apache/flink · error · IllegalStateException

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

Error message

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

What it means

MergingWindowSet tracks the in-flight (active) windows of a merging assigner (session windows) and maps each window to the state window that holds its contents. retireWindow() removes a window from this set and throws IllegalStateException when mapping.remove(window) returns null, i.e. the window was never added or was already retired. When this fires inside a window operator it means the 'merging-window-set' state is inconsistent with the operator's view of active windows, which points to corrupted/half-updated state or a lifecycle bug, not to bad user data.

Source

Thrown at flink-datastream/src/main/java/org/apache/flink/datastream/impl/extension/window/operators/MergingWindowSet.java:120

     * {@code Window} in which we keep the actual state of a given in-flight window. Windows might
     * expand but we keep to original state window for keeping the elements of the window to avoid
     * costly state juggling.
     *
     * @param window The window for which to get the state window.
     */
    public W getStateWindow(W window) {
        return mapping.get(window);
    }

    /**
     * Removes the given window from the set of in-flight windows.
     *
     * @param window The {@code Window} to remove.
     */
    public void retireWindow(W window) {
        W removed = this.mapping.remove(window);
        if (removed == null) {
            throw new IllegalStateException(
                    "Window " + window + " is not in in-flight window set.");
        }
    }

    /**
     * Adds a new {@code Window} to the set of in-flight windows. It might happen that this triggers
     * merging of previously in-flight windows. In that case, the provided {@link MergeFunction} is
     * called.
     *
     * <p>This returns the window that is the representative of the added window after adding. This
     * can either be the new window itself, if no merge occurred, or the newly merged window. Adding
     * an element to a window or calling trigger functions should only happen on the returned
     * representative. This way, we never have to deal with a new window that is immediately
     * swallowed up by another window.
     *
     * <p>If the new window is merged, the {@code MergeFunction} callback arguments also don't
     * contain the new window as part of the list of merged windows.
     *

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Inspect the task logs for an earlier exception (from a WindowProcessFunction, trigger, or MergeFunction) in the same task before this one - a half-applied update is the usual root cause.
2. If a custom MergingWindowAssigner is in use, audit its mergeWindows() contract: merged-away windows and the result window must be reported consistently for identical inputs.
3. Test checkpoint-restore behavior with the same job under load; if the error only appears after restore, verify savepoint compatibility of the 'merging-window-set' state across the Flink versions involved.
4. If the stack trace contains no user code and no prior exception, collect job graph, assigner, state backend and checkpoint history and file a Flink JIRA - this is an internal invariant violation in an @Experimental extension.

// before retiring a window, confirm it is still in flight
if (mergingWindows.getStateWindow(window) != null) {
    mergingWindows.retireWindow(window);
}

When it happens

Trigger: A session-window operator calls mergingWindows.retireWindow(actualWindow) after WindowUtils.isWindowLate(...) flags the window as late; the throw happens when the same window was already retired (double retirement) or was never inserted by MergingWindowSet.addWindow(). Reached with EventTimeSessionWindows / ProcessingTimeSessionWindows when cleanup timers and record processing disagree, or when an earlier exception (from a MergeFunction or user WindowProcessFunction) left the window set partially updated.

Common situations: Restore from a checkpoint/savepoint taken while a merge was in progress; exceptions thrown from user window functions that leave the window set half-updated; custom MergingWindowAssigner implementations whose mergeWindows() results are inconsistent; Flink version changes that altered the 'merging-window-set' state layout.

Related errors

• Window {window} is not in in-flight window set.
• Window {window} is not in in-flight window set.
• Window {window} is not in in-flight window set.
• The window uses a merging assigner, but the window state is
• The end timestamp of an event-time window cannot become earl

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/e8f721b13adfeb35. Report an issue: GitHub.