global_singleton_supervisor.erl

%% Processes supervised by this supervisor will only exist in one
%% place in a cluster. If it goes down on any node, some node will
%% start it back up again.

-module(global_singleton_supervisor).

-behaviour(supervisor).

%% API
-export([start_link/0]).

%% Supervisor callbacks
-export([init/1]).

start_link() ->
    case supervisor:start_link({global, ?MODULE}, ?MODULE, []) of
    {ok, Pid} -> {ok, Pid};
    {error, {already_started, Pid}} ->
        {ok, spawn_link(fun () -> watch(Pid) end)}
    end.

watch(Pid) ->
    process_flag(trap_exit, true),
    erlang:monitor(process, Pid),
    error_logger:info_msg("Monitoring global singleton at ~p~n", [Pid]),
    receive
    LikelyExit ->
        error_logger:info_msg("Global singleton supervisor at ~p exited. Restarting.~n",
                              [Pid])
    end.


init([]) ->
    {ok,{{one_for_all, 5, 5},
         [
          %% Everything in here is run once per entire cluster.  Be careful.
          %% Put all your stuff to be supervised here.
         ]}}.