Question: why we need to validate version in command handler?

[davideicardi]

Thanks for sharing your code! Very interesting!

Just one question:
I see that you do an "optimistic lock" by checking the expected version in the command with the version of the snapshot.
Why you need this?

I mean using Kafka "exactly once" does not guarantee that there is only one "thread" that apply commands on a single aggregate, so why checking the version. I would expect that it always match.

[davideicardi]

I have tried to write a similar POC but without the "optimistic lock" and with some changes:
https://github.com/davideicardi/books-catalog-es

I appreciate any feedback.

The most important part is the event sourcing topology described here:
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Code is here: https://github.com/davideicardi/books-catalog-es/blob/master/sample/src/main/scala/catalog/authors/streaming/AuthorsTopology.scala