The 0MQ team has just released a draft specification of the 0MQ transport layer. It’s a sweet little spec—go take a look! The nice thing about it is that it concentrates on building a flexible transport layer without putting any constraints on the content, routing or meaning of the datagrams it carries.1
Another interesting aspect of it is that messages are structured as a list of segments (frames)—and that structure is exposed to the next layer up! Such a seemingly small decision has large, and favourable, consequences: the next layer up can use the segmented structure of messages to represent many interesting patterns:
- a split between message envelope and message contents;
- a split between message headers and message body;
- a list of addresses to use in routing a message, like the old-school bang paths;
- a stream of content being delivered in chunks as it becomes available; and so on.
In my opinion, the AMQP spec should have been split up into similarly small, flexible pieces as part of the work leading up to the current 1.0 drafts. In defence of the working group’s current approach, a monolithic spec can make it easier to build a coherent whole; but on the minus side, it can make it so difficult to gain experience with the system being designed that coherence remains out of reach. For instance, it is not now (and probably never will be) possible to write a one-line message sender using bash for AMQP.
Well, that’s not quite true: it describes parts of the lowest levels of a few messaging patterns that the transport has been used for in addition to the transport itself; my opinion is that those pieces should be split out from spec:13 and placed in specifications of their own. ↩