2021-08-26 - Hexagonal architecture in Rust #3

This article is part of the following series:

Gather, my fellow soldiers. Today we're going to fight! Who, you tell me? The unspoken devil of these lands, the borrow-checker!

Okay, let's stop with this Lord-of-the-Ring-esque impression, work awaits us :) In the previous articles, we defined our domain entities and implemented a use case and a repository.

src
├── domain
│   ├── create_pokemon.rs
│   ├── entities.rs
│   └── mod.rs
├── repositories
│   ├── mod.rs
│   └── pokemon.rs
└── main.rs

We could have given it to our client, but apart from running the tests, the main.rs file still only outputs an hello world. Today, we are going to transform our project into a nice HTTP API returning JSON.

The HTTP API

If you remember well, I've not used async in the project. This is to focus on the architecture of our application. If you really want to use async, go for it ;) There are not a lot of non-async web framework crates, but some still exists. My choice for this article is rouille, which will handle our use cases nicely.

So first, we open Cargo.toml and add it to our dependencies:

[dependencies]
rouille = "3.2.1"

Let's now create a folder which will contain our API. Inside of it, I'm going to create the usual mod.rs file where we'll add the basic routing logic. I'll also add a simple health.rs file which will handle our first route:

src
└── api
    ├── health.rs
    └── mod.rs

The only use of rouille will be inside of the api folder. If in the future, we want to replace rouille with actix, we'll just have to change the content of api (actually we'll also have to make some functions async but that's orthogonal to the web framework).

Let's now add some code to have a basic working API returning some text when sending a GET request on /health. For this, let us first import the rouille macros in our main.rs file and use some function we'll create afterwards:

mod api;
mod domain;
mod repositories;

#[macro_use]
extern crate rouille;

fn main() {
    api::serve("localhost:8000");
}

Let's now add our serve function to the api/mod.rs:

mod health;

pub fn serve(url: &str) {
    rouille::start_server(url, move |req| {
        router!(req,
            (GET) (/health) => {
                health::serve()
            },
            _ => {
                rouille::Response::from(Status::NotFound)
            }
        )
    });
}

I like to create an HTTP status enum to easily return status codes:

enum Status {
    BadRequest,
    NotFound,
    Conflict,
    InternalServerError,
}

impl From<Status> for rouille::Response {
    fn from(status: Status) -> Self {
        let status_code = match status {
            Status::BadRequest => 400,
            Status::NotFound => 404,
            Status::Conflict => 409,
            Status::InternalServerError => 500,
        };
        Self {
            status_code,
            headers: vec![],
            data: rouille::ResponseBody::empty(),
            upgrade: None,
        }
    }
}

Now, we just have to edit api/health.rs:

use rouille;

pub fn serve() -> rouille::Response {
    rouille::Response::text("Gotta catch them all!")
}

Now you should be able to run the app using cargo run and go to localhost:8000/health with your browser. There, a beautiful message is waiting for you:

Gotta catch them all!

Great! But I told before we want a JSON API. Let's convert this endpoint to return JSON then. serde will help us. rouille already uses some serde traits, let's add the same version as rouille uses in our Cargo.toml. To do that I run cargo tree | grep serde:

├── serde v1.0.129
├── serde_derive v1.0.129 (proc-macro)
├── serde_json v1.0.66
│   └── serde v1.0.129

So let's go with:

[dependencies]
rouille = "3.2.1"
serde = { version = "1.0.129", features = ["derive"] }
serde_json = "1.0.66"

We'll also have to add the import in 'main.rs`:

extern crate serde;

We are done with the dependencies! Now we can edit api/health.rs:

use rouille;
use serde::Serialize;

#[derive(Serialize)]
struct Response {
    message: String,
}

pub fn serve() -> rouille::Response {
    rouille::Response::json(&Response {
        message: String::from("Gotta catch them all!"),
    })
}

Open your browser and tada :D

{
    "message": "Gotta catch them all!"
}

Fetch the request

Our client does not really want a message, even if it's nice. What he wants is being able to create a Pokemon. Let's do it then. First, as our API will be RESTful, here is an example of the HTTP requests we'll work with:

- POST http://localhost:8000
- Headers
    Content-Type: application/json
- Body 
    {
        "number": 4,
        "name": "Charmander",
        "types": ["Fire"]
    }

Now let's go back to api/mod.rs to add a new route:

mod create_pokemon;
mod health;

pub fn serve(url: &str) {
    rouille::start_server(url, move |req| {
        router!(req,
            ...
            (POST) (/) => {
                create_pokemon::serve(req)
            },
            ...
        )
    });
}

Let's create a new file api/create_pokemon.rs and fill it with the following stub:

use rouille;
use serde::Serialize;

