favetelinguis
Message delivery order guarantees for GenServer
I know there is call and cast. The difference is that call blocks the caller until a reply is made and cast returns right away not blocking the caller. However the scenario im interested in is the following.
Given two processes A and B. A send two messages A1 and A2 to B and it want to send them as fast as possible but it must also guarantee that A1 is handled before A2 by B.
Now the problem I have with this is that if I use call A must wait until A1 is processed by B, I want to use the message queue of B to send messages. And if i use cast there is no order guarantees. A2 could get processed before A1.
What is the idiomatic Elixir code for this case?
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kokolegorille
From Erlang’s doc
10.8 Is the order of message reception guaranteed?
Yes, but only within one process.
If there is a live process and you send it message A and then message B, it’s guaranteed that if message B arrived, message A arrived before it.
On the other hand, imagine processes P, Q and R. P sends message A to Q, and then message B to R. There is no guarantee that A arrives before B. (Distributed Erlang would have a pretty tough time if this was required!)
PS. It’s not the same async as in JS 
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