# Coach [![Gem Version](https://badge.fury.io/rb/coach.svg)](http://badge.fury.io/rb/coach) [![CircleCI](https://circleci.com/gh/gocardless/coach.svg?style=svg)](https://circleci.com/gh/gocardless/coach) [![Code Climate](https://codeclimate.com/github/gocardless/coach.svg)](https://codeclimate.com/github/gocardless/coach) Coach improves your controller code by encouraging: - **Modularity** - No more tangled `before_filter`'s and interdependent concerns. Build Middleware that does a single job, and does it well. - **Guarantees** - Work with a simple `provide`/`require` interface to guarantee that your middlewares load data in the right order when you first boot your app. - **Testability** - Test each middleware in isolation, with effortless mocking of test data and natural RSpec matchers. For our policy on compatibility with Ruby and Rails versions, see [COMPATIBILITY.md](docs/COMPATIBILITY.md). # Installation To get started, just add Coach to your `Gemfile`, and then run `bundle`: ```ruby gem 'coach' ``` Coach works with Ruby versions 2.6 and onwards. ## Coach by example The best way to see the benefits of Coach is with a demonstration. ### Mounting an endpoint ```ruby class HelloWorld < Coach::Middleware def call # Middleware return a Rack response [ 200, {}, ['hello world'] ] end end ``` So we've created ourselves a piece of middleware, `HelloWorld`. As you'd expect, `HelloWorld` simply outputs the string `'hello world'`. In an example Rails app, called `Example`, we can mount this route like so... ```ruby Example::Application.routes.draw do match "/hello_world", to: Coach::Handler.new(HelloWorld), via: :get end ``` Once you've booted Rails locally, the following should return `'hello world'`: ```sh $ curl -XGET http://localhost:3000/hello_world ``` ### Zeitwerk The new default autoloader in Rails 6+ is [Zeitwerk](https://github.com/fxn/zeitwerk), which removes support for autoloading constants during app boot, which that example would do - either you have to `require "hello_world"` in your routes file, or avoid referencing the `HelloWorld` constant until the app has booted. To avoid that, you can instead pass the module or middleware _name_ to `Handler.new`, for example: ```ruby Example::Application.routes.draw do match "/hello_world", to: Coach::Handler.new("HelloWorld"), via: :get ``` ### Building chains Suppose we didn't want just anybody to see our `HelloWorld` endpoint. In fact, we'd like to lock it down behind some authentication. Our request will now have two stages, one where we check authentication details and another where we respond with our secret greeting to the world. Let's split into two pieces, one for each of the two subtasks, allowing us to reuse this authentication flow in other middlewares. ```ruby class Authentication < Coach::Middleware def call unless User.exists?(login: params[:login]) return [ 401, {}, ['Access denied'] ] end next_middleware.call end end class HelloWorld < Coach::Middleware uses Authentication def call [ 200, {}, ['hello world'] ] end end ``` Here we detach the authentication logic into its own middleware. `HelloWorld` now `uses` `Authentication`, and will only run if it has been called via `next_middleware.call` from authentication. Notice we also use `params` just like you would in a normal Rails controller. Every middleware class will have access to a `request` object, which is an instance of `ActionDispatch::Request`. ### Passing data through middleware So far we've demonstrated how Coach can help you break your controller code into modular pieces. The big innovation with Coach, however, is the ability to explicitly pass your data through the middleware chain. An example usage here is to create a `HelloUser` endpoint. We want to protect the route by authentication, as we did before, but this time greet the user that is logged in. Making a small modification to the `Authentication` middleware we showed above... ```ruby class Authentication < Coach::Middleware provides :user # declare that Authentication provides :user def call return [ 401, {}, ['Access denied'] ] unless user.present? provide(user: user) next_middleware.call end def user @user ||= User.find_by(login: params[:login]) end end class HelloUser < Coach::Middleware uses Authentication requires :user # state that HelloUser requires this data def call # Can now access `user`, as it's been provided by Authentication [ 200, {}, [ "hello #{user.name}" ] ] end end # Inside config/routes.rb Example::Application.routes.draw do match "/hello_user", to: Coach::Handler.new("HelloUser"), via: :get end ``` Coach analyses your middleware chains whenever a new `Handler` is created, or when the handler is first used if the route is being lazy-loaded (i.e., if you're passing a string name, instead of the route itself). If