RabbitMQ is a very extensible message broker, allowing users to extend the server’s functionality by writing plugins. Many of the broker features are even shipped as plugins that come by default with the broker installation: the Management Plugin, or STOMP support, to name just a couple. While that’s pretty cool, the fact that plugins must be written in Erlang is sometimes a challenge. I decided to see if it was possible to write plugins in another language that targeted the Erlang Virtual Machine (EVM), and in this post I’ll share my progress.
RabbitMQ 2.4.0 introduced an extension that allows publishers to specify multiple routing keys in the CC and BCC message headers. The BCC header is removed from the message prior to delivery. Direct and topic exchanges are the only standard exchange types that make use of routing keys, therefore the routing logic of this feature only works with these exchange types.
Arriving in RabbitMQ 2.1.1, is support for bindings between exchanges. This is an extension of the AMQP specification and making use of this feature will (currently) result in your application only functioning with RabbitMQ, and not the myriad of other AMQP 0-9-1 broker implementations out there. However, this extension brings a massive increase to the expressivity and flexibility of routing topologies, and solves some scalability issues at the same time.
Normal bindings allow exchanges to be bound to queues: messages published to an exchange will, provided the various criteria of the exchange and its bindings are met, pass through the various bindings and be appended to the queue at the end of each binding. That’s fine for a lot of use cases, but there’s very little flexibility there: it’s always just one hop – the message being published to one exchange, with one set of bindings, and consequently one possible set of destinations. If you need something more flexible then you’d have to resort to publishing the same message multiple times. With exchange-to-exchange bindings, a message published once, can flow through any number of exchanges, with different types, and vastly more sophisticated routing topologies than previously possible.