Faking External Services in Tests with Adapters

November 28, 2019

This blog post was originally published in thoughtbot's blog. You can find the original at thoughtbot.com/blog/faking-external-services-in-tests-with-adapters .

When faking external services in tests, I like to start with a simple solution and only progress to a more complex one if I need it. That’s why I usually start with a simple class that acts as the public interface to adapters and make an in-memory adapter for tests. Let’s see a concrete example.

Faking an SMS client

Suppose we need to use Twilio’s SMS service. We don’t want to make a request to Twilio’s api every time we send a message in a test, and in some tests, we’d like to assert that the correct messages are being “sent”.

Let’s first create an SmsClient class that will be the interface for the rest of our code to send messages.

class SmsClient
  cattr_accessor :adapter

  def initialize
    @client = adapter.new
  end

  def send_message(args)
    @client.send_message(args)
  end
end

We use cattr_accessor (that comes with Rails) to have access to a SmsClient.adapter reader and writer methods. If you’re not using Rails, we can instead add an attr_accessor to the eigenclass like this:

class SmsClient
  class << self
    attr_accessor :adapter
  end

  def initialize
    @client = self.class.adapter.new
  end

  def send_message(args)
    @client.send_message(args)
  end
end

The rest of the code is fairly simple. We instantiate our adapter in the initializer and delegate calls to the @client instance.

Now to our adapters:

module SmsAdapters
  class Twilio
    def initialize
      # initialize twilio client with credentials
    end

    def send_message(args)
      # send real message to twilio
    end
  end
end

module SmsAdapters
  class InMemory
    cattr_accessor :messages
    self.messages = []

    def send_message(message)
      self.class.messages << OpenStruct.new(message)
    end
  end
end

We use another cattr_accessor in our in-memory adapter for ease of collecting messages at the class level, but we could have just as easily used an attr_accessor :messages on the eigenclass.

The behavior is much like ActionMailer::Base.deliveries — during tests, action mailer simply collects all emails in an array instead of sending them. In a similar way, our in-memory adapter collects an array of messages instead of sending them.

We can now set the default adapter to be the Twilio adapter:

 class SmsClient
   cattr_accessor :adapter
+  self.adapter = SmsAdapters::Twilio

   def initialize
     @client = adapter.new
   end

   def send_message(args)
     @client.send_message(args)
   end
 end

And we only swap it with our in-memory adapter in tests:

# spec/rails_helper.rb

RSpec.configure do |config|
  config.around do |example|
    # keep old adapter to use after test runs
    old_adapter = SmsClient.adapter

    # set our in-memory adapter
    SmsClient.adapter = SmsAdapters::InMemory

    # run the test
    example.run

    # put back the previous adapter
    SmsClient.adapter = old_adapter
  end
end

Don’t forget to clear that array of messages before every test run. We don’t want some tests polluting other tests’ data and causing intermittent failures.

# spec/rails_helper.rb

RSpec.configure do |config|
  config.before do
    SmsAdapters::InMemory.clear_messages
  end
end

Let’s add that .clear_messages method to our InMemory adapter.

 module SmsAdapters
   class InMemory
     cattr_accessor :messages
     self.messages = []
+
+    def self.clear_messages
+      self.messages = []
+    end

     def send_message(message)
       self.class.messages << OpenStruct.new(message)
     end
   end
 end

Nicely done! Now, all tests that tangentially interact with the SmsClient (but which do not care about the sent messages) continue running normally, being none the wiser. And when we want to assert that an SMS message has been “sent” in an integration test, we can simply query the in-memory adapter:

it 'sends an sms when opening new account' do
  user = create(:user, phone: "+15552223333")

  OnboardUser.run(user)

  sent_messages = SmsClient.adapter.messages
  expect(sent_messages.last).to have_attributes(to: user.phone, body: "Welcome!")
end

Bonus: development adapter

If we also decide not to send SMS messages in development, we can easily create a new adapter that uses local storage — similar to how ActiveStorage’s local adapter just saves attachments to disk.

module SmsAdapters
  class Local
    def send_message(message)
      # save_to_file
    end
  end
end

We could even expose those messages through an /sms_previews route in development. Nice and easy!

What next?

If you don’t like the in-memory type of adapters, or if you find your setup requires something more complex, these blog posts are a good guide on setting up fakes with local servers using Sinatra:

Happy testing!