#[derive(Serialize)]
struct Response {
    message: String,
}

pub fn serve(_req: &rouille::Request) -> rouille::Response {
    rouille::Response::json(&Response {
        message: String::from("Pokemon created!"),
    })
}

Now you can use your favorite REST client (postman, curl, ...) to send a POST request on localhost:8000, body can be anything for now. You should get back the following:

{
    "message": "Pokemon created!"
}

That's nice and all, but it would be better to send back a 400 status code when the request body is not what we want. Let's edit api/create_pokemon.rs a bit:

use crate::api::Status;
use serde::{Deserialize, Serialize};

#[derive(Deserialize)]
struct Request {
    number: u16,
    name: String,
    types: Vec<String>,
}

pub fn serve(req: &rouille::Request) -> rouille::Response {
    match rouille::input::json_input::<Request>(req) {
        Ok(_) => {}
        _ => return rouille::Response::from(Status::BadRequest),
    };
    ...
} 

Now, if you send a request with no name value, or if number is negative, you'll get a 400 status code back.

Plug the repository

Okay, but the Pokemon is actually neither created nor added to a repository. Moreover, the use case is never called! Let's first add an in-memory repository in main.rs:

use repositories::pokemon::InMemoryRepository;

fn main() {
    let repo = InMemoryRepository::new();
    api::serve("localhost:8000", &repo);
}

So we now have to edit accordingly api/mod.rs:

use crate::repositories::pokemon::Repository;

pub fn serve(url: &str, repo: &mut dyn Repository) {
    rouille::start_server(url, move |req| {
        router!(req,
            ...
            (POST) (/) => {
                create_pokemon::serve(repo, req)
            },
            ...
        )
    });
}

Don´t forget to edit api/create_pokemon.rs:

use crate::repositories::pokemon::Repository;

pub fn serve(_repo: &mut dyn Repository, req: &rouille::Request) -> rouille::Response {

You can now run cargo run and it should be w...

error[E0277]: `dyn Repository` cannot be sent between threads safely
= help: the trait `Send` is not implemented for `dyn Repository`
error[E0277]: `dyn Repository` cannot be shared between threads safely
= help: the trait `Sync` is not implemented for `dyn Repository`
error: aborting due to 2 previous errors

I've removed a lot of error log but you have the basics here. Something does not work and it is because of... the borrow-checker. I mean it is because of me, but the borrow-checker got our back :)

Checkmate for the borrow-checker

As usual, the compiler is quite helpful: it tells us to implement Send and Sync on Repository. Let's do it by editing repositories/pokemon.rs:

pub trait Repository: Send + Sync {
    fn insert(&mut self, number: PokemonNumber, name: PokemonName, types: PokemonTypes) -> Insert;
}

Rust is easy, right? Our relief will be very short because as soon as cargo run is run:

error[E0621]: explicit lifetime required in the type of `repo`
 --> src/api/mod.rs:7:5
  |
6 | pub fn serve(url: &str, repo: &mut dyn Repository) {
  |                               ------------------- help: add explicit lifetime `'static` to the type of `repo`: `&'static mut (dyn Repository + 'static)`

Now, the compiler tells us he wants a 'static lifetime on the repository. Let's think about it for a bit. What is really the issue here? We want a reference to the repository to be sent to the various threads which will be spawned for every request. For now we create a repository with our InMemory struct. The thing is, when our app execution will get to the end of the main function, InMemory will be dropped. But maybe some threads will still have a reference to this struct. Hence the compiler error. What we want instead is some way to tell the app to drop InMemory only when the references do not exist anymore. This way is called a reference counter. And we are in luck, Rust provides two types for that, with one especially created to be safe to share between threads. Its name is Arc, and that's what we'll be using.

So let's add the Arc wrapper to our repository in main.rs.

use std::sync::Arc;

fn main() {
    let repo = Arc::new(InMemoryRepository::new());
    api::serve("localhost:8000", repo);
}

You can see that we removed two things: a & and a mut. Arc is actually a pointer so its size is known at compile time. It points to the repository which is located in the heap. Therefore we don't need a reference to it. Secondly, Arc is not mutable so we'll have to use inner mutability. We'll talk about that later.

Let's now edit api/mod.rs:

use std::sync::Arc;

pub fn serve(url: &str, repo: Arc<dyn Repository>) {
    rouille::start_server(url, move |req| {
        router!(req,
            ...
            (POST) (/) => {
                create_pokemon::serve(repo.clone(), req)
            },
            ...
        )
    });
}

And finally, let's edit api/create_pokemon.rs:

use std::sync::Arc;

pub fn serve(_repo: Arc<dyn Repository>, req: &rouille::Request) -> rouille::Response {

It compiles \o/

The domain needs some love too

We have designed our app around a domain with use cases getting data and a repository. Like before, we'll have to replace the mutable reference to an Arc in the use case too. Good thing I've only implemented one use case for now ;) Let's edit the execute function signature in domain/create_pokemon.rs:

use std::sync::Arc;

fn execute(repo: Arc<dyn Repository>, req: Request) -> Response {

Don't forget the tests!

let repo = Arc::new(InMemoryRepository::new());
let res = execute(repo, req);

On cargo run, we stumb upon an issue I've talked about before: an Arc is not mutable.

