worker_pool

% This file is licensed to you under the Apache License,
% Version 2.0 (the "License"); you may not use this file
% except in compliance with the License.  You may obtain
% a copy of the License at
%
% http://www.apache.org/licenses/LICENSE-2.0
%
% Unless required by applicable law or agreed to in writing,
% software distributed under the License is distributed on an
% "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
% KIND, either express or implied.  See the License for the
% specific language governing permissions and limitations
% under the License.
%%% @author Fernando Benavides <elbrujohalcon@inaka.net>
%%% @doc A pool of workers. If you want to put it in your supervisor tree,
%%%      remember it's a supervisor.
-module(wpool_pool).
-author('elbrujohalcon@inaka.net').

-behaviour(supervisor).

%% API
-export([start_link/2, create_table/0]).
-export([ best_worker/1
        , random_worker/1
        , next_worker/1
        , call_available_worker/3
        , hash_worker/2
        ]).
-export([cast_to_available_worker/2]).
-export([stats/1, wpool_size/1, worker_names/1, worker_name/2]).

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

-include("wpool.hrl").

%% ===================================================================
%% API functions
%% ===================================================================
%% @doc Creates the ets table that will hold the information about active pools
-spec create_table() -> ok.
create_table() ->
  case ets:info(?MODULE, named_table) of
    true      -> already_created;
    undefined ->
      error_logger:info_msg("Creating wpool ETS table"),
      ets:new(
        ?MODULE,
        [public, named_table, set,
         {read_concurrency, true}, {keypos, #wpool.name}])
  end,
  ok.

%% @doc Starts a supervisor with several {@link wpool_process}es as its children
-spec start_link(wpool:name(), [wpool:option()]) ->
        {ok, pid()} | {error, {already_started, pid()} | term()}.
start_link(Name, Options) ->
  supervisor:start_link({local, Name}, ?MODULE, {Name, Options}).

%% @doc Picks the worker with the smaller queue of messages.
%% @throws no_workers
-spec best_worker(wpool:name()) -> atom().
best_worker(Sup) ->
  case find_wpool(Sup) of
    undefined -> throw(no_workers);
    Wpool -> min_message_queue(Wpool)
  end.

%% @doc Picks a random worker
%% @throws no_workers

-spec random_worker(wpool:name()) -> atom().
random_worker(Sup) ->
  case wpool_size(Sup) of
    undefined  -> throw(no_workers);
    Wpool_Size ->
      _ = seed_random(),
      worker_name(Sup, random:uniform(Wpool_Size))
  end.

%% @doc Picks the next worker in a round robin fashion
%% @throws no_workers
-spec next_worker(wpool:name()) -> atom().
next_worker(Sup) ->
  case move_wpool(Sup) of
    undefined -> throw(no_workers);
    Next -> worker_name(Sup, Next)
  end.

%% @doc Picks the first available worker and sends the call to it.
%%      The timeout provided includes the time it takes to get a worker
%%      and for it to process the call.
%% @throws no_workers | timeout
-spec call_available_worker(wpool:name(), any(), timeout()) -> any().
call_available_worker(Sup, Call, Timeout) ->
  case wpool_queue_manager:call_available_worker(
        queue_manager_name(Sup), Call, Timeout) of
    noproc  -> throw(no_workers);
    timeout -> throw(timeout);
    Result  -> Result
  end.

%% @doc Picks a worker base on a hash result.
%%      <pre>phash2(Term, Range)</pre> returns hash = integer,
%%      0 &lt;= hash &lt; Range so <pre>1</pre> must be added
%% @throws no_workers
-spec hash_worker(wpool:name(), term()) -> atom().
hash_worker(Sup, HashKey) ->
  case wpool_size(Sup) of
    undefined -> throw(no_workers);
    Wpool_Size ->
      Index = 1 + erlang:phash2(HashKey, Wpool_Size),
      worker_name(Sup, Index)
  end.

%% @doc Casts a message to the first available worker.
%%      Since we can wait forever for a wpool:cast to be delivered
%%      but we don't want the caller to be blocked, this function
%%      just forwards the cast when it gets the worker
-spec cast_to_available_worker(wpool:name(), term()) -> ok.
cast_to_available_worker(Sup, Cast) ->
  wpool_queue_manager:cast_to_available_worker(queue_manager_name(Sup), Cast).

