开发工作中，经常会碰到进程池或者线程池，或者其它的资源池。在这里，用erlang实现一个简单的进程池。
erlang进程是非常轻量级的，这个进程池的主要目的是用一种通用的方式去管理和限制系统中运行的资源占用。当运行的工作者进程数量达到上限，进程池还可以把任务放到队列中，只要进程资源被释放，排队的任务就能获得运行，否则任务只能阻塞。
这是进程池的监督树：
ppool_supersup监督着所有的进程池。一个进程池由ppool_sup、ppool_serv和worker_sup监督的工作者进程池组成。ppool_serv提供对外的进程池调用api，ppool_sup负责监督单个进程池。
下面是实现代码。
%% ppool_supersup
-module(ppool_supersup).
-behavior(supervisor).
-export([start_link/0, stop/0, start_pool/3, stop_pool/1]).
-export([init/1]).
start_link() ->
supervisor:start_link({local, ppool}, ?MODULE, []).
stop() ->
case whereis(ppool) of
P when is_pid(P) ->
exit(P, kill);
_ ->
ok
end.
init([]) ->
MaxRestart = 6,
MaxTime = 3600,
{ok, {{one_for_one, MaxRestart, MaxTime}, []}}.
start_pool(Name, Limit, MFA) ->
%%每个进程池的最大终止时间设置为10500，这个值并没有什么特殊意义，只是保证足够大，所有子进程都有足够的时间终止。如果实在不知道设置为多大，可以试试infinity。
ChildSpec = {Name,
{ppool_sup, start_link, [Name, Limit, MFA]},
permanent, 10500, supervisor, [ppool_sup]},
supervisor:start_child(ppool, ChildSpec).
stop_pool(Name) ->
supervisor:terminate_child(ppool, Name),
supervisor:delete_child(ppool, Name).
%% ppool_sup
-module(ppool_sup).
-export([start_link/3, init/1]).
-behavior(supervisor).
start_link(Name, Limit, MFA) ->
supervisor:start_link(?MODULE, {Name, Limit, MFA}).
init({Name, Limit, MFA}) ->
MaxRestart = 1,
MaxTime = 3600,
{ok, {{one_for_all, MaxRestart, MaxTime},
[{serv,
{ppool_serv, start_link, [Name, Limit, self(), MFA]},
permanent,
5000,
worker,
[ppool_serv]}]}}.
%% ppool_worker_sup
-module(ppool_worker_sup).
-export([start_link/1, init/1]).
-behavior(supervisor).
start_link(MFA = {_, _, _}) ->
supervisor:start_link(?MODULE, MFA).
init({M, F, A}) ->
MaxRestart = 5,
MaxTime = 3600,
{ok, {{simple_one_for_one, MaxRestart, MaxTime},
[{ppool_worker,
{M, F, A},
temporary, 5000, worker, [M]}]}}.
ppool_serv是最复杂的一个模块了。因为ppool_serv对外提供接口，它需要能联系到worker_sup。如果由ppool_sup同时启动ppool_serv和worker_sup，存在乱序的风险，除非都注册进程名。但erlang中对于原子的使用一定要慎重，能少用就少用。所以在这儿，由ppool_serv动态添加worker_sup到ppool_sup。
ppool_serv提供了三种添加任务的方式：
- 如果进程池中有空间，立刻运行；如果已满，给出无法运行的指示。
- 如果进程池中有空间，立刻运行；如果已满，调用者进程阻塞等待，任务入队列。
- 如果进程池中有空间，立刻运行；如果已满，任务入队列，调用者进程不阻塞。
%% ppool_serv
-module(ppool_serv).
-behavior(gen_server).
-export([start/4, start_link/4, run/2, sync_queue/2, async_queue/2, stop/1]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, code_change/3, terminate/2]).
-define(SPEC(MFA),
{worker_sup,
{ppool_worker_sup, start_link, [MFA]},
permanent,
10000,
supervisor,
[ppool_woker_sup]}).
