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ZeroMQ by Pieter Hintjens

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Model Three: Complex and Nasty

The shotgun approach seems too good to be true. Let’s be scientific and work through all the alternatives. We’re going to explore the complex/nasty option, even if it’s only to finally realize that we preferred the brutal approach. Ah, the story of my life.

We can solve the main problems of the client by switching to a ROUTER socket. That lets us send requests to specific servers, avoid servers we know are dead, and in general be as smart as we want to be. We can also solve the main problem of the server (single-threadedness) by switching to a ROUTER socket.

But doing ROUTER to ROUTER between two anonymous sockets (which haven’t set an identity) is not possible. Both sides generate an identity for the other peer only when they receive a first message, and thus neither can talk to the other until it has first received a message. The only way out of this conundrum is to cheat and use hard-coded identities in one direction. The proper way to cheat, in a client/server case, is to let the client “know” the identity of the server. Doing it the other way around would be insane, on top of complex and nasty, because any number of clients should be able to arise independently. Insane, complex, and nasty are great attributes for a genocidal dictator, but terrible ones for software.

Rather than invent yet another concept to manage, we’ll use the connection endpoint as the identity. This is a unique string on which both sides can agree without more prior knowledge ...

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