Creating a Sinatra API with system-wide dependency injection using dry-system and rom-rb

Sinatra is often seen as a tool for simple APIs, but it can also be used to manage big applications. dry-rb libraries can help you create a modular architecture with system wide dependency injection for your application.

Table of contents

• 1. Introduction
• 2. What do we do when our applications start to grow?
• 3. How do we solve these problems? dry-system to the rescue
• 4. Improving our Sinatra application
• 5. Adding dry system and dry auto_inject gems as our dependency injection layer
• 6. Adding database connections with ROM and our modular architecture
• 7. Conclusion

Introduction

Today, among beginners with Ruby, it's common to think about two possible paths when developing an application; if you want a simple single-file API, just use Sinatra and for everything else, use Ruby on Rails. Well, in this article, allow me to provide a way to manage a big application using Sinatra as the HTTP library and dry-rb libraries as the glue to a modular architecture.

Note: All the code produced on this article can be found at: https://github.com/cherryramatisdev/api-with-dry-ruby

What do we do when our applications start to grow?

What do we do when an application with a single Ruby file starts to grow with more dependencies? For me personally, the answer is dependency injection. Basically, I start thinking about how I'll manage the configuration of all these new libraries and use them quickly on my routes, so it's trivial to split routes into services and controller classes in the future.

OK, but how do we do that?

The basic understanding of dependency injection can be seen from the following perspective:

Consider this "service" class:

class SomeService
  def something_important
    # doing something important here

    'information'
  end
end

If we want to inject this service, we can simply instantiate it in our constructor, for example, on a controller:

require_relative 'services/some_services'

class SomeController
  def initialize
    @service = SomeService.new
  end

  def index
    response = @service.something_important

    {result: response}.to_json
  end
end

But what problem does this approach have? Well, this doesn't provide much complexity for a small-scale application, and it's pretty simple to keep all the components isolated and available, but we introduce some annoyances for medium to large-scale applications, such as:

1. Not all providers have simple setups :: Some providers, like ORMs need more configuration, and this can be hard to maintain and make available through the application.

2. Some providers depend on another provider :: It's quite hard to manage by hand when you want one provider for the database connection and another one for the repositories, and this happens a lot.

3. Require hell :: On Ruby, we don't have the habit of importing all our libraries and internal code on every single file; frameworks such as Ruby on Rails provide auto-require for files with business logic, and when you roll an application by hand, it's hard to develop without this feature.

How do we solve these problems? dry system to the rescue

We'll assume a simple Sinatra application and evolve from that by adding dry-system to manage our dependencies; later on, we'll even add a persistence layer using a gem called rom-rb to increase the functionality for a more realistic API example.

A simple Sinatra application

Sinatra is a lightweight library that's quite simple to set up, but let's start with a more structured project, shall we?

DISCLAIMER: This part assumes basic knowledge about ruby language and the sinatra library.

Start a bundle project

mkdir myproject && cd myproject && bundle init

Add our gems

bundle add sinatra puma

Create a router class to encapsulate our execution

Located at config/router.rb

require 'sinatra/base'

class Router < Sinatra::Base
  get '/' do
    {message: 'Hello world'}.to_json
  end
end

1. Add a config.ru to serve as the entry point for our application

Located at config.ru on the project root

require_relative 'config/router'

Router.run!

With this initial setup, we should be able to run the application with bundle exec puma and see a JSON as the response.

Improving our Sinatra application

To make it easier for us to visualize the benefit of dependency injection, let's add some structure to this simple route by creating two simple abstractions: controller and service.

First, we'll create a service located at lib/service/user.rb with the following content:

module Services
  class User
    def index
      'teste'
    end
  end
end

Then let's create a sample controller located at lib/controllers/user.rb with the following content:

require_relative 'lib/services/user'

module Controllers
  class User
    def initialize
      @service = Services::User.new
    end

    def index
      {message: @service.index}.to_json
    end
  end
end

As you can see, we're already instantiating the service the old way, so we can compare by adding dry-system to it!