any middleware `requires :x` when its chain does not provide `:x`, we'll error out before the app even starts with the error: ```ruby Coach::Errors::MiddlewareDependencyNotMet: HelloUser requires keys [user] that are not provided by the middleware chain ``` This static verification eradicates an entire category of errors that stem from implicitly running code before hitting controller methods. It allows you to be confident that the data you require has been loaded, and makes tracing the origin of that data as simple as looking up the chain. ## Configuring middlewares By making use of middleware config hashes, you can build generalised middlewares that can be configured specifically for the chain that they are used in. ```ruby class Logger < Coach::Middleware def call # Logs the incoming request path, with a configured prefix Rails.logger.info("[#{config[:prefix]}] - #{request.path}") next_middleware.call end end class HelloUser < Coach::Middleware uses Logger, prefix: 'HelloUser' uses Authentication def call ... end end ``` The above configures a `Logger` middleware to prefix it's log entries with `'HelloUser'`. This is a contrived example, but at GoCardless we've created middlewares that can act as generalised resource endpoints (show, index, etc) when given the model class and some extra configuration. ## Testing The basic strategy is to test each middleware in isolation, covering all the edge cases, and then create request specs that cover a happy code path, testing each of the middlewares while they work in sequence. Each middleware is encouraged to rely on data passed through the `provide`/`require` syntax exclusively, except in stateful operations (such as database queries). By sticking to this rule, testing becomes as simple as mocking a `context` hash. Coach comes with some RSpec matchers to help simplify your testing, however they aren't required by default. You'll need to run `require 'coach/rspec'`, we recommend putting this in your `spec/spec_helper.rb` or `spec/rails_helper.rb` file. ```ruby require 'spec_helper' describe "/whoami" do let(:user) { FactoryGirl.create(:user, name: 'Clark Kent', token: 'Kryptonite') } context "with correct auth details" do it "responds with user name" do get "/whoami", {}, { 'Authorization' => 'Kryptonite' } expect(response.body).to match(/Clark Kent/) end end end describe Routes::Whoami do subject(:instance) { described_class.new(context) } let(:context) { { authenticated_user: double(name: "Clark Kent") } } it { is_expected.to respond_with_body_that_matches(/Clark Kent/) } end describe Middleware::AuthenticatedUser do subject(:instance) { described_class.new(context) } let(:context) do { request: instance_double(ActionDispatch::Request, headers: headers) } end let(:user) { FactoryGirl.create(:user, name: 'Clark Kent', token: 'Kryptonite') } context "with valid token" do it { is_expected.to call_next_middleware } it { is_expected.to provide(authenticated_user: user) } end context "with invalid token" do it { is_expected.to respond_with_status(401) } it { is_expected.to respond_with_body_that_matches(/access denied/i) } end end ``` ## Routing For routes that represent resource actions, Coach provides some syntactic sugar to allow concise mapping of endpoint to handler in Rails apps. ```ruby # config/routes.rb Example::Application.routes.draw do router = Coach::Router.new(self) router.draw(Routes::Users, base: "/users", actions: [ :index, :show, :create, :update, disable: { method: :post, url: "/:id/actions/disable" } ]) end ``` Default actions that conform to standard REST principles can be easily loaded, with the users resource being mapped to: | Method | URL | Description | |--------|------------------------------|------------------------------------------------| | `GET` | `/users` | Index all users | | `GET` | `/users/:id` | Get user by ID | | `POST` | `/users` | Create new user | | `PUT` | `/users/:id` | Update user details | | `POST` | `/users/:id/actions/disable` | Custom action routed to the given path suffix | If you're using Zeitwerk, you can pass the name of the module to `#draw`, instead of the module itself. ```ruby # config/routes.rb Example::Application.routes.draw do router = Coach::Router.new(self) router.draw("Routes::Users", base: "/users", actions: [ :index, :show, :create, :update, disable: { method: :post, url: "/:id/actions/disable" } ]) end ``` ## Rendering By now you'll probably agree that the rack response format isn't the nicest way to render responses. Coach comes sans renderer, and for a good reason. We initially built a `Coach::Renderer` module, but soon realised that doing so would prevent us from open sourcing. Our `Renderer` was 90% logic specific to the way our APIs function, including handling/formatting of validation errors, logging of unusual events etc. What worked well for us is a standalone `Renderer` class that we could require in all our middleware that needed to format responses. This pattern also led to clearer code - consistent with our preference for explicit code, stating `Renderer.new_resource(...)