How to perform actions periodically with Erlang's gen_server?

Question

I want to start a gen_server that additionally, will perform one action every minute.

What is the best way to schedule that?

Answer 1

To precisely control the timer, you may want to use erlang:start_timer, and save each timer reference you have created.

erlang:start_timer has a tiny difference with erlang:send_after, see http://www.erlang.org/doc/man/erlang.html#start_timer-3 and http://www.erlang.org/doc/man/erlang.html#send_after-3

Example use case:

init(Args) ->
    ...
    TRef = erlang:start_timer(?INTERVAL, self(), trigger),
    State = #state{tref = TRef},
    ...

handle_info({timeout, _Ref, trigger}, State) ->
    %% With this cancel call we are able to manually send the 'trigger' message 
    %% to re-align the timer, and prevent accidentally setting duplicate timers
    erlang:cancel(State#state.tref),
    ...
    TRef = erlang:start_timer(?INTERVAL, self(), trigger),
    NewState = State#state{tref = TRef},
    ...

handle_cast(stop_timer, State) ->
    TRef = State#state.tref,
    erlang:cancel(TRef),

    %% Remove the timeout message that may have been put in our queue just before 
    %% the call to erlang:cancel, so that no timeout message would ever get 
    %% handled after the 'stop_timer' message
    receive
        {timeout, TRef, _} -> void
        after 0 -> void
    end,
    ...