Idempotent consumer pattern – A piece of the eventual consistency puzzle (pradeeploganathan.com)

Keep in mind the race condition in the flow-chart. The algorithm as described will not be safe if the same key is updated concurrently.

I found these two articles on idempotency very well written:

https://brandur.org/http-transactions https://brandur.org/idempotency-keys

A key means of obtaining total order broadcast