Improved LRU with basic reliable queuing

We can apply the just seen lazy pirate pattern (LPP) for the LRU Queue Broker. In this way we make it more reliable inpacting only on the client side of the application.

This approach is called in the ZGuide simple pirate pattern.

What we had, a queue of least recently used workers, managed by a ROUTER to ROUTER 0MQ broker that gets in input requests coming from clients, send them to workers (accordingly to their availability), that do the job, send the result back, through their REQ socket, to the broker, that sends it back to the original client, is matched to the previously seen LPP client.

We have alredy seen about all the code, the only issue is matching it. The full example C++ code is on github, here are just the startup lines:
boost::thread_group threads; // 1
threads.create_thread(std::bind(client, 2500,  3)); //2
threads.create_thread(lruQueue); // 3

boost::this_thread::sleep(boost::posix_time::seconds(20)); // 4
threads.create_thread(std::bind(client, 2500,  3));

1. To simplify the testing, I have put all the components in the same process, each one running in its own thread. This is not realistic, but it is not an issue to refactor the example to run as a multiprocess application.
2. The function client() is the one we have seen in the LPP, see previous post for details.
3. The lruQueue() and worker() components come from the LRU example, the worker has been modified to perform restlessly.
4. Actually, the sample worker is designed to simulate crash after a few seconds, and the client would shutdown if it won't get any feedback. On the other side, the broker is designed to run forever (but it is not protected against interrupts). Here we wait a while, to be sure that the worker crashes, and the client shuts itself down. Then we create another worker and client, and we can check how the system springs back to work.

The code shown on the ZGuide is written for the high level czmq c-binding. This porting could be interesting to you, if you want to see an example of how to do the same in C++. I have used the official C++ wrapper provided for ZeroMQ 2.x, adapted with my extension to zmq::socket_t, called zmq::Socket, that provides some better suppport to multipart and int messages.

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