apache/beam · error

missing corresponding pcollection of named output

Error message

missing corresponding pcollection of named output: %v is expected in %v

What it means

ResolveOutputIsBounded maps each output node of an expanded transform to its corresponding PCollection in the expanded components and propagates boundedness. This panic fires when an output PCollection ID from the expanded transform is not present in the expanded PCollections map.

Solutions

1. Regenerate the expansion via TryCrossLanguage to get a consistent components payload.
2. Check the expansion service logs for errors producing incomplete Pcollections maps.
3. Upgrade the Beam SDK/expansion service to matching versions so protos align.

defer func() {
    if r := recover(); r != nil {
        return fmt.Errorf("expansion components inconsistent: %v", r)
    }
}()

When it happens

Trigger: TryCrossLanguage resolving outputs where expanded.Transform lists an output ID missing from expanded.Components' Pcollections map (inconsistent expansion response).

Common situations: Buggy or mismatched expansion service returning partial components; expansion payload truncated or hand-edited; version drift between the expansion service and SDK protos.

Understand the failure class

Background: "Not found" and "does not exist" errors: why "Task not found", "No such folder", and "Can't find" fire when a lookup comes back empty — this error's family across 14 libraries.

Related errors

• attempted to add namespace to missing coder id
• attempted to add namespace to missing windowing strategy id
• failed to add output coder to coder registry
• failed to expand cross language transform for edge
• failed to make node with node id

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/f4f3994612e92ad1. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/runtime/graphx/xlang.go:184

		var id string
		isBounded := true

		switch tag {
		case graph.UnnamedOutputTag:
			for _, id = range expandedOutputs {
				// easiest way to access map with one entry (key,value)
			}
		default:
			id = expandedOutputs[tag]
		}

		if pcol, exists := expandedPCollections[id]; exists {
			if pcol.GetIsBounded() == pipepb.IsBounded_UNBOUNDED {
				isBounded = false
			}
			isBoundedUpdater(node, isBounded)
		} else {
			panic(errors.Errorf("missing corresponding pcollection of named output: %v is expected in %v", id, expandedPCollections))
		}

	}
}

// AddFakeImpulses adds an impulse transform as the producer for each input to
// the root transform. Inputs need producers to form a correct pipeline.
func AddFakeImpulses(p *pipepb.Pipeline) {
	// For a pipeline consisting of only the external node edge, there will be
	// single root transform which will be the external transform.
	// Adding fake impulses per input to this external transform
	transforms := p.GetComponents().GetTransforms()
	ext := transforms[p.GetRootTransformIds()[0]]

	for tag, id := range ext.GetInputs() {
		key := fmt.Sprintf("%s_%s", "impulse", tag)
		output := map[string]string{"out": id}

View on GitHub (pinned to 12126d8942)