Implement a metrics HTTP endpoint.

README.md

@@ -83,3 +83,11 @@ erldns_zone_cache:put_zone({
     }
   ]}).
 ```
+
+## Metrics
+
+Folsom is used to gather runtime metrics and statistics. There is an HTTP server included that produces a JSON report containing these metrics. Here's an example script that shows how to get the output with curl and pass through Python to format it in a pretty fashion.
+
+```sh
+curl -s http://localhost:8082/ -H "Accept: application/json" | python -mjson.tool
+```

ebin/erldns.app

@@ -1,12 +1,13 @@
 {application,erldns,
              [{description,"Erlang Authoritative DNS Server"},
-              {vsn,"5321c5a"},
+              {vsn,"8bdd3db"},
               {mod,{erldns_app,[]}},
               {applications,[kernel,stdlib,inets,crypto,ssl,folsom]},
               {start_phases,[{post_start,[]}]},
               {modules,[erldns,erldns_app,erldns_axfr,erldns_config,
                         erldns_dnssec,erldns_edns,erldns_encoder,
                         erldns_event_logger,erldns_events,erldns_handler,
+                        erldns_metrics,erldns_metrics_root_handler,
                         erldns_packet_cache,erldns_query_throttle,
                         erldns_records,erldns_resolver,erldns_server_sup,
                         erldns_sup,erldns_tcp_server,erldns_txt,

rebar.config

@@ -14,6 +14,7 @@
 {deps, [
         {lager, ".*", {git, "git://github.com/basho/lager.git", "HEAD"}},
         {folsom, ".*", {git, "git://github.com/boundary/folsom.git", "HEAD"}},
+        {cowboy, ".*", {git, "git://github.com/extend/cowboy.git", {tag, "0.8.6"}}},
         {poolboy, ".*", {git, "git://github.com/devinus/poolboy.git", "HEAD"}},
         {jsx, ".*", {git, "git://github.com/talentdeficit/jsx.git", "HEAD"}},
         {dns, ".*", {git, "git://github.com/aeden/dns_erlang.git", "HEAD"}},

src/erldns.erl

@@ -8,4 +8,6 @@ start() ->
   ssl:start(),
   lager:start(),
   folsom:start(),
+  application:start(ranch),
+  application:start(cowboy),
   application:start(erldns).

src/erldns_app.erl

@@ -4,7 +4,7 @@
 % Application hooks
 -export([start/2, start_phase/3, stop/1]).
 
--export([get_metrics/0, get_stats/0]).
+-export([metrics/0, stats/0]).
 
 start(_Type, _Args) ->
   define_metrics(),
@@ -45,13 +45,13 @@ stop(_State) ->
   lager:info("Stop erldns application"),
   ok.
 
-get_metrics() ->
+metrics() ->
   lists:map(
     fun(Name) ->
         {Name, folsom_metrics:get_metric_value(Name)}
     end, folsom_metrics:get_metrics()).
 
-get_stats() ->
+stats() ->
   Histograms = [udp_handoff_histogram, tcp_handoff_histogram, request_handled_histogram],
   lists:map(
     fun(Name) ->

src/erldns_metrics.erl

@@ -0,0 +1,58 @@
+-module(erldns_metrics).
+
+-behavior(gen_server).
+
+-export([start_link/0]).
+
+-define(DEFAULT_PORT, 8082).
+
+% Gen server hooks
+-export([init/1,
+    handle_call/3,
+    handle_cast/2,
+    handle_info/2,
+    terminate/2,
+    code_change/3
+  ]).
+
+-record(state, {}).
+
+start_link() ->
+  gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
+
+init([]) ->
+  lager:debug("Starting ~p", [?MODULE]),
+
+  Dispatch = cowboy_router:compile(
+    [
+      {'_', 
+        [
+          {"/", erldns_metrics_root_handler, []}
+        ]
+      }
+    ]
+  ),
+
+  {ok, _} = cowboy:start_http(http, 10, [{port, port()}], [{env, [{dispatch, Dispatch}]}]),
+
+  {ok, #state{}}.
+
+handle_call(_Message, _From, State) ->
+  {reply, ok, State}.
+handle_cast(_, State) ->
+  {noreply, State}.
+handle_info(_, State) ->
+  {noreply, State}.
+terminate(_, _) ->
+  ok.
+code_change(_PreviousVersion, State, _Extra) ->
+  {ok, State}.
+
+port() ->
+ proplists:get_value(port, metrics_env(), ?DEFAULT_PORT).
+
+metrics_env() ->
+  case application:get_env(erldns, metrics) of
+    {ok, MetricsEnv} -> MetricsEnv;
+    _ -> []
+  end.

src/erldns_metrics_root_handler.erl

@@ -0,0 +1,59 @@
+-module(erldns_metrics_root_handler).
+
+-export([init/3]).
+-export([content_types_provided/2]).
+-export([to_html/2, to_json/2, to_text/2]).
+
+-export([filter_stats/1]).
+
+init(_Transport, _Req, []) ->
+  {upgrade, protocol, cowboy_rest}.
+
+content_types_provided(Req, State) ->
+  {[
+      {<<"text/html">>, to_html},
+      {<<"text/plain">>, to_text},
+      {<<"application/json">>, to_json}
+    ], Req, State}.
+
+to_html(Req, State) ->
+  {<<"erldns metrics">>, Req, State}.
+
+to_text(Req, State) ->
+  {<<"erldns metrics">>, Req, State}.
+
+to_json(Req, State) ->
+  Body = jsx:encode([{<<"erldns">>, 
+        [
+          {<<"metrics">>, erldns_app:metrics()},
+          {<<"stats">>, filter_stats(erldns_app:stats())}
+        ]
+      }]),
+  {Body, Req, State}.
+
+% Functions to clean up the stats so they can be returned as JSON.
+filter_stats(Stats) ->
+  filter_stats(Stats, []).
+
+filter_stats([], FilteredStats) ->
+  FilteredStats;
+filter_stats([{Name, Stats}|Rest], FilteredStats) ->
+  filter_stats(Rest, FilteredStats ++ [{Name, filter_stat_set(Stats)}]).
+
+filter_stat_set(Stats) ->
+  filter_stat_set(Stats, []).
+
+filter_stat_set([], FilteredStatSet) ->
+  FilteredStatSet;
+filter_stat_set([{percentile, Percentiles}|Rest], FilteredStatSet) ->
+  filter_stat_set(Rest, FilteredStatSet ++ [{percentile, keys_to_strings(Percentiles)}]);
+filter_stat_set([{histogram, _}|Rest], FilteredStatSet) ->
+  filter_stat_set(Rest, FilteredStatSet);
+filter_stat_set([Pair|Rest], FilteredStatSet) ->
+  filter_stat_set(Rest, FilteredStatSet ++ [Pair]).
+
+keys_to_strings(Pairs) ->
+  lists:map(
+    fun({K, V}) ->
+        {list_to_binary(lists:flatten(io_lib:format("~p", [K]))), V}
+    end, Pairs).

src/erldns_sup.erl

@@ -32,6 +32,7 @@ init(_Args) ->
     ?CHILD(erldns_packet_cache, worker, []),
     ?CHILD(erldns_query_throttle, worker, []),
     ?CHILD(erldns_handler, worker, []),
+    ?CHILD(erldns_metrics, worker, []),
 
     ?CHILD(sample_custom_handler, worker, [])
   ],