failed to map main input window to side input window with Wi

Error message

failed to map main input window to side input window with WindowFn %v

What it means

windowMapper.MapWindow maps a main-input window to a side-input window by assigning the element's max timestamp with the pipeline's WindowFn. If assignWindows returns no candidates (the timestamp falls outside all windows defined by the WindowFn), this error is thrown. It indicates the side-input lookup cannot proceed because no window of the side input corresponds to the main input element's event time.

Source

Thrown at sdks/go/pkg/beam/core/runtime/exec/window.go:178

func (m *MapWindows) String() string {
	return fmt.Sprintf("MapWindows[%v]. Out:%v", m.FnUrn, m.Out.ID())
}

// WindowMapper defines an interface maps windows from a main input window space
// to windows from a side input window space. Used during side input materialization.
type WindowMapper interface {
	MapWindow(w typex.Window) (typex.Window, error)
}

type windowMapper struct {
	wfn *window.Fn
}

func (f *windowMapper) MapWindow(w typex.Window) (typex.Window, error) {
	candidates := assignWindows(f.wfn, w.MaxTimestamp())
	if len(candidates) == 0 {
		return nil, fmt.Errorf("failed to map main input window to side input window with WindowFn %v", f.wfn.String())
	}
	// Return earliest candidate window in terms of event time (only relevant for sliding windows)
	// Sliding windows append the latest window first in assignWindows.
	return candidates[len(candidates)-1], nil
}

View on GitHub (pinned to 12126d8942)

Solutions

Verify elements have valid event-time timestamps (via beam.Timestamp) that fall inside your window configuration.
Check the WindowFn's window size/offset/period so the observed timestamps map to at least one window.
Inspect the WindowFn string in the error to confirm the expected windowing matches the pipeline's windowing.
Ensure a Default windowing is not accidentally combined with timestamps outside expected bounds when using custom window assignments.

Example fix

// before
col := beam.ParDo(s, extractFn, col) // elements without timestamps, sliding windows of 1h
// after
col = beam.Window(s, window.NewFixedWindow(time.Hour))
col = beam.ParDo(s, extractFn.WithTimestamp(extractTs), col)

Defensive patterns

Strategy: validation

Validate before calling

func tsInAnyWindow(ts typex.EventTime, wfn *window.Fn) bool {
	return len(assignWindows(wfn, ts)) > 0
}

Try / catch

sideWin, err := mapper.MapWindow(w)
if err != nil {
	return fmt.Errorf("side input lookup failed for ts %v: %w", w.MaxTimestamp(), err)
}

Prevention

Always assign event-time timestamps before windowed side-input reads.
Keep main and side inputs under the same windowing strategy.
Add tests covering boundary timestamps for sliding windows.

When it happens

Trigger: Calling MapWindow (via side-input lookup in exec) when the main input element's max timestamp, fed to assignWindows(f.wfn, ...), yields zero candidate windows — e.g. timestamps outside all configured windows or a malformed/zero timestamp under a windows-based WindowFn.

Common situations: Side-input access with sliding/fixed windows when event timestamps are unset (default Unix epoch far outside window ranges), custom WindowFns that can reject timestamps, or after changing windowing parameters so old timestamps no longer fall in any window.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/cc8166c02aa1c57b. Report an issue: GitHub.