Terminated message ordering during ActorSystem termination with watched children

Question

ContextIn Akka Typed, a parent actor that spawns a child and registers a watch via context.watch receives a Terminated signal when that child stops. The documentation specifies that stopping a parent stops its children before the parent completes its own stop sequence, but it does not guarantee the relative ordering of Terminated delivery versus the completion of ActorSystem.terminate().Unresolved behaviorWhen ActorSystem.terminate() is called, the guardian and its supervision tree begin shutting down. A watched child that stops as a consequence of parent termination may deliver Terminated to its watcher either before or after the termination stage completes. This ordering is version-sensitive and not a documented guarantee, making it unsafe to rely on Terminated arrival for resource cleanup that must happen before the system is considered fully stopped.GoalDetermine whether a configuration option, supervision strategy, or explicit coordination pattern exists that ensures Terminated for watched children is processed before ActorSystem.terminate() completes, without introducing custom shutdown barriers.Specific questions:Is there a supported configuration or API that enforces Terminated delivery before termination stage completion?If not, what is the recommended pattern for coordinating resource release that depends on child termination during system shutdown?

Tasadduq Burney · Answer

Direct AnswerNo, there is no supported configuration or API in Akka that enforces the delivery of a Terminated signal for a watched child before ActorSystem.terminate() completes. The relative ordering between signal delivery and the system termination future completion is implementation-dependent and not guaranteed across versions.Confirmed FactsThe termination sequence involves the guardian and children stopping concurrently with Terminated message delivery.Akka Typed treats Terminated as a standard message delivered to the watcher, not a guaranteed pre-termination signal.No configuration key exists to alter this fundamental timing behavior.Recommended Coordination PatternThe recommended approach for safe resource release dependent on child termination is to utilize Akka's Coordinated Shutdown mechanism.Define a custom shutdown phase that executes prior to the before-actor-system-terminate phase.Within this phase, use a mechanism (like a Promise fulfilled by the watcher actor) to explicitly await receipt of the necessary Terminated messages.Only allow the coordinated shutdown process to proceed to the next phase after all required Terminated signals have been processed by the watcher.Alternative PatternIf custom barriers are disallowed, the parent actor must drive the sequence: explicitly context.stop(child), await the resulting Terminated message, and only then invoke ActorSystem.terminate().Diagnostic Detail RequiredTo refine the recommendation, please confirm if your application utilizes Akka Cluster. If so, cleanup phases should be considered within before-cluster-shutdown or before-actor-system-terminate to respect cluster lifecycle ordering.

Terminated message ordering during ActorSystem termination with watched children

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Unresolved behavior

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Direct Answer

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Recommended Coordination Pattern

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