PoolBoy

source code ：

https://github.com/devinus/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 ->；