Reactive Programming

There's an interesting idea being pushed forward by the Scala community about the way systems should be built, and it's called reactive programming. The reactive manifesto gives a nice summary of their views on the future of distributed applications and how the way we programmed ten years ago is inadequate in this future. Because of the changing landscape of latency expectations, data scale, and hardware improvements, going forward, systems must be event-driven, scalable, resilient, and responsive in order to meet user needs. There is a significant amount of ongoing Scala development in order to build tools that allow programmers to achieve these properties easily. This is the goal of the akka and spray projects.

The reactive programming model being adopted is based on actors. Given that I am a huge fan of the actor model for how it deals with the problems of concurrency, it is great to see an entirely new ecosystem developing around actors. The Scala community has really invested in actors as the new way in which communication between systems (or even within them) will be done in the future. From some slides for a recent talk on akka and spray, actors are part of a different programming paradigm that is "well-suited for building reactive systems" because they inherently shun the root of all concurrency evil, shared mutable state. Specifically, using actors versus the old model of managing threads in servlets gets around the high overhead of such management as well as the difficulties of synchronizing state. In some ways, we can view this as the next step in lightweight request handling: it was once (and still sometimes is) the case that each HTTP request would spawn a process, but that was greatly improved upon by having many threads within a single process handle requests. With actors, a single thread can handle many requests at the same time, allowing the resources to be much more efficiently utilized (this is the premise behind Node.js as well).

Spray is what you would expect the HTTP stack to look like on top of akka. While I haven't played with it myself, the code examples show that requests are mapped directly to actor messages, so everything is under the unified model of message handling. Judging from this, they have done a pretty good job from a performance standpoint as well, indicating just how lightweight request handling is in spray. Akka and spray together form a solid foundation on which you can build an application that conforms to the principles behind reactive programming. It is exciting to see technology evolving to meet the needs of users, programmers, and applications, and I'm sure that in a few years we'll see how far this paradigm can take us.