Add `async_threshold_window` option. #147

README.md

@@ -154,11 +154,13 @@ polls its own mailbox size and toggles the messaging between synchronous and
 asynchronous depending on mailbox size.
 
 ```erlang
-{async_threshold, 20}
+{async_threshold, 20},
+{async_threshold_window, 5}
 ```
 
 This will use async messaging until the mailbox exceeds 20 messages, at which
-point synchronous messaging will be used.
+point synchronous messaging will be used, and switch back to asynchronous, when
+size reduces to `20 - 5 = 15`.
 
 If you wish to disable this behaviour, simply set it to 'undefined'. It defaults
 to a low number to prevent the mailbox growing rapidly beyond the limit and causing

src/lager.app.src

@@ -53,6 +53,9 @@
             %% How many messages per second to allow from error_logger before we start dropping them
             {error_logger_hwm, 50},
             %% How big the gen_event mailbox can get before it is switched into sync mode
-            {async_threshold, 20}
+            {async_threshold, 20},
+            %% Switch back to async mode, when gen_event mailbox size decrease from `async_threshold'
+            %% to async_threshold - async_threshold_window
+            {async_threshold_window, 5}
         ]}
  ]}.

src/lager_app.erl

@@ -42,7 +42,20 @@ start(_StartType, _StartArgs) ->
         {ok, undefined} ->
             undefined;
         {ok, Threshold} when is_integer(Threshold), Threshold >= 0 ->
-            _ = supervisor:start_child(lager_handler_watcher_sup, [lager_event, lager_backend_throttle, Threshold]),
+            DefWindow = erlang:trunc(Threshold * 0.2), % maybe 0?
+            ThresholdWindow =
+                case application:get_env(lager, async_threshold_window) of
+                    undefined ->
+                        DefWindow;
+                    {ok, Window} when is_integer(Window), Window < Threshold, Window >= 0 ->
+                        Window;
+                    {ok, BadWindow} ->
+                        error_logger:error_msg(
+                          "Invalid value for 'async_threshold_window': ~p~n", [BadWindow]),
+                        throw({error, bad_config})
+                end,
+            _ = supervisor:start_child(lager_handler_watcher_sup,
+                                       [lager_event, lager_backend_throttle, [Threshold, ThresholdWindow]]),
             ok;
         {ok, BadThreshold} ->
             error_logger:error_msg("Invalid value for 'async_threshold': ~p~n", [BadThreshold]),

src/lager_backend_throttle.erl

@@ -31,12 +31,13 @@
 
 -record(state, {
         hwm,
+        window_min,
         async = true
     }).
 
-init(Hwm) ->
+init([Hwm, Window]) ->
     lager_config:set(async, true),
-    {ok, #state{hwm=Hwm}}.
+    {ok, #state{hwm=Hwm, window_min=Hwm - Window}}.
 
 
 handle_call(get_loglevel, State) ->
@@ -48,12 +49,12 @@ handle_call(_Request, State) ->
 
 handle_event({log, _Message},State) ->
     {message_queue_len, Len} = erlang:process_info(self(), message_queue_len),
-    case {Len > State#state.hwm, State#state.async} of
-        {true, true} ->
+    case {Len > State#state.hwm, Len < State#state.window_min, State#state.async} of
+        {true, _, true} ->
             %% need to flip to sync mode
             lager_config:set(async, false),
             {ok, State#state{async=false}};
-        {false, false} ->
+        {_, true, false} ->
             %% need to flip to async mode
             lager_config:set(async, true),
             {ok, State#state{async=true}};