In the second tutorial we learned how to use Work Queues to distribute time-consuming tasks among multiple workers.
在第二个指导中，我们学习了如何运用工作队列在多个worker之间分发耗时的任务。
But what if we need to run a function on a remote computer and wait for the result? Well, that's a different story. This pattern is commonly known as Remote Procedure Call or RPC.
但是假使我们需要运行一个远程的函数并等待其返回结果时怎么办？好吧，那是两码事。这种模式通常被称为远程过程调用或者RPC。
In this tutorial we're going to use RabbitMQ to build an RPC system: a client and a scalable RPC server. As we don't have any time-consuming tasks that are worth distributing, we're going to create a dummy RPC service that returns Fibonacci numbers.
这回我们要用RabbitMQ打造一个RPC系统：一个客户端和一个可扩展的RPC服务端。由于我们没有什么值得分发的耗时任务，所以我们将创建一个伪RPC服务，用来返回斐波那契数列。
To illustrate how an RPC service could be used we're going to create a simple client class. It's going to expose a method named call which sends an RPC request and blocks until the answer is received:
我们将创建一个简单的客户端类来阐述RPC服务是如何使用的。这个类将揭示方法调用??发送一个RPC请求并阻塞直至收到回复。
$fibonacci_rpc = new FibonacciRpcClient();$response = $fibonacci_rpc->call(30);echo " [.] Got ", $response, "\n";
Although RPC is a pretty common pattern in computing, it's often criticised. The problems arise when a programmer is not aware whether a function call is local or if it's a slow RPC. Confusions like that result in an unpredictable system and adds unnecessary complexity to debugging. Instead of simplifying software, misused RPC can result in unmaintainable spaghetti code.
尽管RPC在电脑运算中很常见，但它十分挑剔。这个问题出现的原因是程序员不知道是否调用一个本地的方法或是否是一个很慢的RPC。这样的困惑便导致不可预测的系统并增加不必要的调试复杂性。比起简化的软件，误用RPC会导致不可维护的无头绪代码。
Bearing that in mind, consider the following advice:
铭记刚才问题，考虑下面的建议：
When in doubt avoid RPC. If you can, you should use an asynchronous pipeline - instead of RPC-like blocking, results are asynchronously pushed to a next computation stage.
如果有疑问，则尽量避免使用RPC。如果可以话，你应该使用异步管道??而不是RPC??像阻塞，结果被异步推送到下个计算阶段。
In general doing RPC over RabbitMQ is easy. A client sends a request message and a server replies with a response message. In order to receive a response we need to send a 'callback' queue address with the request. We can use the default queue. Let's try it:
一般来讲在RabbitMQ上搞RPC很容易??客户端发送请求消息，服务端回复响应消息。为了收到响应（消息）我们得在发送请求时附带一个回调队列地址。可以试试默认队列：
list($queue_name, ,) = $channel->queue_declare("", false, false, true, false);$msg = new AMQPMessage( $payload, array('reply_to' => $queue_name));$channel->basic_publish($msg, '', 'rpc_queue');# ... then code to read a response message from the callback_queue ...The AMQP protocol predefines a set of 14 properties that go with a message. Most of the properties are rarely used, with the exception of the following:
AMQP协议预定义了14个消息属性。大部分属性都很少用到，下面的除外：
In the method presented above we suggest creating a callback queue for every RPC request. That's pretty inefficient, but fortunately there is a better way - let's create a single callback queue per client.
上面的方法我们暗示为每个RPC请求创建一个回调队列。这非常低效，但幸运的是，有一种更好的方式??我们可以为每个客户端只创建一个回调队列。
That raises a new issue, having received a response in that queue it's not clear to which request the response belongs. That's when the correlation_id property is used. We're going to set it to a unique value for every request. Later, when we receive a message in the callback queue we'll look at this property, and based on that we'll be able to match a response with a request. If we see an unknown correlation_id value, we may safely discard the message - it doesn't belong to our requests.
可这将带来新的问题，队列收到一个响应时，我们不清楚它属于哪个请求。这正是correlation_id发挥作用的地方。我们将为每一个请求设置一个唯一correlation_id。之后，当在回调队列中收到（响应）消息的时候我们来查看这个属性，基于它我们就可以把请求和响应进行匹配。要是我们检测到未知correlation_id，可能会安全丢弃这条消息??因为它不属于我们的请求嘛。
You may ask, why should we ignore unknown messages in the callback queue, rather than failing with an error? It's due to a possibility of a race condition on the server side. Although unlikely, it is possible that the RPC server will die just after sending us the answer, but before sending an acknowledgment message for the request. If that happens, the restarted RPC server will process the request again. That's why on the client we must handle the duplicate responses gracefully, and the RPC should ideally be idempotent.
