Amendments & Clarifications

Posted on February 04, 2014 - Subscribe - Home

I published the last post in the Joy of Design series about a week ago. Since then I've received many questions, concerns, and various feedback. This post addresses the common ones. I may add more follow up posts or simply add onto this post.

What's a Boundary?

I had a lengthy discussion with a coworker about this term. Good software architecture contains layers. Each layer builds upon the next. The OSI Model is an example. Each layer is ignorant to what's above or below it. Dependencies flow one way. A boundary is where one layer meets another. A boundary is there if one side can be swapped for a completely different implementation without having to change the other. That's to say there is no knowledge about the other. The repository pattern is a great example. Objects ask the repository for objects. The repository does whatever to provide the objects. The repository object is a boundary. The repository's implementation could change without callers knowing about it. Ars Technica has more on this topic.

Uniqueness Validations

I received a question about uniqueness validations. How should a unique email/username be implemented? Then what if the uniqueness depends on an account or other value? I was happy that someone asked this question because it forces you to make a design decision. Implementing the validation is easy. Create a validator that queries the repository with the given values and test there are no matches. The real question is: where should this be implemented? Should it go into the form or into a use case?

Now we're off on a topic. Form objects should be context free. They should focus on input collection and validation. They should not need to interact with the outside world to do their job. That means they should have no collaborators. But wait, uniques validation is an input concern is it not? Well perhaps. Take the simple case where uniqueness simply checks against one value. Is that context free? What about a more complex case where uniqueness takes three or four other objects? There comes a point where it seems the validations are domain model specific and not input specific. Consider a simple validations like required values or email address formats. These are context fee. The form does not need to know how this value is used. Now switch back to the uniqueness problem. The form validations would contain domain specific validation. How does the form know that email address should be unique to a given project/customer or account? The correct answer is that it should not. This logic should be implemented in the contextual object in the system: the use case. I prefer to keep this these use case specific concerns in the use cases themselves. This ensures the form object have no collaborators. However this adds more logic to the use cases and increase their number of collaborator. This is a trade-off I'm willing to make. Yes, it is possible to implement the validation in the form but implementing it in the use case makes more sense in more cases and thusly is a better common solution.

Now the only question is how to do the validation. This is straight forward using the repository. Query the repository and check the results are empty? or nil. I'm still using ActiveModel::Validations (nothing better yet). I prefer to decorate the form with extra validators. I create a validator class for the use case, validate the form, then run the extra validations.

class CreateUser
  class FormValidator
    class UserNameValidator < ActiveModel::Validator
      def validate(form)
        existing_users = UserRepo.named form.user_name
        record.errors.add :base, "user name is already taken!" if !existing_users.empty?

    include Validations
    include ActiveModel::Validations

    validates_with :user_name, FormValidator

    def initialize(form)
      @form = form

    def user_name

  def run!
    # pass in other things to check uniqueness with here
    # In this case "FormValidator" handles use case specific
    # validations!

    # other stuff

Test Suites

The blog posts contained example tests, but never anything real about the entire test suite. How did I miss that? Well, here's my attempt at making it right.

My test suites contain multiple types of tests designed to run in multiple modes. There are two modes: fast and slow. "fast" is default. "fast" runs all the tests using fake implementations of all external things. "slow" uses real implementations of all external services. I use the slow mode when I'm developing to ensure that each class has the correct collaborator and that data is moving correctly through the system. Once that is correct, I run the tests in slow mode to test the data is hitting the external world correctly. In practice "slow" is for CI.

The tests are broken up into categories. They all build off MiniTest::Unit::TestCase. All adapters/caches/services use null implementations. This makes it impossible to touch the outside world. Interaction with these services should not generally happen in unit tests--but if they do it doesn't matter. Next come integration tests. These tests use fake implementations. IE the repository uses an in memory implementation. This way I can test data flowing through the entire system without slowing down the tests. Next acceptance tests. Acceptance tests use fake implementations as well, but they go through the delivery mechanism. In practice it works out like this:

The "fake" implementations are switched in CI mode. So the in memory adapter would be replaces with a redis implementation. The cache would talk to memcached etc.

The various adapters are tested in isolation. IE, I can test the repository is writing to postgresql correctly.

Here are some rake tasks:

I run use this for ci: CI=true rake test:smoke test:ci. This runs the most important smoke tests using real implementations. If everything's passing then run the entire suite against real implementations. Note, travis will set the CI environment variable for you.

This setup is perfect. Following boundary principles keeps the test suite insanely fast. Also there is no bullshit framework to load so even loading ruby is fast. Then through a combination of unit tests and end-to-end integration tests I have more confidence in my applications then I ever did before. This takes almost all the fear out of continuous delivery.

One final note about testing. I want to share some information about code paths. It is important that the application only has a single code path. The implementation is an object and not checking a setting then behaving in two different ways. Rails conventions fail absolutely in this case. Rails disabled caching in the test environment by default. This causes all controller and view related code to act in two different ways (if caching is enabled to this, otherwise do this). This is horrible practice. The active support cache implementation contains a null and in memory implementation. Instead of switching caching to off, set the cache to a null implementation. This is also horrible practice because the code that runs in test is not the code that runs in production. The entire point of tests is to ensure code performs correctly in production. You must make the correct decisions to make this goal. I cannot tell you how many random bugs I found in Radium related to caching with memcache and marshaling objects. I could have only found these bugs by running the tests using a real cache. So please, customize behavior through objects and not through settings. This will make the code better and give you more confidence its in true behavior.

— Adam Hawkins