[erlang-questions] Order of message processing by a gen_server

Jack Orenstein <>
Wed Dec 5 07:22:52 CET 2007


On Dec 5, 2007, at 12:40 AM, Lev Walkin wrote:

> Jack Orenstein wrote:
>> Suppose I have a gen_server process whose queue contains {a} and  
>> {b}.  Does the order of the handle_call function clauses determine  
>> which  message is processed first? I.e., if the function clauses are
>>      handle_call({a}, From, State) -> ...
>>      handle_call({b}, From, State) -> ...
>> is it guaranteed that {a} will be processed first?
>
> Yes, but this is implementation detail and is not guaranteed
> by the standard.

What I am trying to do is to write a server that will process a  
series of requests, always the same requests (phase1, phase2, phase3)  
in the same order. But I want to be able to cancel the sequence of  
requests at any point, resetting the process state to be ready for  
phase1 again. If I write my own function with a receive, I can do this:

          loop(State) ->
              receive
                  cancel -> ...
                  phase1 -> ...
                  phase2 -> ...
                  phase3 -> ...
              end.

Normally this server will receive messages phase1, phase2, phase3 in  
order. But if a cancel message shows up, e.g. after phase2, it takes  
priority (if I understand receive correctly).

I'd like to use a gen_server, but it sounds like I can't do anything  
that is guaranteed by the language (not the implementation) to handle  
the cancel message with higher priority than any of the other  
messages. I.e.,

         handle_call({cancel}, From, State) -> ...
         handle_call({phase1}, From, State) -> ...
         handle_call({phase2}, From, State) -> ...
         handle_call({phase3}, From, State) -> ...

isn't guaranteed to work similarly to the loop function above.

Any advice on how to proceed?

Jack Orenstein




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