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PoolBoy
Checkout
ready({checkout, Block, Timeout}, {FromPid, _}=From, State) ->
#state{supervisor = Sup,
workers = Workers,
monitors = Monitors,
max_overflow = MaxOverflow} = State,
case queue:out(Workers) of
{{value, Pid}, Left} ->
Ref = erlang:monitor(process, FromPid),
true = ets:insert(Monitors, {Pid, Ref}),
NextState = case queue:is_empty(Left) of
true when MaxOverflow < 1 -> full;
true -> overflow;
false -> ready
end,
{reply, Pid, NextState, State#state{workers=Left}};
{empty, Empty} when MaxOverflow > 0 ->
{Pid, Ref} = new_worker(Sup, FromPid),
true = ets:insert(Monitors, {Pid, Ref}),
{reply, Pid, overflow, State#state{workers=Empty, overflow=1}};
{empty, Empty} when Block =:= false ->
{reply, full, full, State#state{workers=Empty}};
{empty, Empty} ->
Waiting = add_waiting(From, Timeout, State#state.waiting),
{next_state, full, State#state{workers=Empty, waiting=Waiting}}
end;
Checkout出一个worker从worker Queue中，假设有,则monitor（ets）这个worker，然后依据队列的容量和MaxOverflow的值来确定下一状态为full。overflow，ready （ready + overflow <= full）;假设没有。而MaxOverFlow的值大于0，则新建一个worker，并将其增加monitor。最后重置状态项worker = empty, overflow = 1；假设没有，而且MaxOverflow 小于1， Block == false，则{reply, full, full, State#state{workers=Empty}};如过没有。MaxOverflow < 1, Block == true,则 {next_state, full, State#state{workers=Empty, waiting=Waiting}}。
Checkin
ready({checkin, Pid}, State) ->
Monitors = State#state.monitors,
case ets:lookup(Monitors, Pid) of
[{Pid, Ref}] ->
true = erlang:demonitor(Ref),
true = ets:delete(Monitors, Pid),
Workers = queue:in(Pid, State#state.workers),
{next_state, ready, State#state{workers=Workers}};
[] ->
{next_state, ready, State}
end;
从Monitor中剔除相应的worker。然后回收到worker queue中去。
状态转变
状态转变的计算：
worker_queue_size(当前size) + maxoverflow 。
ready仅仅与当前worker_queque_size有关，overflow 和worker_queue_size(0)和maxoverflow>0有关。full 和work_queue_size(0), overfllow = maxoverflow有关。
worker的来源
全部的worker要么在初始化时创建平；要么调用checkout时。经过poolboy创建。但此时创建的worker没有进到worker queue。要想进到worker queue，仅仅能调用checkin。
work pid 回收到worker_queue中
checkin_while_full –》{empty, Empty} when MaxOverflow < 1 ->；
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