Question

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.

Answer 1

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

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