NgRx Effects concurrency mapping for race condition prevention

Direct Answer

exhaustMap is the appropriate flattening operator when you want to ignore new incoming actions while an asynchronous operation is already in progress. It prevents duplicate submissions without requiring manual loading-state tracking in the reducer.

How exhaustMap Differs from switchMap

Operator	Behavior on New Action During Active Request	Typical Use Case
exhaustMap	Ignores the new action; current request continues uninterrupted	Form submissions, save buttons, navigation triggers where duplicates must be dropped
switchMap	Cancels the in-flight request and starts a new one	Search/typeahead, filter changes where only the latest request matters

Implementation Pattern

// Functional effect (NgRx 14+)
export const saveData$ = createEffect(() =>
  actions$.pipe(
    ofType(saveDataAction),
    exhaustMap(({ payload }) =>
      dataService.save(payload).pipe(
        map(result => saveSuccess({ result })),
        catchError(error => of(saveFailure({ error })))
      )
    )
  )
);

The exhaustMap operator subscribes to the inner observable (the HTTP call) and ignores any further saveDataAction emissions until that inner observable completes or errors. This satisfies both constraints: unidirectional flow is preserved because the effect only dispatches new actions (saveSuccess or saveFailure), and no loading flag is needed in the reducer.

Verification Steps

1. Open the Network tab in DevTools.
2. Dispatch the same action rapidly (e.g., double-click a save button).
3. Confirm only one request appears and subsequent clicks produce no network activity until the first completes.

One Diagnostic Detail That Could Change the Recommendation

If the user must be notified that their action was ignored (e.g., toast message "Already saving"), exhaustMap alone cannot emit a notification for dropped actions. In that case, a custom operator or a concatMap with a guard in the service layer would be needed. Does your UI require feedback for ignored duplicate triggers?