{"record":{"id":"cc8166c02aa1c57b","repo":"apache/beam","slug":"failed-to-map-main-input-window-to-side-input-window-with","errorCode":null,"errorMessage":"failed to map main input window to side input window with WindowFn %v","messagePattern":"failed to map main input window to side input window with WindowFn (.+?)","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"sdks/go/pkg/beam/core/runtime/exec/window.go","lineNumber":178,"sourceCode":"\nfunc (m *MapWindows) String() string {\n\treturn fmt.Sprintf(\"MapWindows[%v]. Out:%v\", m.FnUrn, m.Out.ID())\n}\n\n// WindowMapper defines an interface maps windows from a main input window space\n// to windows from a side input window space. Used during side input materialization.\ntype WindowMapper interface {\n\tMapWindow(w typex.Window) (typex.Window, error)\n}\n\ntype windowMapper struct {\n\twfn *window.Fn\n}\n\nfunc (f *windowMapper) MapWindow(w typex.Window) (typex.Window, error) {\n\tcandidates := assignWindows(f.wfn, w.MaxTimestamp())\n\tif len(candidates) == 0 {\n\t\treturn nil, fmt.Errorf(\"failed to map main input window to side input window with WindowFn %v\", f.wfn.String())\n\t}\n\t// Return earliest candidate window in terms of event time (only relevant for sliding windows)\n\t// Sliding windows append the latest window first in assignWindows.\n\treturn candidates[len(candidates)-1], nil\n}\n","sourceCodeStart":160,"sourceCodeEnd":184,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/go/pkg/beam/core/runtime/exec/window.go#L160-L184","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","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."],"exampleFix":"// before\ncol := beam.ParDo(s, extractFn, col) // elements without timestamps, sliding windows of 1h\n// after\ncol = beam.Window(s, window.NewFixedWindow(time.Hour))\ncol = beam.ParDo(s, extractFn.WithTimestamp(extractTs), col)","handlingStrategy":"validation","validationCode":"func tsInAnyWindow(ts typex.EventTime, wfn *window.Fn) bool {\n\treturn len(assignWindows(wfn, ts)) > 0\n}","typeGuard":null,"tryCatchPattern":"sideWin, err := mapper.MapWindow(w)\nif err != nil {\n\treturn fmt.Errorf(\"side input lookup failed for ts %v: %w\", w.MaxTimestamp(), err)\n}","preventionTips":["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."],"tags":["windowing","side-input","beam","go"],"backgroundTag":"internal-invariant-violation","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T16:17:12.679Z"}