Github user markhamstra commented on the issue: https://github.com/apache/spark/pull/16291 > You are right, that's why the executor service is single threaded which guarantees ordered processing of the events per listener. But this is still substantially different from the ordering guarantees in the current implementation, and I'm not sure that this PR wouldn't invalidate some assumptions made by current listeners. Events will end up in the same order in each `ListenerEventExecutor` (at least by my understanding, which someone else should double-check), but there is no synchronization between `ListenerEventExecutor`s of the processing of their `eventQueue`s; so it is entirely possible. for example, for one `ListenerEventExecutor` to process a task end event for a particular task before another `ListenerEventExecutor` has worked sufficiently through its `eventQueue` to have even seen the corresponding task start event. That is quite a bit different than the prior ordering guarantee implied by the comment "`onPostEvent` is guaranteed to be called in the same thread for all listeners." Whether this actually constitutes a problem or not, I'm not certain; but serializing event processing within each `ListenerEventExecutor` isn't enough if `Listener`s on different `ListenerEventExecutor` threads expect to share mutable state.
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