eLog/src/backend/rumBackendThrottle.erl

%% @doc A simple gen_event backend used to monitor mailbox size and
%% switch log messages between synchronous and asynchronous modes.
%% A gen_event handler is used because a process getting its own mailbox
%% size doesn't involve getting a lock, and gen_event handlers run in their
%% parent's process.

-module(rumBackendThrottle).

-behaviour(gen_event).

-include("eRum.hrl").

-export([
   init/1
   , handle_call/2
   , handle_event/2
   , handle_info/2
   , terminate/2,
   code_change/3
]).

-record(state, {
   sink :: atom(),
   hwm :: non_neg_integer(),
   window_min :: non_neg_integer(),
   async = true :: boolean()
}).

init([{sink, Sink}, Hwm, Window]) ->
   rumConfig:set({Sink, async}, true),
   {ok, #state{sink = Sink, hwm = Hwm, window_min = Hwm - Window}}.

handle_call(get_loglevel, State) ->
   {ok, {mask, ?LOG_NONE}, State};
handle_call({set_loglevel, _Level}, State) ->
   {ok, ok, State};
handle_call(_Request, State) ->
   {ok, ok, State}.

handle_event({mWriteLog, _Message}, State) ->
   {message_queue_len, Len} = erlang:process_info(self(), message_queue_len),
   case {Len > State#state.hwm, Len < State#state.window_min, State#state.async} of
      {true, _, true} ->
         %% need to flip to sync mode
         rumConfig:set({State#state.sink, async}, false),
         {ok, State#state{async = false}};
      {_, true, false} ->
         %% need to flip to async mode
         rumConfig:set({State#state.sink, async}, true),
         {ok, State#state{async = true}};
      _ ->
         %% nothing needs to change
         {ok, State}
   end;
handle_event(_Event, State) ->
   {ok, State}.

handle_info(_Info, State) ->
   {ok, State}.

%% @private
terminate(_Reason, _State) ->
   ok.

%% @private
code_change(_OldVsn, State, _Extra) ->
   {ok, State}.