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Consider you are having two or more instances of a the same service, in a microservices setup using java/spring/hibernate. To make this example a bit more clear let’s assume we have:
booksservice, which creates/updates books
book-detailsservice which will process details of the books
tasksservice which will create some task for the created/updated books
What I’m trying to solve and find out the best way to handle this is related to the following scenarios:
- The same event (for example book 1 was updated) is emitted multiple times
book 1 updatedupdated event is picked by
book-detailsservice which after its processing will emit
book 1 details updated event. Both those events will be handled in
tasks will eventually do is to do a check if exists and create or remove task for this subject (book 1 in this case). This can be a complex operation that cannot be an atomic db transaction.
Given that multiple instances subscribe to the same topic, the same event can be processed at the same time by multiple instances causing integrity constraint errors (due to unique indexes).
How this can be properly handled? The ideal would be having
book 1 processing by instance #1 but that not blocking
book 2 being processed by instance #2. While same
book 1 events being queued up for the same instance.
You can use distributed locks and maps in your case. Hazelcast for example.
- Get lock by event(or event id..) – to prevent execution on differents instances
- Check map ( as example
IMap<BookEvent, boolean wasProcessed>), if this event was already processed on another instance – return
- Process your event
- Put event and status to map