Question

In Erlang is there any way that a message sender can wait on a response, so it only continues execution once the message has been processed?

And I mean something like this:

Actor ! DoSomething
Continue to this next line of code when DoSomething has been processed

I know a callback can be made by sending the Pid of the sender, but is there any other way to wait?

Answer 1

First thing to understand is that Erlang was built to deal with asynchronous message passing. As such, the only way to have synchronous message passing is to implement something akin to an acknowledgement.

Imagine two processes, P1 and P2. P1 might run the following code:

%% process P1 takes the Pid of P2 as a parameter
%% and a Message to pass on to P2
p1(P2, Message) ->
    P2 ! {self(), Message},
    receive
        {P2, ok}
    after 5000 -> % this section is optional, times out after 5s
        exit("P2 didn t process this!") % this kills P1
    end.

P2, on its side might just run the following:

p2() ->
    receive
        {From, Message} ->
            io:format("P2 received message ~p~n",[Message]),
            %% processing is done!
            From ! {self(), ok}
    end.

So then you might spawn p2 as a new process. This one will sit waiting for any message. When you then call p1, it sends a message to P2, which then processes it (io:format/2) and replies to P1. Because P1 was waiting for a reply, no additional code was run inside that process.

That s the basic and only way to implement blocking calls. The suggestions to use gen_server:call roughly implement what I ve just shown. It s hidden from the programmer, though.

Answer 2

You could use a receive block:

http://www.erlang.org/doc/reference_manual/expressions.html#id2270724

Reading from the doc:

receive never fails. Execution is suspended, possibly indefinitely, until a message arrives that does match one of the patterns and with a true guard sequence.

In other words, send a message and wait for a reply.

Answer 3

If the receiving process is a gen_server, you can use gen_server:call. E.g.:

gen_server:call(Pid, Message),
% At this point, we know that the other process has answered.

Answer 4

No, there is only asynchronous message passing.

If you want to be a little philosophical then it is very difficult to automatically define when a message has been processed. Is it when the message has arrived at the process, been received but not yet acted upon or at sometime when it has been acted upon by the receiving process. It is similar to getting automatic notification when someone has "read" my mail. Yes, they have seen it but they have really read it?

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