apache/flink · error · IllegalStateException

Window is not in in-flight window set.

Error message

Window {window} is not in in-flight window set.

What it means

In TwoOutputWindowProcessOperator, after addWindow returns the post-merge representative window (actualWindow), getStateWindow(actualWindow) must yield the state window holding its contents; null means the in-flight window set is inconsistent with addWindow's contract and IllegalStateException is thrown. As elsewhere, the message reports the originally assigned 'window', not the actualWindow used in the failed lookup.

Solutions

1. Fix any earlier exception in the same task logs first.
2. Audit custom window equals()/hashCode() against the window serializer.
3. Use a fresh checkpoint chain after changing assigner/window semantics.
4. Report invariant breaks without user-code involvement as Flink bugs with full context.

try {
    env.execute("two-output-session-job");
} catch (Exception e) {
    if (e instanceof IllegalStateException
            && String.valueOf(e.getMessage()).contains("not in in-flight window set")) {
        haltWithDiagnostics(jobId, lastCheckpoint, e); // do not blind-restart
    }
    throw e;
}

When it happens

Trigger: A record's session merge completes but the 'merging-window-set' has no mapping for the returned window: half-applied state from an earlier exception, restore-state mismatch with the window serializer/assigner, or an extension bug.

Common situations: Custom windows with equals/hashCode inconsistent with serialization; savepoint restore across assigner changes; exceptions from two-output window functions during merges; version skew.

Related errors

• Window is not in in-flight window set.
• Window is not in in-flight window set.
• Window is not in in-flight window set.
• The end timestamp of a processing-time window cannot become…
• The end timestamp of an event-time window cannot become…

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/ac67bade12cada34. Report an issue: GitHub.

Appendix: source

Thrown at flink-datastream/src/main/java/org/apache/flink/datastream/impl/extension/window/operators/TwoOutputWindowProcessOperator.java:316

                                        // merge the merged state windows into the newly resulting
                                        // state window
                                        windowMergingState.mergeNamespaces(
                                                stateWindowResult, mergedStateWindows);
                                    }
                                });

                // drop if the window is already late
                if (WindowUtils.isWindowLate(
                        actualWindow, windowAssigner, internalTimerService, allowedLateness)) {
                    mergingWindows.retireWindow(actualWindow);
                    continue;
                }
                isSkippedElement = false;

                W stateWindow = mergingWindows.getStateWindow(actualWindow);
                if (stateWindow == null) {
                    throw new IllegalStateException(
                            "Window " + window + " is not in in-flight window set.");
                }

                windowState.setCurrentNamespace(stateWindow);
                mainCollector.setTimestamp(window.maxTimestamp());
                sideCollector.setTimestamp(window.maxTimestamp());
                windowFunctionContext.setWindow(window);
                windowProcessFunction.onRecord(
                        element.getValue(),
                        mainCollector,
                        sideCollector,
                        partitionedContext,
                        windowFunctionContext);

                triggerContext.setKey(key);
                triggerContext.setWindow(actualWindow);

                TriggerResult triggerResult = triggerContext.onElement(element);

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