你可能会问，为喵我们该忽略回调队列中的未知消息呢，而不是置为处理失败并返回一个错误？是因为存在服务端紊乱的可能性。尽管几率很小，可还是有可能??RPC服务在给我们发送完响应后宕掉，但还没来得进行消息确认。那样的话，重启的RPC服务会再次处理这个请求。 这也就是为喵客户端必须优雅地处理重复响应，而RPC服务最好的幂等的。
Our RPC will work like this:
RPC运行流程：
The Fibonacci task:
斐波那契任务：
function fib($n) {
if ($n == 0) return 0; if ($n == 1) return 1; return fib($n-1) + fib($n-2);}
We declare our fibonacci function. It assumes only valid positive integer input. (Don't expect this one to work for big numbers, and it's probably the slowest recursive implementation possible).
声明斐波那契函数。假定它只接收正整数。（别指望它能处理很大的数字，它可能是最慢的递归实现）
The code for our RPC server rpc_server.php looks like this:
rpc_server.php代码：
<?phprequire_once __DIR__ . '/vendor/autoload.php';use PhpAmqpLib\Connection\AMQPConnection;use PhpAmqpLib\Message\AMQPMessage;$connection = new AMQPConnection('localhost', 5672, 'guest', 'guest');$channel = $connection->channel();$channel->queue_declare('rpc_queue', false, false, false, false);function fib($n) { if ($n == 0) return 0; if ($n == 1) return 1; return fib($n-1) + fib($n-2);}echo " [x] Awaiting RPC requests\n";$callback = function($req) { $n = intval($req->body); echo " [.] fib(", $n, ")\n"; $msg = new AMQPMessage( (string) fib($n), array('correlation_id' => $req->get('correlation_id')) ); $req->delivery_info['channel']->basic_publish( $msg, '', $req->get('reply_to')); $req->delivery_info['channel']->basic_ack( $req->delivery_info['delivery_tag']);};$channel->basic_qos(null, 1, null);$channel->basic_consume('rpc_queue', '', false, false, false, false, $callback);while(count($channel->callbacks)) { $channel->wait();}$channel->close();$connection->close();?>The server code is rather straightforward:
服务端代码相当直白：
The code for our RPC client rpc_client.php:
rpc_client.php代码：
connection = new AMQPConnection( 'localhost', 5672, 'guest', 'guest'); $this->channel = $this->connection->channel(); list($this->callback_queue, ,) = $this->channel->queue_declare( "", false, false, true, false); $this->channel->basic_consume( $this->callback_queue, '', false, false, false, false, array($this, 'on_response')); } public function on_response($rep) { if($rep->get('correlation_id') == $this->corr_id) { $this->response = $rep->body; } } public function call($n) { $this->response = null; $this->corr_id = uniqid(); $msg = new AMQPMessage( (string) $n, array('correlation_id' => $this->corr_id, 'reply_to' => $this->callback_queue) ); $this->channel->basic_publish($msg, '', 'rpc_queue'); while(!$this->response) { $this->channel->wait(); } return intval($this->response); }};$fibonacci_rpc = new FibonacciRpcClient();$response = $fibonacci_rpc->call(30);echo " [.] Got ", $response, "\n";?>Now is a good time to take a look at our full example source code for rpc_client.php and rpc_server.php.
是时候看看整个例子的源码了rpc_client.php and rpc_server.php.
Our RPC service is now ready. We can start the server:
RPC服务准备就绪，启动！
$ php rpc_server.php [x] Awaiting RPC requests
To request a fibonacci number run the client:
运行客户端请求一个斐波那契数字：
$ php rpc_client.php [x] Requesting fib(30)
The design presented here is not the only possible implementation of a RPC service, but it has some important advantages:
这里展现的设计不是RPC服务的唯一可能实现，但它却有一些重要的优势：
Our code is still pretty simplistic and doesn't try to solve more complex (but important) problems, like:
我们的代码还是忒简化，并没有想解决更为复杂（但重要）的问题，像：
If you want to experiment, you may find the rabbitmq-management plugin useful for viewing the queues.
想尝试吗？ rabbitmq-management plugin 这里你可能会发现一些有用的插件来查看队列。
