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No such thing as exactly-once delivery
The differences between at-most-once, at-least-once, and exactly-once delivery in messaging systems.
If you have an open transaction with your database and recently verified your TCP connection to your receiver is still good, you've really improved your odds. If one message will cause 10 side effects, be mindful the worker can crash at any point during that process and leave your system in an inconsistent state. The idea is that if the system doesn't hear back from a worker, it will assume it crashed, and so redeliver the message.
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