To wrap up, just update the content of config/router.rb file:

require 'sinatra/base'
require_relative 'lib/controllers/user'

class Router < Sinatra::Base
  get '/' do
    Controllers::User.new.index
  end
end

Simple right? Now everything should work fine, but we won't stop there, so let's start integrating dry-system into it and seeing the benefits.

Adding dry system and dry auto inject gems as our dependency injection layer

Adding our gems

bundle add dry-system dry-auto_inject zeitwerk

Making our application REPL work

A REPL (Read-Eval-Print Loop) is a very important tool for Ruby developers. Both the Rails and Hanami frameworks provide one, so we'll set up a simple REPL for our application. This will allow us to further integrate the dependency injection layer, which will make our code more modular and easier to test.

To do this, we'll create a file called config/boot.rb and add the following code:

ENV['APP_ENV'] ||= 'development'

require 'bundler'
Bundler.setup(:default, ENV.fetch('APP_ENV', nil))

After that create a script file under bin/console with the following content:

#!/usr/bin/env ruby
require 'irb'

IRB.start

To make it executable you can run chmod +x ./bin/console

Now we should have a working REPL for the application!

Creating our main container

This container will be used to register all the other components of our application

Create a file under config/application.rb with the following:

require 'dry/system'

class Application < Dry::System::Container
  configure do |config|
    config.root = Pathname('.')

    config.component_dirs.loader = Dry::System::Loader::Autoloading

    config.component_dirs.add 'lib'
    config.component_dirs.add 'config'
  end
end

loader = Zeitwerk::Loader.new
loader.push_dir(Application.config.root.join('lib').realpath)
loader.push_dir(Application.config.root.join('config').realpath)
loader.setup

You can see with this code that we're already solving one of the problems; the component_dirs.add method and the Zeitwerk instance will automatically require all our code inside the lib and config folders.

Note: The zeitwerk gem is doing the lazy loading for us.

Let's include this in our entry points to make it work right away.

On the config.ru and on the bin/console we'll add the following:

require_relative 'config/application'

Application.finalize!

The finalize! method makes the Application instance variable available for the whole application and lazy-loads our files under the lib and config folders.

Tip: You can and it's encouraged to remove the require_relative from your controller and router file

Now you can run bin/console and check the application instance by typing Application on the REPL.

Adding a sample service as a provider

Now that we have our main container, it's just a matter of registering providers to it, just like the following:

Create a file located at config/providers/services.rb with the following content:

Application.register_provider(:services) do
  start do
    register('services.user', Services::User.new)
  end
end

And after creating this provider, we'll load it on our entry point files; these are the only places where we'll require files.

On config.ru:

require_relative 'config/providers/services'

And on bin/console:

require_relative '../config/providers/services'

Enjoying the benefits of our work

Going back to our controller class, we can rewrite it like this:

module Controllers
  class User
    def initialize
      @service = Application['services.user']
    end

    def index
      {message: @service.index}.to_json
    end
  end

See how the controller class doesn't know anything about which class it's getting from Application['services.user']? This is so cool because if you want to change your service completely, you can simply change the class instantiation on the provider file.

This initial purpose already works for us, right? But we'll keep going further.

Adding database connections with ROM and our modular architecture

Now that we have a basic understanding of how dry-system works to modularize our application, let's add a database layer using this knowledge while levering rom-rb with it.

Adding our gems

bundle add rom rom-repository rom-sql pg

Registering a database connection as a provider for our system

Since we're already using dry-system up to this point, let's work with it by adding the database connection as a provider:

Create a file located at config/providers/db.rb with the following content

Disclaimer: This assumes you're running a PostgreSQL database.

Application.register_provider(:db) do
  prepare do
    require 'rom'
    require 'rom-sql'
  end

  start do
    connection = Sequel.connect('postgres://postgres:postgres@localhost:5432/example_database', extensions: %i[pg_timestamptz])
    register('db.connection', connection)
    register('db.config', ROM::Configuration.new(:sql, connection))
  end
end

As you can see, the register_provider method provides a simple DSL that we can use to isolate our whole setup by requiring the correct libraries on prepare and then instantiating or registering the objects on start.

