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

After MergingWindowSet.addWindow(...) returns the post-merge representative window (actualWindow) and the lateness check passes, OneInputWindowProcessOperator looks up the state window holding that window's contents via getStateWindow(actualWindow). A null result means the in-flight window set has no mapping for a window that addWindow just returned - an internal invariant violation indicating inconsistent 'merging-window-set' state. Note the message prints the originally assigned 'window' while the lookup used 'actualWindow', so the reported window can be misleading.

Solutions

1. Check the task logs for an earlier exception (user WindowProcessFunction, trigger, MergeFunction) - the half-applied state update is the root cause; fix that first.
2. If the window type is custom, verify its equals() and hashCode() are consistent with the window serializer used for the 'merging-window-set' state.
3. Do not restore state taken with a different window assigner configuration (changed session gap/size or window class); start a fresh checkpoint chain instead.
4. Otherwise capture job graph, assigner, backend and checkpoint history and file a Flink JIRA; do not blind-retry as restart loops will re-hit the same inconsistent state.

// the throw happens inside the task; surface it with context instead of blind restarts
try {
    env.execute("session-job");
} catch (IllegalStateException e) {
    if (e.getMessage() != null && e.getMessage().contains("not in in-flight window set")) {
        // invariant break: capture job id + last checkpoint, alert, and stop
        throw new IllegalStateException("window set inconsistent - see checkpoint " + lastCpId, e);
    }
    throw e;
}

When it happens

Trigger: getStateWindow(actualWindow) returns null after addWindow promised a mapping. Reached when a previous merge callback or user function threw after mutating state (half-applied update), when the restored 'merging-window-set' state does not match the current assigner/window serializer (equals/hashCode mismatch), or via bugs in the @Experimental window extension itself.

Common situations: Custom window types whose equals()/hashCode() disagree with the window serializer so map lookups miss; restoring a savepoint taken with a different session gap or window class; exceptions from user code near the merge path leaving partial updates; Flink version skew between checkpoint producer and consumer.

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 a processing-time window cannot become…

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/4d14e901f79a726b. Report an issue: GitHub.

Appendix: source

Thrown at flink-datastream/src/main/java/org/apache/flink/datastream/impl/extension/window/operators/OneInputWindowProcessOperator.java:307

                                        // 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);
                outputCollector.setTimestamp(window.maxTimestamp());
                windowFunctionContext.setWindow(window);
                windowProcessFunction.onRecord(
                        element.getValue(),
                        outputCollector,
                        partitionedContext,
                        windowFunctionContext);

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

                TriggerResult triggerResult = triggerContext.onElement(element);

                if (triggerResult.isFire()) {

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