apache/beam · error

not a Window Value, got %T

Error message

not a Window Value, got %T

What it means

In exec/window.go, MapWindows' ProcessElement expects the second element of a KV-style windowed element (Elm2) to be a typex.Window. If Elm2 is non-nil but does not implement typex.Window, the SDK cannot determine the source window to remap and throws this error.

Solutions

1. Verify the PCollection feeding MapWindows uses the correct windowed-value coder so Elm2 decodes as typex.Window.
2. Check that upstream transforms emit elements in the expected (value, window) form and are not reordering components.
3. Regenerate/refresh coders after changing windowing strategy (e.g. switching from GlobalWindows to FixedWindows).
4. If constructing FullValues manually, set Elm2 to the actual typex.Window value or leave it nil to use Windows[0].

Example fix

// before (manual FullValue construction)
beam.FullValue{Elm: val, Elm2: timestamp} // Elm2 is a time, not a Window
// after
beam.FullValue{Elm: val, Windows: []typex.Window{w}} // put window in Windows, leave Elm2 nil

if fv.Elm2 != nil {
    if _, ok := fv.Elm2.(typex.Window); !ok {
        return fmt.Errorf("element second component is %T, not a typex.Window", fv.Elm2)
    }
}

func isWindow(v any) bool {
    _, ok := v.(typex.Window)
    return ok
}

win, err := mapWindowsResult(...)
if err != nil && strings.Contains(err.Error(), "not a Window Value") {
    log.Printf("coder/window mismatch: %v", err)
}

When it happens

Trigger: A MapWindows transform receives an element whose second component is not a typex.Window — typically from a coder mismatch where the element decoded with the wrong coder, or a hand-built FullValue whose Elm2 is a non-window value.

Common situations: Custom coders that emit FullValues with two components where the second is data rather than a window; runner/coder incompatibilities after changing windowing or coder configuration; combining MapWindows with transforms producing non-KV/non-windowed pairs.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• coder type must be identical to node type
• Nested FullValues must be nested as pointers.
• not a float type
• not a signed integer type
• not a unsigned integer type

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/6319e56486bdf5e8. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/runtime/exec/window.go:133

}

// Up does nothing
func (m *MapWindows) Up(_ context.Context) error {
	return nil
}

func (m *MapWindows) StartBundle(ctx context.Context, id string, data DataContext) error {
	return m.Out.StartBundle(ctx, id, data)
}

func (m *MapWindows) ProcessElement(ctx context.Context, elm *FullValue, values ...ReStream) error {
	// MapWindows ends up with the wrappedDecode path, which can pass the value window through the
	// Window field. Use that as the default for resilience to a change to match the coder correctly.
	win := elm.Windows[0]
	if elm.Elm2 != nil {
		w, ok := elm.Elm2.(typex.Window)
		if !ok {
			return errors.Errorf("not a Window Value, got %T", elm.Elm2)
		}
		win = w
	}
	newW, err := m.Fn.MapWindow(win)
	if err != nil {
		return err
	}
	out := &FullValue{
		Elm:       elm.Elm,
		Elm2:      newW,
		Timestamp: elm.Timestamp,
		Windows:   elm.Windows,
		Pane:      elm.Pane,
	}
	return m.Out.ProcessElement(ctx, out, values...)
}

// FinishBundle propagates finish bundle to downstream nodes.

View on GitHub (pinned to 12126d8942)