0f1ed0d03ef5. Report an issue: GitHub.