-record(state, {limit = 0,
sup,
refs,
queue = queue:new()}).
start(Name, Limit, Sup, MFA) when is_atom(Name), is_integer(Limit) ->
gen_server:start({local, Name}, ?MODULE, {Limit, MFA, Sup}, []).
start_link(Name, Limit, Sup, MFA) when is_atom(Name), is_integer(Limit) ->
gen_server:start_link({local, Name}, ?MODULE, {Limit, MFA, Sup}, []).
run(Name, Args) ->
gen_server:call(Name, {run, Args}).
sync_queue(Name, Args) ->
gen_server:call(Name, {sync, Args}, infinity).
async_queue(Name, Args) ->
gen_server:cast(Name, {async, Args}).
stop(Name) ->
gen_server:call(Name, stop).
init({Limit, MFA, Sup}) ->
self() ! {start_worker_supervisor, Sup, MFA},
{ok, #state{limit = Limit, refs = gb_sets:empty()}}.
handle_info({'DOWN', Ref, process, _Pid, _}, S = #state{refs = Refs}) ->
case gb_sets:is_element(Ref, Refs) of
true ->
handle_down_worker(Ref, S);
false ->
{noreply, S}
end;
handle_info({start_worker_supervisor, Sup, MFA}, S = #state{}) ->
{ok, Pid} = supervisor:start_child(Sup, ?SPEC(MFA)),
link(Pid),
{noreply, S#state{sup = Pid}};
handle_info(_Msg, State) ->
{noreply, State}.
handle_call({run, Args}, _From, S = #state{limit = N, sup = Sup, refs = R}) when N > 0 ->
{ok, Pid} = supervisor:start_child(Sup, Args),
Ref = erlang:monitor(process, Pid),
{reply, {ok, Pid}, S#state{limit = N - 1, refs = gb_sets:add(Ref, R)}};
handle_call({run, _Args}, _From, S = #state{limit = N}) when N =< 0 ->
{reply, noalloc, S};
handle_call({sync, Args}, _From, S = #state{limit=N, sup=Sup, refs=R}) when N > 0 ->
{ok, Pid} = supervisor:start_child(Sup, Args),
Ref = erlang:monitor(process, Pid),
{reply, {ok,Pid}, S#state{limit=N-1, refs=gb_sets:add(Ref,R)}};
handle_call({sync, Args}, From, S = #state{queue=Q}) ->
{noreply, S#state{queue=queue:in({From, Args}, Q)}};
handle_call(stop, _From, State) ->
{stop, normal, ok, State};
handle_call(_Msg, _From, State) ->
{noreply, State}.
handle_cast({async, Args}, S=#state{limit=N, sup=Sup, refs=R}) when N > 0 ->
{ok, Pid} = supervisor:start_child(Sup, Args),
Ref = erlang:monitor(process, Pid),
{noreply, S#state{limit=N-1, refs=gb_sets:add(Ref,R)}};
handle_cast({async, Args}, S=#state{limit=N, queue=Q}) when N =< 0 ->
{noreply, S#state{queue=queue:in(Args,Q)}};
handle_cast(_Msg, State) ->
{noreply, State}.
code_change(_OldVsn, State, _Extra) ->
{ok, State}.
terminate(_Reason, _State) ->
ok.
handle_down_worker(Ref, S = #state{limit = L, sup = Sup, refs = Refs}) ->
case queue:out(S#state.queue) of
{{value, {From, Args}}, Q} ->
{ok, Pid} = supervisor:start_child(Sup, Args),
NewRef = erlang:monitor(process, Pid),
NewRefs = gb_sets:insert(NewRef, gb_sets:delete(Ref, Refs)),
gen_server:reply(From, {ok, Pid}),
{noreply, S#state{refs = NewRefs, queue = Q}};
{{value, Args}, Q} ->
{ok, Pid} = supervisor:start_child(Sup, Args),
NewRef = erlang:monitor(process, Pid),
NewRefs = gb_sets:insert(NewRef, gb_sets:delete(Ref, Refs)),
{noreply, S#state{refs = NewRefs, queue = Q}};
{empty, _} ->
{noreply, S#state{limit = L + 1, refs = gb_sets:delete(Ref, Refs)}}
end.