ZeroMq LRU算法中间件

前一段时间2014北京PyCon大会吐槽颇多，所以我就到InfoQ上找了找2013的大会视频，对网络射击手游High Noon 2基于Python的服务器架构的视频挺感兴趣，尤其是游戏服务器中的0 downtime，原理他们底层不是原生的socket，

而是基于ZeroMq的socket,由于ZeroMq的短线自动重连可以满足游戏服务器的热启动，不需要代码层面的热启动，热更新，当更新代码完成后直接重启服务器，之前未处理的请求会继续处理。瞬间觉得非常高大上，于是最近一段时间回家一直研究ZeroMq，在guide的LRU这边停留了较长时间，本篇就是谈谈ZeroMq LRU算法中间件。

对普通的请求回复代理，也就是使用了ROUTER-DEALER模式比较好理解，但这种方式有个天生缺点，DEALER会使用负载均衡的方式将客户端的请求转发给服务器端，由于服务器的处理能力各不相同，就会导致有些服务器很忙，有些很闲，这不是我们想看到的，我们希望能压榨所有服务器能力，所以就出现了通用型的LRU算法。

这里使用ROUTER-ROUTER模式，刚开始你可能很诧异，且听我慢慢道来。

那如何能充分使用每台服务器性能呢？

1.当woker启动时告诉backend自己已准备好，将该worker加入work_queue可用队列中

2.当client请求时，从work_queue中取出一个work，把请求交给该work处理

3.当work处理好后把响应发给client，并把自己再次加入到work_queue中

也就是使用work队列，记录可用work，只要work完成请求就代表已空闲，再次加入work队列。

那为什么backend需要使用ROUTER模式呢？关键点是当我要把client来的请求发给固定的work，而只有ROUTER模式具有路由标识功能，明白了这点，代码也就容易了。

"""

   Least-recently used (LRU) queue device
   Clients and workers are shown here in-process

   Author: Guillaume Aubert (gaubert) <guillaume(dot)aubert(at)gmail(dot)com>

"""
from __future__ import print_function

import threading
import time
import zmq

NBR_CLIENTS = 10
NBR_WORKERS = 3

def worker_thread(worker_url, context, i):
    """ Worker using REQ socket to do LRU routing 
    work的响应消息格式为
    --------------------
    |Frame0| Client-idntity
    --------------------
    |Frame1|
    --------------------
    |Frame2| OK
    """

    socket = context.socket(zmq.REQ)

    # Set the worker identity
    socket.identity = (u"Worker-%d" % (i)).encode('ascii')

    socket.connect(worker_url)

    # Tell the borker we are ready for work
    socket.send(b"READY")

    try:
        while True:

            address = socket.recv()
            empty = socket.recv()
            request = socket.recv()

            print("%s: %s\n" % (socket.identity.decode('ascii'), request.decode('ascii')), end='')

            socket.send(address, zmq.SNDMORE)
            socket.send(b"", zmq.SNDMORE)
            socket.send(b"OK")

    except zmq.ContextTerminated:
        # context terminated so quit silently
        return


def client_thread(client_url, context, i):
    """ Basic request-reply client using REQ socket
    client的消息格式为
    --------------------
    |Frame0| Client-1
    --------------------
    |Frame1|
    --------------------
    |Frame2| HELLO
    """

    socket = context.socket(zmq.REQ)

    socket.identity = (u"Client-%d" % (i)).encode('ascii')

    socket.connect(client_url)

    #  Send request, get reply
    socket.send(b"HELLO")
    reply = socket.recv()

    print("%s: %s\n" % (socket.identity.decode('ascii'), reply.decode('ascii')), end='')


def main():
    """ main method """

    url_worker = "inproc://workers"
    url_client = "inproc://clients"
    client_nbr = NBR_CLIENTS

    # Prepare our context and sockets
    context = zmq.Context()
    frontend = context.socket(zmq.ROUTER)
    frontend.bind(url_client)
    backend = context.socket(zmq.ROUTER)
    backend.bind(url_worker)



    # create workers and clients threads
    for i in range(NBR_WORKERS):
        thread = threading.Thread(target=worker_thread, args=(url_worker, context, i, ))
        thread.start()

    for i in range(NBR_CLIENTS):
        thread_c = threading.Thread(target=client_thread, args=(url_client, context, i, ))
        thread_c.start()

    # Logic of LRU loop
    # - Poll backend always, frontend only if 1+ worker ready
    # - If worker replies, queue worker as ready and forward reply
    # to client if necessary
    # - If client requests, pop next worker and send request to it

    # Queue of available workers
    available_workers = 0
    workers_list      = []

    # init poller
    poller = zmq.Poller()

    # Always poll for worker activity on backend
    poller.register(backend, zmq.POLLIN)

    # Poll front-end only if we have available workers
    poller.register(frontend, zmq.POLLIN)

    while True:

        socks = dict(poller.poll())

        # Handle worker activity on backend
        if (backend in socks and socks[backend] == zmq.POLLIN):

            # Queue worker address for LRU routing
            worker_addr  = backend.recv()

            assert available_workers < NBR_WORKERS

            # add worker back to the list of workers
            available_workers += 1
            workers_list.append(worker_addr)

            #   Second frame is empty
            empty = backend.recv()
            assert empty == b""

            # Third frame is READY or else a client reply address
            client_addr = backend.recv()

            # If client reply, send rest back to frontend
            if client_addr != b"READY":

                # Following frame is empty
                empty = backend.recv()
                assert empty == b""

                reply = backend.recv()

                frontend.send(client_addr, zmq.SNDMORE)
                frontend.send(b"", zmq.SNDMORE)
                frontend.send(reply)

                client_nbr -= 1

                if client_nbr == 0:
                    break  # Exit after N messages

        # poll on frontend only if workers are available
        if available_workers > 0:

            if (frontend in socks and socks[frontend] == zmq.POLLIN):
                # Now get next client request, route to LRU worker
                # Client request is [address][empty][request]
                client_addr = frontend.recv()

                empty = frontend.recv()
                assert empty == b""

                request = frontend.recv()

                #  Dequeue and drop the next worker address
                available_workers -= 1
                worker_id = workers_list.pop()
                """worker_id就是work的标识，也就是需要发给worker_id，所以backend需要使用ROUTER模式
                   在所有消息之间zmq需要一个空消息作为标识，当work接受到请求时会直接读到第一个空消息，
                   也就是work接受的第一个消息就是client_addr，然后work再把处理的响应发给client_addr，
                   当backend收到消息后，直接通过forentend转发给client_addr，这也是forentend也需要是ROUTER
                   模式的原因
                """
                backend.send(worker_id, zmq.SNDMORE)
                backend.send(b"", zmq.SNDMORE)
                backend.send(client_addr, zmq.SNDMORE)
                backend.send(b"", zmq.SNDMORE)
                backend.send(request)


    # Out of infinite loop: do some housekeeping

    frontend.close()
    backend.close()
    context.term()


if __name__ == "__main__":
    main()

"""
当需要具体转发的时候，就是ROUTER大显身手的时候
"""