Akka.NET is described as “a toolkit and runtime for building highly concurrent, distributed, and fault tolerant event-driven applications on .NET & Mono.” It is a port of the Akka framework for the JVM written in Scala. Its initial release was in April 2015, not long after Microsoft’s similar cloud-oriented Project Orleans (February 2015). Orleans is described as “a framework that provides a straightforward approach to building distributed high-scale computing applications, without the need to learn and apply complex concurrency or other scaling patterns.”
Each of these frameworks is based on the Actor Model of concurrency of which more later.
I first heard of Akka via a polyglot developer colleague who has extensive experience of both Java and .NET. He happened to get into some Scala development and was fortunate enough to get some experience with Akka. Later on I started encountering various references to .NET Actor frameworks/libraries, almost all in their very early stages. In February 2014 I came across a link to Roger Johansson’s Pigeon project in Github that later became Akka.NET. A year later via my F# Weekly feed I saw that Akka.NET was in beta, so I browsed to the site and was amazed at how much information was there. There was also a Visual Studio Nuget package that I tried and it “just worked,” no faffing around with configuration. That’s not always the case with open source projects. Then a few weeks after that it reached 1.0.
The Actor Model of Concurrency
The Actor Model in computer science is “a mathematical model of concurrent computation that treats ‘actors’ as the universal primitives of concurrent computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message received.”
“According to Carl Hewitt, unlike previous models of computation, the Actor model was inspired by physics, including general relativity and quantum mechanics.”
Wow! But don't worry. You don't need to understand general relativity and quantum mechanics to get started!
One way of thinking about the Actor Model is by analogy to garbage collection or other automated memory management schemes. You can view garbage collection as providing a high-level abstraction over manual memory management. Similarly you can view the Actor Model as providing a high-level abstraction over manual thread management and synchronization. The reason why the Actor Model is attracting a lot of attention now is due to the rise of multiple processors and multi-cores combined with the growth of the internet and highly distributed computing. Actor-based frameworks such as Akka and Orleans are more easily able to handle these scenarios, freeing the developer to concentrate on solving business problems rather than getting bogged down in “low-level” concurrency issues.
Akka.NET provides an actor system that the user typically arranges into a hierarchy (tree) of actors that communicate with each other via immutable messages. Actors supervise the actors directly below them in the tree and are responsible for handling their failures. When an actor crashes, its parent can either restart or stop it, or escalate the failure up the hierarchy of actors. It is this that enables “self-healing” – fault tolerance and resilience.
Each actor has its own state that is not shared with other actors. Actors send messages to other actors asynchronously so that they don’t block. Actors process received messages one at a time. They can also determine how to respond to the next message received. This is called switchable behaviour. Supervision and switchable behaviours are two of the “killer” features of the Actor Model.
Well, that’s the basic idea. There are a lot more features available but I hope this gives you a flavour. Apart from the Akka.NET site you can also find some excellent, well-written blog posts by Petabridge. (one of the creators of the framework). They also provide a free online Bootcamp. If you have a subscription to Pluralsight then, at the time of writing, there are four excellent courses on Akka.NET.