During the past couple of months I gave a talk named "Functional Core, Reactive Shell" (or something similar) at a number of (3) events. I think is my best talk so far, and I'd like to share a summary of its contents and ideas here.

The purpose of the talk is to introduce a somehow different way to look at software architecture and how manage its complexity.

You can see the video from one of the talks here. And these are the slides:

Spaghetti, Lasagna and Pizza

We're all too familiar with so called spaghetti code, source code so entangled and messy to remind of a bowl of spaghetti, minus the deliciousness.

The opposite of spaghetti code is lasagna code, software that follows the single responsibility principle and composed of multiple layers of abstraction, each segregating parts of the code from the other making the system easier to reason about despite its size.

Another famous, and yummy, Italian food that can be used to describe software architecture is... Pizza. To understand the details of pizza code we need to look back at the lasagna for a moment.

From the tests' point of view

From the developer point of view lasagna code is great, every component has clear dependencies, and operates on a specific layer of abstraction. At the top we work with domain objects, at the bottom with database drivers and network clients.

But when writing tests for these component we bump in two issues. The first is that objects from a certain layer depend on objects from the layer below, which depend on objects from the layer below them, and so on. The higher the lasagna the taller is the dependency tree that we need to build to initialize our system under test.

The second issue is that the behaviour of some components depends on what the objects it depends upon do. For example a persistence service which only deals with domain objects in its public interface, internally relies on a database controller for a certain vendor to persist that data to disk. To test that the persistence service behaves as expected we need to test how it commands the database.

Test Doubles

Pasta lovers and test enthusiast have found a neat solution for these limitation of lasagna software, using test doubles.

Test doubles is the name of a group of testing patter based on the idea of making a special copy, a double, of an object for the purpose of facilitating testing.

We can use a stub, a test double allowing us to control the indirect inputs provided to the system under test, to replace dependencies trees that would otherwise be expensive to setup with a single copy of the top level dependency. The methods of this stub when called won't perform any logic but simply return values that we have configured at the start.

Alongside making the tests easier to setup, another advantage of using stub is that now we can easily control the inputs provided to our system, and so we can easily test any kind of scenario.

We can use a mock, at test double allowing us to control the indirect outputs produced by the system under test, to verify that top level objects depending on other at lower level consume them as intended.

Going back to our persistence service example, we could initialize it with a mock database, which doesn't really persist data to disk, but simply records the method calls it receives. Now testing the behaviour of the persistence service is just a matter of verifying it calls the expected methods, with the expected arguments.

The issue with Mocks

Mocks sound very promising on paper, but they have an issue which you'll soon come familiar with when using them. Mocks are liars. Mocks are not production code, they make you test your system with components that are not going to be used for real.

There are too many stories of software projects with test suites relying heavily on mocking going wrong despite all the tests being green.

I myself am guilty of shipping a build to testers crashing due to a networking issue that my tests didn't scam because they were relying on a mock.

"Well, didn't you test the real version of the mocked dependency?" you could ask. I had. "Clearly not well enough.". Correct, but that misses the point.

The point is that mocks make us test the wrong things. A test should assert behaviours in the form of outputs for a given input, not by verifying if a certain method is called or not. Mocks focus on implementation, while the results are what we actually care about.

Isn't one of the points of tests to enable us to change the implementation of our components without changing the system's behaviour?

If you were to test a method with this signature:

func sum (a: Int, b: Int) -> Int

Would you assert that + is called, or that given 1 and 2 the result is 3?

Smarter people than me have tackled the issue of mocking, one of those is Ken Scambler in his post "To Kill a Mockingtest". I highly recommend you read it for a deeper and more practical look at the issue, and how to solve it.

The root of the problem

Mocks are not evil, yes they are pathological liar, but at the end of the day they are just a tool, and quite a smart one actually.

The root of the problem is with the lasagna architecture itself. Layers upon layers of objects interacting with and depending upon each other make it so that a change in one layer could ripple to the others.

From Objects to Values

As I said, smarter people than me have thought about these issues, this is the beauty of software development, and science, we stand on the shoulders of giants.

In his talk "Boundaries" Gary Bernhardt proposes to shift the focus of our software design from the objects and how they interact, to the values they exchange.

By shifting towards values we can leverage concept from the functional programming world, in particular immutability and pure functions.

Immutable values cannot be changed. This makes it easy to pass them around without having to worry that some other component will change them, invalidating our assumptions.

Pure functions do not have side effects, which means that they don't produce mutations, nor they interact with the outside world, no networks calls nor file-system operations.

Testing pure functions is incredibly straightforward: provide inputs and assert that the outputs are what you expect. No need to setup stubs or mocks.

If we write our software by focusing on the data it deals with then we can have pipelines of pure functions that get values in and give values out.

But what about I/O?! As we said, a pure function does not have side effects, and as such cannot perform I/O. So how do we actually show stuff to the users or gather their inputs?

Decision and Actions

A software designed "value first" can manage side effects by... making them values as well!

The idea is to split the code that takes decision, what side effect to perform, from the code taking action, what actually performs the effect.

