Drops stale events from the current payload to only include events that reflect an entity state that is more recent than previously received events.

Helps to ignore events received out-of-order (e.g. an update event about en entity received after the delete event for that same entity), given that the Routemaster event bus server makes no guarantee of in-order delivery of events.

Implementation Details

First it removes duplicates by URL from the current payload and only preserves the most recent event for each URL. Then it compares the timestamp of the remaining events with the Redis-backed Routemaster.Drain.EventState data, to check if the event is newer than the latest known state of the resource at that URL. If an event is indeed fresher than the latest known state, the state is updated in Redis with the timestamp of the fresh event.

Since the output of this middleware should be a single event per URL, filtering the events first and only checking the most recent one will reduce the number of Redis calls.

The Redis calls to read and save the event states are executed one by one. While reading them in bulk would be more efficient, it would not work correctly because multiple batches can be received and processed concurrently, and out-of-order events for the same resources could be spread in more than one batch.