Building Azure Service Fabric Actors with F# – Part 2


In Part 1, I provided an overview of what Service Fabric (SF) is, and provided some step-by-step guidance on how to get up and running with the Service Fabric local installation. In this post, I want to move from the infrastructure to the code, and show how we can use F# with an Actor model designed primarily for C# and VB .NET, whilst still retaining an idiomatic F# feel where possible.

All code for the full sample used as the basis for this series is available here.

Actors in Service Fabric

Firstly, I’ll show you some elided examples of how we modelled some features of my cat as an Actor in Service Fabric. Every cat has some state which is affected by actions it does, and needs to be persisted across calls. In Service Fabric, we call this a “stateful” actor. After every “state-updating” action (in SF terms, this equates to a method call on the actor), SF will automatically persist your state back to disk and automatically replicate to other nodes in the SF cluster (typically at least two others); if your primary node goes down, one of the secondaries will immediately take over and the failed node will be silently replaced in the background. You can also have so-called “read only” actions, which do not modify state but typically return some payload to the caller. You can typically think of these as “getter” methods / properties on a class. You’ll normally have a mix of both state-mutating and read-only methods on a given actor.

Implementing Stateful Actors in F#

Every stateful Actor in SF inherits from the type Actor<T>, where T is the state that needs to be persisted. It shows up as a member property on the actor, State. Service Fabric will automatically create one of these when starting every given actor, and silently persist / load it across calls etc.

We’ll start by modelling the state on the Actor by default with a standard OO class in F# – see below. Notice the DataContract and DataMember attributes – these are used by the persistence layer of SF to de/re-hydrate state to an Actor. Personally I’m not particularly fond of these attributes – there are plenty of serialization frameworks out there that seem to work just fine without decorating every single property, so why are we stuck with this old-school approach? Perhaps there’s a way to replace the serialization in SF – I haven’t tried yet.

Anyway, here’s an example method on Cat, called Jump(). It takes in a destination of where the cat is jumping to, and depending on the destination, this affects the cat – and the owner’s – Happiness (in a more fully featured model, the owner themselves would probably be an actor with their own state). The cat will also work up an appetite by Jumping(). Hunger can be alleviated by Feeding() the cat.

On the one hand, F# works nicely with interfaces – we still don’t have to specify types, as they are inferred from the interface we’re implementing. However, this sample is still somewhat unsatisfactory to me as an F#-first person: I’m used to creating copies of data from other data, not mutating it. I also don’t like this approach of modifying state in several places arbitrarily – I feel uneasy when seeing code like this. It seems very statement oriented, with side effects everywhere – something I struggle to reason about easily. There must be something better!

Use immutable data structures on Actors

As it turns out, there is. Notice that up until now we’ve basically written everything in an OO style, using standard C#/ VB constructs like classes etc. – we’ve not used any F# types. We can actually use many F# features without too much fuss, and they can quickly help us out in our quest to getting back to sane and easy-to-reason-about code.

Firstly, we can change the way we model our state from a class to an F# record. This actually works without any problem, once you do the same WCF-style attribute decoration, and add the [<CLIMutable>] attribute – this is necessary as although Records boil down to standard Classes, by default there’s no public setter on any properties, so SF can’t rehydrate state by default. We can also add in other F#-only features, like units of measure, if we want – as these are a compile-only feature, there’s no issue with serialization of them.

On their own, using records within SF only works up to a point – we’re forced to make copies of state, rather than mutating the single attributes of the State member multiple times, which is a good thing. However, it still looks undesirable – we’re now just mutating the State member property on the Actor instead! Plus it’s not clear when and where we should replace the contents of the State member within the method – every time? Once at the end of the method call? Something in between?

Adapting functional patterns into Actors

Let’s take a step back and think about the two types of methods I mentioned earlier on – state-updating and read-only calls. The former intends to do some processing, and update the State of the actor. The latter typically reads from the State and returns some data to the caller (I’m setting aside things like calling external dependencies etc. which for simplifies’ sake we can ignore – plus it really doesn’t affect us here as we would partially apply our functions with dependencies). We can formally specify such actions and implement them with something like this: –

Notice how now our functions are much simpler – Jump is made up of a single expression that generates the new State of the Actor, based on the input state and distance – we’re no longer mutating state multiple times, or even once. And because State is an immutable record, it’s impossible to modify the supplied input State ever.

Plugging pure functions into Actors

Now that we’ve formalised how we see our actor methods working, we can re-write our earlier code from the anything-goes, mutate-everywhere C# style to one that is easier to test, easier to reason about and more idiomatic from an FP, F# point of view. You’ll notice that the implementation code above is back in a module – so how do we plug this into our OO Actor model?

There are a few ways, but the easiest one is with the help of a couple of shim functions that tightly control the mutation of the Actor State, whilst delegating control to our purely functional code for business logic. Our core code is kept free from worrying about the mutation of state and is performed in a consistent manner; our SF Actor model simply delegates to them.

A word on Read-Only Service Fabric methods

Another point worth mentioning are Read Only methods in Service Fabric. These are methods that you, as the developer, tell the SF runtime “I will never amend state in this method – don’t try to persist state at the end of the call”. This is achieved in SF simply by placing the [<Readonly>] attribute on the method. I don’t like this much for two reasons. Firstly, the attribute differs from the System.ComponentModel [<ReadOnly>] attribute simply by virtue of the fact that it has a different casing on one of the characters in the type. Use the wrong one accidentally and things will quickly go pop with your actor (believe me – I did it during the creation of the code referenced in this post; the error that you get isn’t helpful either). The other, more dangerous issue is that there is no compile time safety around the use of the [<Readonly>] attribute. If you decide to start changing state in one of these calls – tough. You won’t get any support from the compiler, nor from the runtime. Your method simply won’t update state and you’ll be left wondering why your application isn’t behaving correctly.

With the “adapt to a functional style” approach, whilst we don’t eliminate the issue completely – you still have to decorate the methods appropriately – we at least get compile-time checking on read-only functions, because they don’t allow us to return state; you therefore can’t accidentally modify the state of an actor. In addition, because we’re now using records, which are themselves immutable, it’s impossible for us to modify the state that was supplied to us.

For a simple example like the one supplied, one could argue that the extra delegation and modules etc. complicates matters compared to e.g. C# / OO. However, once you start writing even mildly complicate business logic, it quickly becomes a tiny cost compared to the simplification you benefit from through immutability, records etc.. as well as the usual other benefits of F#.

Taking it further

You can take this approach even further – in other actor frameworks, rather than adopting the “method-per-action” approach, a more functional approach is to have a single message which is itself a discriminated union containing all the different messages ; we then pattern match on this in order to process the message appropriately. We can apply this sort of pattern for updating-state messages, although it isn’t exactly idiomatic SF actor code (I’ve supplied an example in the source code).

Another alternative might be to create a custom Computation Expression (perhaps similar to the Writer monad that Tomas Petricek blogged about many moons ago) in order to make this modification to state even more succinct. Perhaps someone could write one 😉

Conclusion

We’ve seen how we can marry up some features inherent to the F# type system in order to enforce a cleaner way of reasoning about the code that our actors have to implement, through a couple of simple function signatures and some simple adaptors. We’ve also seen how F#, and typical FP paradigms, can be used in an reliable and distributable framework designed for a mutable-first OO consumer.

In part three, I want to illustrate how we can quickly and easily host arbitrary services on top of Service Fabric in F# for just about any code you might want to write, and how we can easily scale it to large volume.

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