ZeroMQ

What’s This Good For?

To be honest, the use cases for strict request-reply or extended request-reply are somewhat limited. For one thing, there’s no easy way to recover from common failures like the server crashing due to buggy application code (we’ll see more about this in Chapter 4). However, once you grasp the way these four sockets deal with envelopes, and how they talk to each other, you can do very useful things. We saw how ROUTER uses the reply envelope to decide which client REQ socket to route a reply back to. Now let’s express this another way:

• Each time ROUTER gives you a message, it tells you what peer that message came from, as an identity.

• You can use this with a hash table (with the identity as the key) to track new peers as they arrive.

• ROUTER will route messages asynchronously to any peer connected to it, if you prefix the identity as the first frame of the message.

ROUTER sockets don’t care about the whole envelope. They don’t know anything about the empty delimiter. All they care about is that one identity frame that lets them figure out which connection to send a message to.