Request-Reply Combinations

We have four request-reply sockets, each with a certain behavior. We’ve seen how they connect in simple and extended request-reply patterns. But these sockets are building blocks that you can use to solve many problems.

These are the legal combinations:

  • REQ to REP

  • DEALER to REP

  • REQ to ROUTER

  • DEALER to ROUTER

  • DEALER to DEALER

  • ROUTER to ROUTER

And these combinations are invalid (I’ll explain why):

  • REQ to REQ

  • REQ to DEALER

  • REP to REP

  • REP to ROUTER

Here are some tips for remembering the semantics. DEALER is like an asynchronous REQ socket, and ROUTER is like an asynchronous REP socket. Where we use a REQ socket, we can use a DEALER; we just have to read and write the envelope ourselves. Where we use a REP socket, we can stick a ROUTER; we just need to manage the identities ourselves.

Think of REQ and DEALER sockets as “clients” and REP and ROUTER sockets as “servers.” Mostly, you’ll want to bind REP and ROUTER sockets, and connect REQ and DEALER sockets to them. It’s not always going to be this simple, but it is a clean and memorable place to start.

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