ZeroMQ

Using Sockets to Carry Data

To send and receive messages, you use the zmq_msg_send() and zmq_msg_recv() methods. The names are conventional, but ØMQ’s I/O model is different enough from the TCP model (Figure 2-1) that you will need time to get your head around it.

Figure 2-1. TCP sockets are 1-to-1

Let’s look at the main differences between TCP sockets and ØMQ sockets when it comes to working with data:

• ØMQ sockets carry messages, like UDP, rather than a stream of bytes as TCP does. A ØMQ message is length-specified binary data. We’ll come to messages shortly; their design is optimized for performance and so a little tricky.

• ØMQ sockets do their I/O in a background thread. This means that messages arrive in local input queues and are sent from local output queues, no matter what your application is busy doing.

• ØMQ sockets have 1-to-N routing behavior built in, according to the socket type.

The zmq_msg_send() method does not actually send the message to the socket connection(s). It queues the message so that the I/O thread can send it asynchronously. It does not block except in some exception cases. So, the message is not necessarily sent when zmq_msg_send() returns to your application.