"Trying to insert an Event for an aggregate with a sequence number that is already present in the Event Store".

[Joseph Sintim-Amoah]

We are beginning to see to these errors in our logs:

     "Trying to insert an Event for an aggregate with a sequence number that is already present in the Event Store".

@Allard
             If Allard or anyone else have come across these error before, let me know what did you do to resolve it. Anyone?
             Thanks.
    

[Allard Buijze]

Hi Joseph,

this is quite common of you have a multi-node setup, where commands are divided between the nodes randomly (or at least inconsistently). I have also seen this error pop up in cases where only a single node runs the application. It had to do with the database's transaction isolation settings. Setting it to read-committed (or read-uncomitted, if you prefer) resolved that issue.

Some background:

Axon uses locking. Imagine Thread 1 and Thread 2 sending a command that routes to the same aggregate instance. The command bus starts a transaction for both (usually this means a connection is also acquired). Thread 1 loads the aggregate, while Thread 2 blocks at the lock. Thread 1 finishes processing and publishes the results. Thread 2 is released and loads the aggregate. Because the transaction was started before Thread 1 committed, it is possible that the database doesn't show Thread 1's results because of the isolation level.

If you use Spring, I also recommend using the LazyConnectionDataSourceProxy, as it will postpone getting the actual connection until it's really used.

Cheers,

Allard

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[Joseph Sintim-Amoah]

Thanks Allard. My team and I try the pointers provided.

Have a great day,

Joseph

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Joseph Sintim-Amoah
Senior Software Engineer, Consumer Platforms

One LifeWay Plaza
Nashville, TN 37234

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http://tech.lifeway.com