Adding support for migration commands

Now that we have our base connection done, let's create a Rakefile on the root of our project with the following content:

require 'rom-sql'
require 'rom/sql/rake_task'

require_relative 'config/boot'
require_relative 'config/application'
require_relative 'config/providers/db'

namespace :db do
  task :setup do
    Application.start(:db)
    config = Application['db.config']
    config.gateways[:default].use_logger(Logger.new($stdout))
  end
end

As you can see, we can use the start method as an alternative to the finalize! that injects all our providers. That way, we only enable the database layer through the :db symbol. This allows us to inject on the :setup task.

Now we should be able to run the following command:

rake "db:create_migration[create_users]"

This should create a file located at db/migrate/3128932189_create_users.rb, on this file, we can complement the following DSL to create a sample table for our application:

ROM::SQL.migration do
  change do
    create_table :users do
      primary_key :id
      column :name, String
      column :email, String
    end
  end
end

And finally, by running the following command, we can persist this migration on the Postgres database:

rake db:migrate

Defining our relations and repositories

In the rom-rb gem, we define our main classes as relations and repositories. Relations mimic the structure of our Postgres table, while repositories define our actions on that relation.

First, we'll define a relation to represent the new table we created. To do this, we'll create a file called lib/relations/users.rb and add the following code:

module Relations
  class Users < ROM::Relation[:sql]
    schema(:users) do
      attribute :id, Types::Integer
      attribute :name, Types::String
      attribute :email, Types::String

      primary_key :id
    end
  end
end

Here we're using the simple DSL provided by ROM::Relation class to mimic our migration with the correct types for each attribute.

Now for the repository, we can create a file at lib/repos/user.rb with the following content:

require 'rom-repository'

module Repos
  class User < ROM::Repository[:users]
    commands :create

    # @param limit Integer
    def all(limit = 10)
      users.limit(limit).to_a
    end
  end
end

Repositories on ROM have sample commands for common actions, such as
creating, updating, and deleting records. However, for more complex queries, we need to write our own methods. In this case, I have provided a simple all method that returns all the users, limited to a certain number.

Making our code available through the codebase

Since we defined two new components for our applications, we'll create two new providers on the system.

First, let's create a provider at config/providers/persistence.rb with the following content:

Application.register_provider(:persistence) do
  start do
    target.start :db

    config = target['db.config']

    config.register_relation(Relations::Users)

    register('container', ROM.container(config))
  end
end

Similar to our Rakefile we're using the start method to make the db provider available when we're instantiating our relation class.

Then let's create another provider at config/providers/repos.rb with the following content:

Application.register_provider(:repos) do
  start do
    target.start :persistence

    register('repos.user', Repos::User.new(target['container']))
  end
end

See how we start the persistence provider we defined previously ? We don't need to start the db provider because dry-system will go to the persistence provider and start there, so we can have as many co-dependent providers as we want.

Since we added new providers, we'll update our entry points files as usual:

At config.ru:

require_relative 'config/providers/persistence'
require_relative 'config/providers/repos'

And at bin/console:

require_relative '../config/providers/persistence'
require_relative '../config/providers/repos'

Refactor time, shall we?

Now that we've defined our required providers, it's just a matter of using them on the layer we want; this layer will be the service class for us.

On the service class at lib/services/user.rb, we'll rewrite to use the repository:

module Services
  class User
    def initialize
      @repo = Application['repos.user']
    end

    def list_all
      users = @repo.all

      users.map do |user|
        { id: user.id, name: user.name, email: user.email }
      end.to_json
    end
  end
end

The refactor is done! Pretty easy, right? Our route should now use the database to provide a list of users.

Conclusion

I hope this article is useful for anyone who ends up reading it. I tried to demonstrate how easy it is to decouple application parts and manage them, even when each part requires complex setups.

Furthermore, I'm always available to help with any doubt or just to chat about cool Ruby stuff. May the force be with you!