I need to keep a gen_mod process running as it loops every minute and does some cleanup. However once every few days it will crash and I'll have to manually start it back up again.
I could use a basic example of implementing a supervisor into ejabberd_sup so it can keep going. I am struggling to understand the examples that use gen_server.
Thanks for the help.
Here's an example module combining ejabberd's gen_mod and OTP's gen_server. Explanation is inlined in the code.
-module(cleaner).
-behaviour(gen_server).
-behaviour(gen_mod).
%% gen_mod requires these exports
-export([start/2, stop/1]).
%% these are exports for gen_server
-export([start_link/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
terminate/2, code_change/3]).
-define(INTERVAL, timer:minutes(1)).
-record(state, {}).
%% ejabberd calls this function when this module is loaded
%% basically it adds gen_server defined by this module to
%% ejabberd main supervisor
start(Host, Opts) ->
Proc = gen_mod:get_module_proc(Host, ?MODULE),
ChildSpec = {Proc,
{?MODULE, start_link, [Host, Opts]},
permanent,
1000,
worker,
[?MODULE]},
supervisor:start_child(ejabberd_sup, ChildSpec).
%% this is called by ejabberd when module is unloaded, so it
%% does the opposite of start/2 :)
stop(Host) ->
Proc = gen_mod:get_module_proc(Host, ?MODULE),
supervisor:terminate_child(ejabberd_sup, Proc),
supervisor:delete_child(ejabberd_sup, Proc).
%% it will be called by supervisor when it is time to start
%% this gen_server under control of supervisor
start_link(_Host, _Opts) ->
gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
%% it is an initialization function for gen_server
%% it starts a timer, which sends 'tick' message periodically to itself
init(_) ->
timer:send_interval(?INTERVAL, self(), tick),
{ok, #state{}}.
handle_call(_Request, _From, State) ->
Reply = ok,
{reply, Reply, State}.
handle_cast(_Msg, State) ->
{noreply, State}.
%% this function is called whenever gen_server receives a 'tick' message
handle_info(tick, State) ->
State2 = do_cleanup(State),
{noreply, State2};
handle_info(_Info, State) ->
{noreply, State}.
terminate(_Reason, _State) ->
ok.
code_change(_OldVsn, State, _Extra) ->
{ok, State}.
%% this function is called by handle_info/2 when tick message is received
%% so put all cleanup code here
do_cleanup(State) ->
%% do all cleanup work here
State.
This blog post gives a good explanation how gen_servers work. Of course make sure to re-read OTP design principles on gen_server and on supervisor.
Ejabberd's module developement is described here
