correct usage of erlang spawn_monitor

Question

Still working through Joe's book, and having hard time fully understanding monitors in general and spawn_monitor in particular. Here's the code I have; the exercise is asking to write a function that will start a process whose job is to print a heartbeat every 5 seconds, and then a function to monitor the above process and restart it. I didn't get to a restart part, because my monitor fails to even detect the process keeling over.

% simple "working" loop
loop_5_print() ->
    receive
    after 5000 ->
            io:format("I'm still alive~n"),
            loop_5_print()
    end.

% function to spawn and register a named worker
create_reg_keep_alive(Name) when not is_atom(Name) ->
    {error, badargs};
create_reg_keep_alive(Name) ->
    Pid = spawn(ex, loop_5_print, []),
    register(Name, Pid),
    {Pid, Name}.

% a simple monitor loop
monitor_loop(AName) ->
    Pid = whereis(AName),
    io:format("monitoring PID ~p~n", [Pid]),
    receive
        {'DOWN', _Ref, process, Pid, Why} ->
            io:format("~p died because ~p~n",[AName, Why]),
            % add the restart logic
            monitor_loop(AName)
    end.

% function to bootstrapma monitor
my_monitor(AName) ->
    case whereis(AName) of
        undefined -> {error, no_such_registration};

        _Pid -> spawn_monitor(ex, monitor_loop, [AName])
    end.

And here's me playing with in:

39> c("ex.erl").                    
{ok,ex}
40> ex:create_reg_keep_alive(myjob).
{<0.147.0>,myjob}
I'm still alive                     
I'm still alive          
41> ex:my_monitor(myjob).
monitoring PID <0.147.0>
{<0.149.0>,#Ref<0.230612052.2032402433.56637>}
I'm still alive
I'm still alive                     
42> exit(whereis(myjob), stop).
true
43>

It sure stopped the loop_5_print "worker" - but where's the line that the monitor was supposed to print? The only explanation that I see is that the message emitted by a process quitting in this manner isn't of the pattern on which I am matching inside monitor loop's receive. But that's the only pattern introduced in the book in this chapter, so I'm not buying this explanation..

user_320615 · Answer

spawn_monitor is not what you want here. spawn_monitor spawns a process and immediately starts monitoring it. When the spawned process dies, the process that called spawn_monitor gets a message that the process is dead. You need to call erlang:monitor/2 from the process that you want to receive the DOWN messages in, with the second argument being the Pid to monitor.

Just add:

monitor(process, Pid),

after:

Pid = whereis(AName),

and it works:

1> c(ex).
{ok,ex}
2> ex:create_reg_keep_alive(myjob).
{<0.67.0>,myjob}
I'm still alive
I'm still alive
I'm still alive
3> ex:my_monitor(myjob).
monitoring PID <0.67.0>
{<0.69.0>,#Ref<0.2696002348.2586050567.188678>}
I'm still alive
I'm still alive
I'm still alive
4> exit(whereis(myjob), stop).
myjob died because stop
true
monitoring PID undefined

correct usage of erlang spawn_monitor

1 Answers