%% @doc Retrieves a snapshot of the pool stats
%% @throws no_workers
-spec stats(wpool:name()) -> wpool:stats().
stats(Sup) ->
  case find_wpool(Sup) of
    undefined -> throw(no_workers);
    Wpool ->
      stats(Wpool, Sup)
  end.

stats(Wpool, Sup) ->
  {Total, WorkerStats} =
    lists:foldl(
      fun(N, {T, L}) ->
        Worker = erlang:whereis(worker_name(Sup, N)),
        [{message_queue_len, MQL} = MQLT,
         Memory, Function, Location, {dictionary, Dictionary}] =
          erlang:process_info(
            Worker,
            [message_queue_len, memory, current_function,
             current_location, dictionary]),
        Time =
          calendar:datetime_to_gregorian_seconds(calendar:universal_time()),
        WS =
          case {Function, proplists:get_value(wpool_task, Dictionary)} of
            {{current_function, {gen_server, loop, 6}}, undefined} ->
              [MQLT, Memory];
            {{current_function, {erlang, hibernate, _}}, undefined} ->
              [MQLT, Memory];
            {_, undefined} ->
              [MQLT, Memory, Function, Location];
            {_, {_TaskId, Started, Task}} ->
              [MQLT, Memory, Function, Location,
               {task, Task}, {runtime, Time - Started}]
          end,
        {T + MQL, [{N, WS} | L]}
      end, {0, []}, lists:seq(1, Wpool#wpool.size)),
  ManagerStats = wpool_queue_manager:stats(Wpool#wpool.name),
  PendingTasks = proplists:get_value(pending_tasks, ManagerStats),
  [{pool,                     Sup},
   {supervisor,               erlang:whereis(Sup)},
   {options,                  Wpool#wpool.opts},
   {size,                     Wpool#wpool.size},
   {next_worker,              Wpool#wpool.next},
   {total_message_queue_len,  Total + PendingTasks},
   {workers,                  WorkerStats}].

%% @doc Returns the names of the workers in the pool
-spec worker_names(wpool:name()) -> [atom()].
worker_names(Pool_Name) ->
  case find_wpool(Pool_Name) of
    undefined -> [];
    #wpool{size=Size} ->
      [worker_name(Pool_Name, N) || N <- lists:seq(1, Size)]
  end.

%% @doc the number of workers in the pool
-spec wpool_size(atom()) -> non_neg_integer() | undefined.
wpool_size(Name) ->
  try ets:update_counter(?MODULE, Name, {#wpool.size, 0}) of
    Wpool_Size ->
      case erlang:whereis(Name) of
        undefined ->
          ets:delete(?MODULE, Name),
          undefined;
        _ ->
          Wpool_Size
      end
  catch
    _:badarg ->
      case build_wpool(Name) of
        undefined -> undefined;
        Wpool -> Wpool#wpool.size
      end
  end.

%% ===================================================================
%% Supervisor callbacks
%% ===================================================================
%% @private
-spec init({wpool:name(), [wpool:option()]}) ->
        {ok, {supervisor:sup_flags(), [supervisor:child_spec()]}}.
init({Name, Options}) ->
  Workers = proplists:get_value(workers, Options, 100),
  OverrunHandler =
    proplists:get_value(
      overrun_handler, Options, {error_logger, warning_report}),
  TimeChecker = time_checker_name(Name),
  QueueManager = queue_manager_name(Name),
  ProcessSup = process_sup_name(Name),
  _Wpool =
    store_wpool(
      #wpool{ name = Name
            , size = Workers
            , next = 1
            , opts = Options
            , qmanager = QueueManager
            }),
  TimeCheckerSpec =
    {TimeChecker,
     {wpool_time_checker, start_link, [Name, TimeChecker, OverrunHandler]},
     permanent, brutal_kill, worker, [wpool_time_checker]},
  QueueManagerSpec =
    {QueueManager,
     {wpool_queue_manager, start_link, [Name, QueueManager]},
     permanent, brutal_kill, worker, [wpool_queue_manager]},

  WorkerOpts =
    [{queue_manager, QueueManager}, {time_checker, TimeChecker} | Options],
  ProcessSupSpec =
    {ProcessSup,
     {wpool_process_sup, start_link, [Name, ProcessSup, WorkerOpts]},
     permanent, brutal_kill, supervisor, [wpool_process_sup]},

  SupStrategy = {one_for_all, 5, 60},
  {ok, {SupStrategy, [TimeCheckerSpec, QueueManagerSpec, ProcessSupSpec]}}.