` is instantly more debuggable than an inherited method on all middlewares. ## Instrumentation Coach uses `ActiveSupport::Notifications` to issue events that can be used to profile middleware. Information for how to use `ActiveSupport`s notifications can be found [here](http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html). | Event | Arguments | |-------------------------------|------------------------------------------------------- | | `start_handler.coach` | `event(:middleware, :request)` | | `start_middleware.coach` | `event(:middleware, :request)` | | `finish_middleware.coach` | `start`, `finish`, `id`, `event(:middleware, :request)`| | `finish_handler.coach` | `start`, `finish`, `id`, `event(:middleware, :request)`| | `request.coach` | `event` containing request data and benchmarking | Of special interest is `request.coach`, which publishes statistics on an entire middleware chain and request. This data is particularly useful for logging, and is our solution to Rails `process_action.action_controller` event emitted on controller requests. The benchmarking data includes information on how long each middleware took to process, along with the total duration of the chain. For coach to emit `request.coach` events, it first needs to be subscribed to handler/middleware events: ```ruby Coach::Notifications.subscribe! # Now you can subscribe to and use request.coach events, e.g. ActiveSupport::Notifications.subscribe("request.coach") do |_, event| Rails.logger.info(event) end ``` You can add additional metadata to the notifications published by Coach by calling the `log_metadata` method from inside your Coach middlewares. ```ruby class Tracking < Coach::Middleware requires :user def call log_metadata(user_id: user.id) next_middleware.call end end ``` # Coach CLI As well as the library, the Coach gem comes with a command line tool - `coach`. When working in a large codebase that uses Coach, one of the challenges you may run into is understanding the `provide`/`require` graph made up of all the middleware chains you've built. While the library enforces the correctness of those chains at boot time, it doesn't help you understand those dependencies. That's where the `coach` CLI comes in! Currently, the `coach` CLI supports two commands. ## `find-provider` `find-provider` is the simpler of the two commands. Given the name of a Coach middleware and a value that it requires, it outputs the name of the middleware that provides it. ```bash $ bundle exec coach find-provider HelloUser user Value `user` is provided to `HelloUser` by: Authentication ``` If there are multiple middlewares in the chain that provide the same value, all of them will be listed. ## `find-chain` `find-chain` is the more advanced of the two commands, and is most useful in larger codebases. Given the name of a Coach middleware and a value it requires, it outputs the chains of middleware between the specified middleware and the one that provides the required value. ```bash # Note that we've assumed an intermediate middleware - `UserDecorator` exists in this # example to make the functionality of the command clearer. $ bundle exec coach find-chain HelloUser user Value `user` is provided to `HelloUser` by: HelloUser -> UserDecorator -> Authentication ``` If there are multiple paths to a middleware that provides that value, all of them will be listed. Similarly, if multiple middlewares provide the same value, all of them will be listed. ## Spring integration Given that the Coach CLI is mostly aimed at large Rails apps using Coach, it would be an oversight for us not to integrate it with [Spring](https://github.com/rails/spring/). To enable the use of Spring with the Coach CLI, add the following to `config/spring.rb` or an equivalent Rails config file. ```ruby require "spring/commands/coach" ``` On GoCardless' main Rails app, using Spring reduces the time to run `coach` commands from around 15s to 1s. ## Future work While we think the commands we've already built are useful, we do have some ideas to go further, including: - Better formatting of provider chains - Outputting DOT format files to visualise with Graphviz - Editor integrations (e.g. showing the provider chains when hovering a `requires` statement) # License & Contributing * Coach is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT). * Bug reports and pull requests are welcome on GitHub at https://github.com/gocardless/coach. GoCardless ♥ open source. If you do too, come [join us](https://gocardless.com/about/jobs).