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

You have two easy alternatives, use timer:send_interval/2 or erlang:send_after/3. send_interval is easier to setup, while send_after (when used in the Erlang module) is more reliable since it is a built-in function, see the Efficiency Guide.

Using send_after also ensures that the gen_server process is not overloaded. If you were using the send_interval function you would get a message regardless if the process can keep up or not. With send_after being called just before the return in handle_info you only schedule a new message once you handled the previous one. If you want more accurate time tracking you can still schedule a send_after with the time set dynamically to something lower than ?INTERVAL (or even 0) to catch up.

I would recommend something along the following lines in your gen_server:

-define(INTERVAL, 60000). % One minute

init(Args) ->
   ... % Start first timer
   erlang:send_after(?INTERVAL, self(), trigger),
   ...

handle_info(trigger, State) ->
   ... % Do the action
   ... % Start new timer
   erlang:send_after(?INTERVAL, self(), trigger),
   ...

Instead of trigger you could send something with a state if it is needed, like {trigger, Count} or something.

Answer 2

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,
    ...

Answer 3

There is actually a built-in mechanism within gen_server to accomplish the same thing. If the third element of response tuple of the init, handle_call, handle_cast or handle_info methods in the gen_server is an integer, a timeout message wil be sent to the process after that period of time in millisecs... which should be handled using handle_info. For eg :

init(Args) ->
   ... % Start first timer
   {ok, SomeState, 20000}. %% 20000 is the timeout interval

handle_call(Input, From, State) ->
   ... % Do something
   ... % Do something else
   {reply, SomeState, 20000}. %% 20000 is the timeout interval

handle_cast(Input, State) ->
   ... % Do something
   ... % Do something else
   {noreply, SomeState, 20000}. %% 20000 is the timeout interval


%% A timeout message is sent to the gen_server to be handled in handle_info %%
handle_info(timeout, State) ->
   ... % Do the action
   ... % Start new timer
   {noreply, SomeState, 20000}. %% "timeout" can be sent again after 20000 ms