how to create a keep-alive process in Erlang

Question

I'm currently reading Programming Erlang! , at the end of Chapter 13, we want to create a keep-alive process, the example likes:

on_exit(Pid, Fun) ->
    spawn(fun() ->
            Ref = monitor(process, Pid),
            receive
                {'DOWN', Ref, process, Pid, Info} ->
                    Fun(Info)
            end
    end).
keep_alive(Name, Fun) ->
    register(Name, Pid = spawn(Fun)),
    on_exit(Pid, fun(_Why) -> keep_alive(Name, Fun) end).

but when between register/2 and on_exit/2 the process maybe exit, so the monitor will failed, I changed the keep_alive/2 like this:

keep_alive(Name, Fun) ->
    {Pid, Ref} = spawn_monitor(Fun),
    register(Name, Pid),
    receive
        {'DOWN', Ref, process, Pid, _Info} ->
            keep_alive(Name, Fun)
end.

There also an bug, between spawn_monitor/2 and register/2, the process maybe exit. How could this come to run successfully? Thanks.

Answer 1

I'm not sure that you have a problem that needs solving. Monitor/2 will succeed even if your process exits after register/2. Monitor/2 will send a 'DOWN' message whose Info component will be noproc. Per the documentation:

A 'DOWN' message will be sent to the monitoring process if Item dies, if Item does not exist, or if the connection is lost to the node which Item resides on. (see http://www.erlang.org/doc/man/erlang.html#monitor-2).

So, in your original code

1. register assocates Name to the Pid
2. Pid dies
3. on_exit is called and monitor/2 is executed
4. monitor immediately sends a 'DOWN' message which is received by the function spawned by on_exit
5. the Fun(Info) of the received statement is executed calling keep_alive/2

I think all is good.

Answer 2

So why you did't want to use erlang supervisor behaviour? it's provides useful functions for creating and restarting keep-alive processes.

See here the example: http://www.erlang.org/doc/design_principles/sup_princ.html