What causes "invalid status for combine merge %v: %v" in apache/beam?

Ensure StartBundle was called successfully before processing accumulator elements.

invalid status for combine merge %v: %v

Error message

invalid status for combine merge %v: %v

What it means

MergeAccumulators.ProcessElement (the GBK merge side of a combine) requires the node to be Active within a started bundle. Elements arriving while the node is in any other status indicate an out-of-order lifecycle invocation.

Source

Thrown at sdks/go/pkg/beam/core/runtime/exec/combine.go:577

	}
	c.cache = nil
	return nil
}

// MergeAccumulators is an executor for merging accumulators from a lifted combine.
type MergeAccumulators struct {
	*Combine
}

func (n *MergeAccumulators) String() string {
	return fmt.Sprintf("MergeAccumulators[%v] Keyed:%v Out:%v", path.Base(n.Fn.Name()), n.UsesKey, n.Out.ID())
}

// ProcessElement accepts a stream of accumulator values with the same key and
// runs the MergeAccumulatorsFn over them repeatedly.
func (n *MergeAccumulators) ProcessElement(ctx context.Context, value *FullValue, values ...ReStream) error {
	if n.status != Active {
		return errors.Errorf("invalid status for combine merge %v: %v", n.UID, n.status)
	}
	n.Combine.states.Set(n.Combine.ctx, metrics.ProcessBundle)
	a, err := n.newAccum(n.Combine.ctx, value.Elm)
	if err != nil {
		return n.fail(err)
	}
	first := true

	stream, err := values[0].Open()
	if err != nil {
		return n.fail(err)
	}
	defer stream.Close()
	for {
		v, err := stream.Read()
		if err != nil {
			if err == io.EOF {
				break

View on GitHub (pinned to 12126d8942)

Solutions

Ensure StartBundle was called successfully before processing accumulator elements.
Check for swallowed errors from earlier lifecycle calls (a failed StartBundle leaves status Up).
Rebuild the plan node if its status is stale.

Defensive patterns

Strategy: validation

Try / catch

if err := merge.ProcessElement(ctx, fv, values); err != nil {
	if strings.Contains(err.Error(), "invalid status for combine merge") {
		// ensure the merge node was started (Up + StartBundle)
	}
	return err
}

Prevention

Verify StartBundle succeeded before streaming accumulators.
Fail fast on earlier lifecycle errors instead of continuing to process.
Use standard plan construction in tests.

When it happens

Trigger: Calling MergeAccumulators.ProcessElement before StartBundle or after FinishBundle.

Common situations: Tests feeding accumulator streams directly to the merge node; runners whose bundle-start step failed silently but continued to emit data.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/5abbd530afeb9dc0. Report an issue: GitHub.