In what order does an Erlang process consume messages?

Question

Are messages processed in a first-come-first-serve basis or are they sorted by timestamp or something like that?

Answer 1

Order of messages is preserved between a process and another one. Reading from the FAQ:

10.9 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!)

@knutin is right regarding how you can consume messages within a process. As an addition, note that you might use two subsequent receive statements to ensure that a certain message is consumed after another one:

receive
  first ->
    do_first()
end,
receive
  second ->
    do_second()
end

The receive statement is blocking. This will ensure that you never do_second() before you do_first(). The difference from @knutin's second solution is that, in that case, if something not important arrives just before an important one, you queue the important bit.