apache/beam · error

mismatched CoGBK windowing strategies: %v, want %v

Error message

mismatched CoGBK windowing strategies: %v, want %v

What it means

Error returned by NewCoGBK when an input's windowing strategy differs from the first input's (w). All inputs to a CoGBK must share the same windowing strategy so per-window grouping is well defined; the message shows the mismatched strategy versus the expected one.

Source

Thrown at sdks/go/pkg/beam/core/graph/edge.go:228

	}

	// (1) Create CoGBK result type: KV<T,U>, .., KV<T,Z> -> CoGBK<T,U,..,Z>.

	c := ns[0].Coder.Components[0]
	w := inputWindow(ns)
	bounded := inputBounded(ns)
	comp := []typex.FullType{c.T, ns[0].Type().Components()[1]}

	for i := 1; i < len(ns); i++ {
		n := ns[i]
		if !typex.IsKV(n.Type()) {
			return nil, addContext(errors.Errorf("input type must be KV: %v", n), s)
		}
		if !n.Coder.Components[0].Equals(c) {
			return nil, addContext(errors.Errorf("key coder for %v is %v, want %v", n, n.Coder.Components[0], c), s)
		}
		if !w.Equals(n.WindowingStrategy()) {
			return nil, addContext(errors.Errorf("mismatched CoGBK windowing strategies: %v, want %v", n.WindowingStrategy(), w), s)
		}
		if bounded != n.Bounded() {
			return nil, addContext(errors.Errorf("unmatched CoGBK boundedness: %v, want %v", n.Bounded(), bounded), s)
		}

		comp = append(comp, n.Type().Components()[1])
	}

	t := typex.NewCoGBK(comp...)
	out := g.NewNode(t, w, bounded)

	// (2) Add CoGBK edge

	edge := g.NewEdge(s)
	edge.Op = CoGBK
	for i := 0; i < len(ns); i++ {
		edge.Input = append(edge.Input, &Inbound{Kind: Main, From: ns[i], Type: ns[i].Type()})
	}

View on GitHub (pinned to 12126d8942)

Solutions

1. Apply the same windowing (beam.WindowInto with identical strategy) to all inputs before CoGroupByKey
2. Align boundedness: re-window streaming inputs or re-read batch inputs consistently
3. Inspect each PCollection's WindowingStrategy in tests to confirm equality

Example fix

// before
wA := beam.WindowInto(s, window.NewFixedWindows(60), a)
beam.CoGroupByKey(s, wA, b) // b global windows
// after
wB := beam.WindowInto(s, window.NewFixedWindows(60), b)
beam.CoGroupByKey(s, wA, wB)

strategy := inputs[0].WindowingStrategy()
for i, c := range inputs[1:] {
    if !strategy.Equals(c.WindowingStrategy()) {
        return fmt.Errorf("input %d windowing mismatch", i+1)
    }
}

if err := beam.TryCoGroupByKey(s, a, b); err != nil {
    return fmt.Errorf("CoGroupByKey windowing strategies differ: %w", err)
}

When it happens

Trigger: CoGroupByKey combining a bounded/unbounded input, or inputs with different triggers, allowed lateness, or window assignments (e.g. fixed vs global windows).

Common situations: Mixing batch and streaming sources in one CoGroupByKey, or applying custom windowing to only some inputs.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

• error encoding pane %v: non-speculative index value must be
• failed to map main input window to side input window with Wi
• input type must be KV: %v
• key coder for %v is %v, want %v
• failed to decode union value '%v' for key %v

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/78928c9ec5ba3eeb. Report an issue: GitHub.