ZeroMQ

ØMQ Is Not a Neutral Carrier

A common question that newcomers to ØMQ ask (it’s one I’ve asked myself) is, “How do I write an XYZ server in ØMQ?” For example, “How do I write an HTTP server in ØMQ?” The implication is that if we use normal sockets to carry HTTP requests and responses, we should be able to use ØMQ sockets to do the same, only much faster and better.

The answer used to be, “This is not how it works.” ØMQ is not a neutral carrier; it imposes a framing on the transport protocols it uses. This framing is not compatible with existing protocols, which tend to use their own framing. For example, compare an HTTP request (Figure 2-2) and a ØMQ request (Figure 2-3), both over TCP/IP. The HTTP request uses CRLF (carriage return line feed) as its simplest framing delimiter, whereas ØMQ uses a length-specified frame.

Figure 2-2. HTTP on the wire

Figure 2-3. ØMQ on the wire

So, you could write an HTTP-like protocol using ØMQ, using for example the request-reply socket pattern. But it would not be HTTP.

Since ØMQ v3.3, however, ØMQ has a socket option called ZMQ_ROUTER_RAW that lets you read and write data without the ØMQ framing. You could use this to read and write proper HTTP requests and responses. Hardeep Singh contributed this change so that he could ...