For example when storing some data in the database we could have a DatabaseAction value:

enum DatabaseActionMode { case Delete case Insert case Update } struct DatabaseAction { let objects: [ Persistable ] let mode: DatabaseActionMode }

Testing code that interacts with the database would then become a matter of testing the logic, pure functions, producing DatabaseAction values.

The action can then be performed by something like this:

class DatabaseService { func performAction (action: DatabaseAction) { switch action.mode { case . Delete : case . Insert : case . Update : } } }

Testing DatabaseService would be easy as well, just create a simple DatabaseAction and verify that the objects it carries are modified in database according to the value of its mode property.

Functional Core, Imperative Shell

Our software now boils down to a pipeline of purely functional values transformations that eventually produces side effects which are sent to objects that know how to perform them.

This is what Gary Bernhardt calls "Functional Core, Imperative Shell".

The functional core is made by the values and the purely functional logic, the imperative shell is the straightforward glue that takes value from one function, passes them to the next, and eventually provides a side effect value to its performer.

And that's the pizza! Values and functions are our toppings, the imperative glue code is the crust.

Functional Core, Reactive Shell

Gary Bernhardt talk is dated 2012, and he was working in Ruby. Some years have passed and some paradigms have become mainstream, so I'd like to introduce my spin on the concept, with the background of an iOS developer working in Swift.

The Functional Reactive Programming flavour introduced by RxJS and implemented in Swift by frameworks such as RxSwift and ReactiveCocoa marries very well with the concept of looking to software as a pipeline of values transformation.

By leveraging FRP we can replace most of the imperative glue code with the scaffold provided by signal transformations. We can go from "Functional Core, Imperative Shell", to "Functional Core, Reactive Shell".

Input sources, like a button the user touches, a read from the file-system, a network request, etc. can all be treated as signal producers. We can transform the input signals by using the FRP scaffolding tools like map , flatMap , zip , etc. operators apply our pure transformations, for example going from a JSON dictionary to a view update action with embedded view model. An finally we can observe those signals, and perform the side effects they produce.

This approach reduces the amount of actual business logic we need to write, boiling it down to a number of functions that we can test with confidence.

And speaking of testing we can test the behaviour of the whole reactive shell quite easily too, just provide our own source of input signals to the shell, and subscribe to them to assert the side effect values are configured as expected.

Is this pizza actually edible?

While there's little doubt that real Italian food, and Pizza in particular, is delicious, you might be wondering if the "Functional Core, Reactive Shell" is actually a good idea.

I'll be honest. There are some downsides. First of all the FRP style and its tools come with a bit of a learning curve, and not all developers are familiar with them.

Even if you and your team are prolific with this style, to be effective all your code needs to buy into this style. This could be a problem later on if you want to change architecture, or if the original team is replaced by one less familiar with FRP.

Finally, another considerable disadvantage is that the reactive shell code, the crust of the pizza, will look long and odd. Like this:

SignalProducer (values: [ databaseService.allStuff() . map { $ 0 . map { Stuff (realmObject: $ 0 ) } }, networkService.performRequest(toEndpoint: . GetStuff ) .flatMap(. Latest ) { JSON in return Stuff .stuffProducer(withJSON: JSON ) } ]) .flatten(. Merge ) . map { stuffArray in stuffArray. map { CellViewModel (stuff: $ 0 ) } } . map { viewModels in Effect . UpdateView (viewModels: viewModels) } .observeOn( UIScheduler ()) .on( failed: { [ weak self ] error in self ?.performEffect( Effect . PresentAlert (error: error)) }, next: { [ weak self ] effect in self ?.performEffect(effect) } ) .start()

You can see more code here.

My personal opinion is that these issues are balanced up by what you get out of this approach. The awkward and hard to read "reactive shell" is balanced up by very small and easy to understand toppings, and that's actually where the majority of your business logic will be.

On top of that the shell is the tells you the story of the whole system, no need to jump up and down abstraction layers and managers classes that call controllers that call other managers.

Furthermore, this approach simplifies making changes to the system. It easy to add or remove steps to the pipeline, and since each transformation is pure and isolated from the others changing them is even easier.

Finally, I see the fact that these techniques are not mainstream, and not straightforward to pick up as a non-issue. I've been very lucky to work with many software developers that embrace learning and realise that any new pattern, paradigm, or technology they learn enriches them.

Wrapping it up

I am not here to tell you that "Functional Core, Imperative/Reactive Shell" is the silver bullet of software architecture. Each project is different, and each team working on it is different as well.

I personally find this approach very interesting, and have been working in a "Functional Core, Reactive Shell" way on some side projects. Working with it is fun and rewarding, and I find it easy to pick the code up even after weeks and find my way around it straightaway.

To be honest I haven't had the chance to use it on a "real world" app, and due to that I probably have yet to bump into issues that could arise at scale, or with a larger team working on it.

I hope you'll give this approach a shot, even if sandboxed to a new small feature. If you do, please get in touch with me on Twitter @mokagio or leave a comment below. I'd love to know if you find it useful, or if you think this is a silly idea.

Leave the codebase better than you found it.