rabbitmq之work_pool
worker_pool_worker的作用是用来完成数据操作。
如何获取worker是从worker_pool里获取，并由worker_pool管理。
起动时间：
-rabbit_boot_step({worker_pool,
[{description, "worker pool"},
{mfa, {rabbit_sup, start_supervisor_child,
[worker_pool_sup]}},
{requires, pre_boot},
{enables, external_infrastructure}]}).
在起动顺序中，work_pool是由pre_boot之后，external_infrastructure之后起动的。起动一个worker_pool进程，并起动Wcount个work_pool_worker进程，Wcount是由erlang:system_info(schedulers)决定的。
init([WCount]) ->
{ok, {{one_for_one, 10, 10},
[{worker_pool, {worker_pool, start_link, []}, transient,
16#ffffffff, worker, [worker_pool]} |
[{N, {worker_pool_worker, start_link, []}, transient, 16#ffffffff,
worker, [worker_pool_worker]} || N <- lists:seq(1, Wcount)]]}}.
worker_pool_worker起动的时候会将通过worke_pool:read(self())来监控起来，并将此进程放置于state中avaliable列表中。当worker_pool_worker down掉时，worker_pool则会将此进程从avaliable队表中删除掉。
init([]) ->
ok = file_handle_cache:register_callback(?MODULE, set_maximum_since_use,
[self()]),
ok = worker_pool:ready(self()),
put(worker_pool_worker, true),
{ok, undefined, hibernate,
{backoff, ?HIBERNATE_AFTER_MIN, ?HIBERNATE_AFTER_MIN, ?DESIRED_HIBERNATE}}.
worker_pool.erl
ready(WPid) -> gen_server2:cast(?SERVER, {ready, Wpid}).
handle_cast({ready, WPid}, State) ->
erlang:monitor(process, WPid),
handle_cast({idle, WPid}, State);
handle_cast({idle, WPid}, State = #state { available = Avail,
pending = Pending }) ->
{noreply,
case queue:out(Pending) of
{empty, _Pending} ->
State #state { available = ordsets:add_element(WPid, Avail) };
{{value, {next_free, From, CPid}}, Pending1} ->
worker_pool_worker:next_job_from(WPid, CPid),
gen_server2:reply(From, WPid),
State #state { pending = Pending1 };
{{value, {run_async, Fun}}, Pending1} ->
worker_pool_worker:submit_async(WPid, Fun),
State #state { pending = Pending1 }
end, hibernate};
handle_info({'DOWN', _MRef, process, WPid, _Reason},
State = #state { available = Avail }) ->
{noreply, State #state { available = ordsets:del_element(WPid, Avail) },
hibernate};
如果有submit动作，获取空闲的worker，并作
submit(Fun, ProcessModel) ->
case get(worker_pool_worker) of
true -> worker_pool_worker:run(Fun);
_ -> Pid = gen_server2:call(?SERVER, {next_free, self()}, infinity),
worker_pool_worker:submit(Pid, Fun, ProcessModel)
end.
此时应获取一个空闲的worker_pool_worker，并让此worker装备一下，返回worker Pid。
handle_call({next_free, CPid}, _From, State = #state { available =
[WPid | Avail1] }) ->
worker_pool_worker:next_job_from(WPid, CPid),
{reply, WPid, State #state { available = Avail1 }, hibernate};
worker_pool_worker准备工作
next_job_from(Pid, CPid) ->
gen_server2:cast(Pid, {next_job_from, Cpid}).
work_pool_worke刚起来的时候state是undefined，当需要自己去完成工作的时候，worker会将work_pool进程监控起来，以便pool在使用自己执行操作的时候且pool异常时能够释放自己。
handle_cast({next_job_from, CPid}, undefined) ->
MRef = erlang:monitor(process, CPid),
{noreply, {from, CPid, MRef}, hibernate};
submit(Pid, Fun, ProcessModel) ->
gen_server2:call(Pid, {submit, Fun, self(), ProcessModel}, infinity).
将pool解监控，返回pool执行结果，释放自己，state重回undefined。
handle_call({submit, Fun, CPid, ProcessModel}, From, {from, CPid, MRef}) ->
erlang:demonitor(MRef),
gen_server2:reply(From, run(Fun, ProcessModel)),
ok = worker_pool:idle(self()),
{noreply, undefined, hibernate};
参考文献：
Erlang：RabbitMQ源码分析 5. worker pool 实现分析. http://m.blog.csdn.net/blog/liaosongbo/39317829