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Engine-based Rails Apps: Pros & Cons
Headshot 128 Ben, January 25, 2016

Within the Rails community, there's a growing number of applications that are being built as a collection of Rails engines, rather than a single, monolithic application. At Cultivate Labs, we've been using this pattern to build Cultivate Forecasts, a fantasy money prediction market. Along the way, we learned a great deal about building applications using this pattern, so I wanted to share some of our takeaways.

Pros

Code Organization

In my opinion, the biggest benefit inherent in the engine-based Rails app (EBRA) pattern is that it forces you to organize and separate your code. By doing so, it forces you to think through where to put each piece of code that you write.

A great example of this is the user model. In many Rails apps, the user model ends up being a catch-all place for any code looking for a home, resulting in a long, overly complex chunk of code. In practice, this frequently results in the user model having structural knowledge of almost every other piece of the system. Based on The Law of Demeter, we know that this spreading of structural knowledge results in an application that is tightly coupled and difficult to maintain -- an environment that makes future development slow and error-prone.

If you set up your dependencies properly in an EBRA, then your user model doesn't "know" about most of the other components (n.b. component == engine, here) in the system. This forces you to put methods/code in their proper place, rather than just dumping them in a user model.

For example, say we want a method that returns a list of questions where the user has made a prediction. Sometimes you'll see something like:

class User < ActiveRecord::Base
  def questions_where_user_has_made_prediction
    Question.joins(:predictions).where(predictions: { user_id: id })
  end
end

This might seem innocuous, but (1) these tend to pile up en-masse in the user model and (2) we're introducing knowledge of the question concept into the user model.

When building an EBRA, the accounts/users component woudn't "know" about questions or predictions. This forces us to put this code in a more appropriate place:

class Question < ActiveRecord::Base
  def self.with_prediction_by_user(user)
    joins(:predictions).where(predictions: { user_id: user.id })
  end
end

Decreased Conflicts

Another benefit that we noticed along the way was a marked decrease in the number of merge conflicts we were experiencing. I think that there are two main reasons for this (which are, in a way, kind of the same thing):

  1. The improved code organization mentioned above. If we're being forced to properly isolate our code rather than dumping it all into a giant user model, then there are going to be fewer conflicts in the user model.

  2. Since the components tend to encapsulate a functional section of the application, it's likely that most of the code you're working on for a feature will fall into one or two components. When one developer is working in the admin component and another is working in the reporting component, they're unlikely to overlap and conflict.

Code Re-Use

When starting out, we expected code reuse to be a major benefit of the engine-based approach. In practice, we didn't end up with as many reusable components as we expected and most of our engines were pretty application specific. However, we did end up with some pieces that we could reuse in other projects, including a commenting component.

Cons

Associations Can be a Pain

If you're setting up your dependencies appropriately, you won't have circular dependencies between components. So, in our case, the forecasting component knows about the user/account component, but not vice versa.

This can make associations a pain. If the user component doesn't "know" about the forecasting component (which contains a Question model), then we really shouldn't be putting forecasting-related associations in the user model. So something like this would be a no-go:

class User
  has_many :questions
end

Instead, we'd do something like:

class Question
  scope :for_user, ->(user){ where(user_id: user.id) }
end

# Use it like this:
Forecast::Question.for_user(user)

This takes some getting used to, feels less clean than a normal association, and is still not my favorite solution.

Component Isolation is Difficult to Enforce

When building a Rails app, pretty much everything gets loaded up and is available to you in any other place in the app -- no import "filename" needed. This is convenient, but also makes it trivial to pollute a component with knowledge of another, even if they should be isolated.

For example, I mentioned above that we have an accounts component and a forecasting component. Technically, the accounts component has no dependency on the forecasting component (and does not declare one in the gemspec). However, something like this will "just work" if you do it:

module Accounts
  class User
    def do_something_with_a_question
      question = Forecast::Question.first
      question.do_something
    end
  end
end

If we want the account/users component to be an isolated component that could be extracted & reused, then it should be agnostic of the Forecast component. For better or worse, it's easy to accidentally violate this isolation.

Test Environment Setup is Challenging

Ideally, all of your components would be able to run their test suites in isolation. If an engine didn't declare (via its gemspec) a dependency on another component, then that component wouldn't be loaded in the test environment for that engine.

To make this happen, you need a dummy app inside every engine. This can get cumbersome very quickly as the number of engines you have grows. For example, in Cultivate Forecasts, we have an FeedUI component that contains an activity feed. We also have a commenting component, which is utilized by FeedUI to allow users to comment on activity feed stories. So we built out the code in the FeedUI which leverages the commenting component.

Naturally, we want to have test coverage for the commenting functionality, including feature/integration tests. We just built out an endpoint in FeedUI that could be used in a feature spec to test creation & listing/viewing of comments. But the commenting gem has no dependency on FeedUI and technically doesn't even know it exists, so putting a feature spec inside the commenting engine that goes and does visit "/activity" seems like a bad idea.

The solution then becomes to build something into the commenting component's dummy app that also utilizes the commenting features, essentially duplicating the work we just did on the activity feed. This tends to happen fairly frequently.

To help address this, we decided to eliminate dummy apps from the engines and instead use the host app to do testing. This helped eliminate some of the duplication, but has the downside of polluting components with knowledge of other components where there should be no dependency. Frankly, I haven't yet found or seen an approach to this issue that solves both of these issues.

Increased Developer Ramp-Up Time

Probably the least surprising of the cons we encountered with this pattern was increased developer ramp up time. The pattern is relatively new and plenty of Rails developers don't have a ton of experience with building engines or gems. There's also a period of learning where to find things inside the app, since you can no longer just go to app/models to find every model in the app.

The flip side of this is that developers can be productive in a single component without having to understand every piece of the app. If someone learns the commenting engine well, they can make progress on comment-related features without having to understand all of the other components.

If you enjoyed this post, let me know on twitter: @bcroesch

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