架构：Hexagonal Architecture Guidelines for Rails

原文地址：http://theaudaciouscodeexperiment.com/blog/2014/03/17/hexagonal-architecture-guidelines-for-rails/。

TL;DR

Good application design is hard and there’s no one “right” way to do it.

I often get asked the how I design decoupled applications and while there’s no easy answer I’m going to lay down the rules that have worked for me.

Background

Rails gives you a very minimal architecture of three parts (MVC) all three of which have proved inadequate to contain any real amount of complexity. Remember fat models, skinny controllers?

This is in stark contrast to ‘enterprise’ frameworks which is often criticised for providing too much complexity up front.

The key is to start with just a couple of extra layers and keep those layers decoupled so that more can be added, allowing you application to scale in complexity with ease.

The Rules

• Controllers actions are allowed a single line of code
• The application doesn’t return anything to controllers
• The application has no knowledge of the framework
• No inheritance or mixins (with two exceptions)
• Domain objects have no knowledge of persistence
• Separate your “wiring”

The skinniest of controllers

Your controllers should look like this

1 class ThingsController
2   def show
3     app_of_things.show_thing(rails_adapter)
4   end
5 
6   def create
7     app_of_things.create_thing(rails_adapter)
8   end
9 end

Nothing more, just that single line. This is an example of where “Tell don’t ask” really shines.

A note on “Tell don’t ask”

As soon as an object’s method returns data back to its caller it relinquishes control of the program.

“Here’s the data, you decide what to do with it.”

In hexagon land the application is in charge until the bitter end it never returns data back to its caller, it sends messages and calls all the shots.

Don’t return anything to controllers

So how does the view or data get rendered? It’s your app’s responsibility to send a message back to the web layer instructing it to render.

The mysterious rails_adapter will be a wrap the Rails controller defining a narrow interface that your app will depend on.

I recommend you implement methods such as #success, #created, #not_updated and map these to redirects or appropriate HTTP status codes in your adapter.

The adapter is not part of your application, it’s the mediator that translates results or events from your application into something Rails can understand. It can and will have knowledge of the web or framework but must not leak it into the application.

The application has no knowledge of the framework

Define your own APIs for everything your application touches, do this by wrapping foreign objects in scar tissue. Use SimpleDegator or Forwardable to make proxy object quickly and easily.

No inheritance or mixins

Predictably I make exceptions for SimpleDegator and Forwardable. The Gang of Four said “Prefer composition over inheritance” so just prefer it, all the time! These two standard library tools will help you build composed objects easily.

Domain objects have no knowledge of persistence

Use a combination of the repository pattern with a simple data mapper to get your data into some PORO or Struct objects.

Writing a general purpose data mappers is hard, Ruby doesn’t have one yet and the enterprise solutions like Hibernate are not exactly loved. My advice is to write your own data mapper without making it general purpose or feature rich. If it only works just enough with your data it shouldn’t be too complex. Add the features as you need them, optimise as lazily as possible.

Separate your wiring (or inject dependencies)

Have an object whose responsibility is to wire up all the others, it will make your code simpler and more flexible. I’ve written and spoke about this before so won’t re-iterate here.

That’s it

That’s my hexagonal prescription. If you want to get in touch to nerd out and discuss these ideas I’d be more than happy to chat.

Where’s the sample app?

In the works :)