在云计算环境中，很多时候需要用它其他机器的计算资源，我们有可能会在接收到Message进行处理时，会把一部分计算任务分配到其他节点来完成。那么，RabbitMQ如何使用RPC呢？在本篇文章中，我们将会通过其它节点求来斐波纳契完成示例。

1. 客户端接口 Client interface

为了展示一个RPC服务是如何使用的，我们将创建一段很简单的客户端class。它将会向外提供名字为call的函数，这个call会发送RPC请求并且阻塞知道收到RPC运算的结果。代码如下：

fibonacci_rpc = FibonacciRpcClient()
result = fibonacci_rpc.call(4)
print "fib(4) is %r" % (result,)

fibonacci_rpc = FibonacciRpcClient()

result = fibonacci_rpc.call(4)

print "fib(4) is %r" % (result,)

2. 回调函数队列 Callback queue

总体来说，在RabbitMQ进行RPC远程调用是比较容易的。client发送请求的Message然后server返回响应结果。为了收到响应client在publish message时需要提供一个”callback“（回调）的queue地址。code如下:

result = channel.queue_declare(exclusive=True)
callback_queue = result.method.queue
channel.basic_publish(exchange='',
routing_key='rpc_queue',
properties=pika.BasicProperties(
reply_to = callback_queue,
),
body=request)
# ... and some code to read a response message from the callback_queue ...

result = channel.queue_declare(exclusive=True)

callback_queue = result.method.queue

channel.basic_publish(exchange='',

                      routing_key='rpc_queue',

                      properties=pika.BasicProperties(

                            reply_to = callback_queue,

                            ),

                      body=request)

# ... and some code to read a response message from the callback_queue ...

2.1 Message properties

AMQP 预定义了14个属性。它们中的绝大多很少会用到。以下几个是平时用的比较多的：

delivery_mode: 持久化一个Message（通过设定值为2）。其他任意值都是非持久化。请移步RabbitMQ消息队列（三）：任务分发机制

content_type: 描述mime-type 的encoding。比如设置为JSON编码：设置该property为application/json。

reply_to: 一般用来指明用于回调的queue（Commonly used to name a callback queue）。

correlation_id: 在请求中关联处理RPC响应（correlate RPC responses with requests）。

3. 相关id Correlation id

在上个小节里，实现方法是对每个RPC请求都会创建一个callback queue。这是不高效的。幸运的是，在这里有一个解决方法：为每个client创建唯一的callback queue。

这又有其他问题了：收到响应后它无法确定是否是它的，因为所有的响应都写到同一个queue了。上一小节的correlation_id在这种情况下就派上用场了：对于每个request，都设置唯一的一个值，在收到响应后，通过这个值就可以判断是否是自己的响应。如果不是自己的响应，就不去处理。

4. 总结

工作流程：

当客户端启动时，它创建了匿名的exclusive callback queue.
客户端的RPC请求时将同时设置两个properties： reply_to设置为callback queue；correlation_id设置为每个request一个独一无二的值.
请求将被发送到an rpc_queue queue.
RPC端或者说server一直在等待那个queue的请求。当请求到达时，它将通过在reply_to指定的queue回复一个message给client。
client一直等待callback queue的数据。当message到达时，它将检查correlation_id的值，如果值和它request发送时的一致那么就将返回响应。

5. 最终实现

The code for rpc_server.py:

#!/usr/bin/env python
import pika
connection = pika.BlockingConnection(pika.ConnectionParameters(
host='localhost'))
channel = connection.channel()
channel.queue_declare(queue='rpc_queue')
def fib(n):
if n == 0:
return 0
elif n == 1:
return 1
else:
return fib(n-1) + fib(n-2)
def on_request(ch, method, props, body):
n = int(body)
print " [.] fib(%s)" % (n,)
response = fib(n)
ch.basic_publish(exchange='',
routing_key=props.reply_to,
properties=pika.BasicProperties(correlation_id = \
props.correlation_id),
body=str(response))
ch.basic_ack(delivery_tag = method.delivery_tag)
channel.basic_qos(prefetch_count=1)
channel.basic_consume(on_request, queue='rpc_queue')
print " [x] Awaiting RPC requests"
channel.start_consuming()

#!/usr/bin/env python

import pika

connection = pika.BlockingConnection(pika.ConnectionParameters(

        host='localhost'))

channel = connection.channel()

channel.queue_declare(queue='rpc_queue')

def fib(n):

    if n == 0:

        return 0

    elif n == 1:

        return 1

    else:

        return fib(n-1) + fib(n-2)

def on_request(ch, method, props, body):

    n = int(body)

    print " [.] fib(%s)"  % (n,)

    response = fib(n)

    ch.basic_publish(exchange='',

                     routing_key=props.reply_to,

                     properties=pika.BasicProperties(correlation_id = \

                                                     props.correlation_id),

                     body=str(response))

    ch.basic_ack(delivery_tag = method.delivery_tag)

channel.basic_qos(prefetch_count=1)

channel.basic_consume(on_request, queue='rpc_queue')

print " [x] Awaiting RPC requests"

channel.start_consuming()