%% @private
-spec worker_name(wpool:name(), pos_integer()) -> atom().
worker_name(Sup, I) ->
  list_to_atom(
    ?MODULE_STRING ++ [$-|atom_to_list(Sup)] ++ [$-| integer_to_list(I)]).

%% ===================================================================
%% Private functions
%% ===================================================================
process_sup_name(Sup) ->
  list_to_atom(?MODULE_STRING ++ [$-|atom_to_list(Sup)] ++ "-process-sup").
time_checker_name(Sup) ->
  list_to_atom(
    ?MODULE_STRING ++ [$-|atom_to_list(Sup)] ++ "-time-checker").
queue_manager_name(Sup) ->
  list_to_atom(?MODULE_STRING ++ [$-|atom_to_list(Sup)] ++ "-queue-manager").

min_message_queue(Wpool) ->
  %% Moving the beginning of the list to a random point to ensure that clients
  %% do not always start asking for process_info to the processes that are most
  %% likely to have bigger message queues
  First = random:uniform(Wpool#wpool.size),
  min_message_queue(0, Wpool#wpool{next = First}, []).
min_message_queue(Size, #wpool{size = Size}, Found) ->
  {_, Worker} = lists:min(Found),
  Worker;
min_message_queue(Checked, Wpool, Found) ->
  Worker = worker_name(Wpool#wpool.name, Wpool#wpool.next),
  case erlang:process_info(erlang:whereis(Worker), message_queue_len) of
    {message_queue_len, 0} -> Worker;
    {message_queue_len, L} ->
      NextWpool =
        Wpool#wpool{next = (Wpool#wpool.next rem Wpool#wpool.size) + 1},
      min_message_queue(Checked + 1, NextWpool, [{L, Worker} | Found]);
    Error -> throw(Error)
  end.

%% ===================================================================
%% ETS functions
%% ===================================================================
store_wpool(Wpool) ->
  true = ets:insert(?MODULE, Wpool),
  Wpool.

move_wpool(Name) ->
  try
    Wpool_Size = ets:update_counter(?MODULE, Name, {#wpool.size, 0}),
    ets:update_counter(?MODULE, Name, {#wpool.next, 1, Wpool_Size, 1})
  catch
    _:badarg ->
      case build_wpool(Name) of
        undefined -> undefined;
        Wpool -> Wpool#wpool.next
      end
  end.

find_wpool(Name) ->
  try ets:lookup(?MODULE, Name) of
    [Wpool | _] ->
      case erlang:whereis(Name) of
        undefined ->
          ets:delete(?MODULE, Name),
          undefined;
        _ ->
          Wpool
      end;
    _ -> build_wpool(Name)
  catch
    _:badarg ->
      build_wpool(Name)
  end.

%% @doc We use this function not to report an error if for some reason we've
%%      lost the record on the ets table. This SHOULDN'T be called too much
build_wpool(Name) ->
  error_logger:warning_msg(
    "Building a #wpool record for ~p. Something must have failed.", [Name]),
  try supervisor:count_children(process_sup_name(Name)) of
    Children ->
      case proplists:get_value(active, Children, 0) of
        0 -> undefined;
        Size ->
          Wpool = #wpool{name = Name, size = Size, next = 1, opts = []},
          store_wpool(Wpool)
      end
  catch
    _:Error ->
      error_logger:warning_msg("Wpool ~p not found: ~p", [Name, Error]),
      undefined
  end.

-ifdef(r_18).
  seed_random() -> random:seed(erlang:timestamp()).
-else.
  seed_random() -> random:seed(now()).
-endif.