25 |         (Ok(number), Ok(name), Ok(types)) => match repo.insert(number, name, types) {
   |                                                    ^^^^ cannot borrow as mutable

If we check Repository in repositories/pokemon.rs, we can actually see that the insert function expects the repository to be mutable:

pub trait Repository: Send + Sync {
    fn insert(&mut self, number: PokemonNumber, name: PokemonName, types: PokemonTypes) -> Insert;
}

So we'll remove this mut in the trait and in our implementation :) Let's run cargo run:

36 |     fn insert(&self, number: PokemonNumber, name: PokemonName, types: PokemonTypes) -> Insert {
   |               ----- help: consider changing this to be a mutable reference: `&mut self`
...
46 |         self.pokemons.push(Pokemon::new(number_clone, name, types));
   |         ^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable

Ouch, this error message is not very helping. We just removed mut and now the compiler wants us to add it back. Actually that's logical, the compiler does not know the repository is inside an Arc.

The interesting thing here is that the issue is not on the trait anymore but only on our InMemory implementation. We need to be able to mutate pokemons without self being mutable. That's interior mutability. And, again, Rust provides some primitives for that! We'll choose the Mutex primitive as it is designed for data being shared between threads. So let's wrap pokemons into a Mutex:

use std::sync::Mutex;

pub struct InMemoryRepository {
    error: bool,
    pokemons: Mutex<Vec<Pokemon>>,
}

impl InMemoryRepository {
    pub fn new() -> Self {
        let pokemons: Mutex<Vec<Pokemon>> = Mutex::new(vec![]);
        Self {
            error: false,
            pokemons,
        }
    }
}

Now, we'll have to lock the Mutex in order to read or write pokemons. Locking a Mutex means that the theads wait in turns to read or write the data it holds so only one thread accesses the data at all times.

impl Repository for InMemoryRepository {
    fn insert(&self, number: PokemonNumber, name: PokemonName, types: PokemonTypes) -> Insert {
        if self.error {
            return Insert::Error;
        }

        let mut lock = match self.pokemons.lock() {
            Ok(lock) => lock,
            _ => return Insert::Error,
        };

        if lock.iter().any(|pokemon| pokemon.number == number) {
            return Insert::Conflict;
        }

        let number_clone = number.clone();
        lock.push(Pokemon::new(number_clone, name, types));
        Insert::Ok(number)
    }
}

Now it compiles and all the tests still pass!

API + domain = <3

It is the time to connect the API to the domain. Let's edit api/create_pokemon.rs:

use crate::domain::create_pokemon;

pub fn serve(repo: Arc<dyn Repository>, req: &rouille::Request) -> rouille::Response {
    let req = match rouille::input::json_input::<Request>(req) {
        Ok(req) => create_pokemon::Request {
            number: req.number,
            name: req.name,
            types: req.types,
        },
        _ => return rouille::Response::from(Status::BadRequest),
    };
    match create_pokemon::execute(repo, req) {
        create_pokemon::Response::Ok(number) => rouille::Response::json(&Response { number }),
        create_pokemon::Response::BadRequest => rouille::Response::from(Status::BadRequest),
        create_pokemon::Response::Conflict => rouille::Response::from(Status::Conflict),
        create_pokemon::Response::Error => rouille::Response::from(Status::InternalServerError),
    }
}

And publish the needed things from domain:

// domain/mod.rs
pub mod create_pokemon;

// domain/create_pokemon.rs
pub struct Request {
    pub number: u16,
    pub name: String,
    pub types: Vec<String>,
}

pub enum Response {
    ...
}

pub fn execute(repo: Arc<dyn Repository>, req: Request) -> Response {
    ...
}

You can run cargo run and send a valid request to the create_pokemon route. You should get a valid response:

{
    "number": 30
}

\o/

Next steps

That was longer than expected, sorry for that. I hope it was useful to you :) In the next article, I'll implement the other use cases (the client is tired of waiting for me to explain everything, bad client :p). After that I'll implement other front-end and repositories to get a better understanding of the power of the hexagonal architecture. If you want to be notified of the release of the next article, feel free to subscribe to the rss feed or to my twitter.

As usual, the code is accessible on github.

Thanks