The server code is rather straightforward:

(4) As usual we start by establishing the connection and declaring the queue.
(11) We declare our fibonacci function. It assumes only valid positive integer input. (Don't expect this one to work for big numbers, it's probably the slowest recursive implementation possible).
(19) We declare a callback for basic_consume, the core of the RPC server. It's executed when the request is received. It does the work and sends the response back.
(32) We might want to run more than one server process. In order to spread the load equally over multiple servers we need to set theprefetch_count setting.

The code for rpc_client.py:

#!/usr/bin/env python
import pika
import uuid
class FibonacciRpcClient(object):
def __init__(self):
self.connection = pika.BlockingConnection(pika.ConnectionParameters(
host='localhost'))
self.channel = self.connection.channel()
result = self.channel.queue_declare(exclusive=True)
self.callback_queue = result.method.queue
self.channel.basic_consume(self.on_response, no_ack=True,
queue=self.callback_queue)
def on_response(self, ch, method, props, body):
if self.corr_id == props.correlation_id:
self.response = body
def call(self, n):
self.response = None
self.corr_id = str(uuid.uuid4())
self.channel.basic_publish(exchange='',
routing_key='rpc_queue',
properties=pika.BasicProperties(
reply_to = self.callback_queue,
correlation_id = self.corr_id,
),
body=str(n))
while self.response is None:
self.connection.process_data_events()
return int(self.response)
fibonacci_rpc = FibonacciRpcClient()
print " [x] Requesting fib(30)"
response = fibonacci_rpc.call(30)
print " [.] Got %r" % (response,)

#!/usr/bin/env python

import pika

import uuid

class FibonacciRpcClient(object):

    def __init__(self):

        self.connection = pika.BlockingConnection(pika.ConnectionParameters(

                host='localhost'))

        self.channel = self.connection.channel()

        result = self.channel.queue_declare(exclusive=True)

        self.callback_queue = result.method.queue

        self.channel.basic_consume(self.on_response, no_ack=True,

                                   queue=self.callback_queue)

    def on_response(self, ch, method, props, body):

        if self.corr_id == props.correlation_id:

            self.response = body

    def call(self, n):

        self.response = None

        self.corr_id = str(uuid.uuid4())

        self.channel.basic_publish(exchange='',

                                   routing_key='rpc_queue',

                                   properties=pika.BasicProperties(

                                         reply_to = self.callback_queue,

                                         correlation_id = self.corr_id,

                                         ),

                                   body=str(n))

        while self.response is None:

            self.connection.process_data_events()

        return int(self.response)

fibonacci_rpc = FibonacciRpcClient()

print " [x] Requesting fib(30)"

response = fibonacci_rpc.call(30)

print " [.] Got %r" % (response,)

The client code is slightly more involved:

(7) We establish a connection, channel and declare an exclusive 'callback' queue for replies.
(16) We subscribe to the 'callback' queue, so that we can receive RPC responses.
(18) The 'on_response' callback executed on every response is doing a very simple job, for every response message it checks if thecorrelation_id is the one we're looking for. If so, it saves the response inself.response and breaks the consuming loop.
(23) Next, we define our main call method - it does the actual RPC request.
(24) In this method, first we generate a unique correlation_id number and save it - the 'on_response' callback function will use this value to catch the appropriate response.
(25) Next, we publish the request message, with two properties: reply_to and correlation_id.
(32) At this point we can sit back and wait until the proper response arrives.
(33) And finally we return the response back to the user.

开始rpc_server.py：

$ python rpc_server.py
[x] Awaiting RPC requests

$ python rpc_server.py

 [x] Awaiting RPC requests

通过client来请求fibonacci数:

$ python rpc_client.py
[x] Requesting fib(30)

$ python rpc_client.py

 [x] Requesting fib(30)

现在这个设计并不是唯一的，但是这个实现有以下优势：

如何RPC server太慢，你可以扩展它:启动另外一个RPC server。
在client端，无所进行加锁能同步操作，他所作的就是发送请求等待响应。

我们的code还是挺简单的，并没有尝试去解决更复杂和重要的问题，比如：

如果没有server在运行，client需要怎么做？
RPC应该设置超时机制吗?
如果server运行出错并且抛出了异常，需要将这个问题转发到client吗？
需要边界检查吗？

转载自： anzhsoft：http://blog.csdn.net/anzhsoft/article/details/19633107

参考资料：

1. http://www.rabbitmq.com/tutorials/tutorial-six-python.html

秒客网

RabbitMQ 适用于云计算集群的远程调用(RPC)