Testing External APIs with VCR in Rails

Testing external APIs can be a time-consuming task, VCR is a ruby gem that allows you to record test suite's HTTP interactions and replay them during future test runs for fast, deterministic, accurate tests.

How VCR works?

When we make the first API call the request goes through full request and response cycle. VCR records the API request and response, which it saves as a cassette. In other words, VCR stubs it for future use. When the test is run again, no API call is made. Instead, VCR stubs the response. The test will run much faster as a result.

Installation

VCR works together with webmock a library for stubbing and setting expectations of HTTP requests. Add the latest version of VCR and Webmock in your Gemfile. Run bundle install to update dependencies.

gem 'vcr', '~> 5.0'
gem 'webmock', '~> 3.7', '>= 3.7.6'

Its good to add them in test group gems

Add them as development dependencies if you are developing a gem.

Configuration

VCR implements a configure block for different configuration. Add the below block in your test_helper.rb file.

VCR.configure do |config|
  config.allow_http_connections_when_no_cassette = false
  config.cassette_library_dir = File.expand_path('cassettes', __dir__)
  config.hook_into :webmock
  config.ignore_request { ENV['DISABLE_VCR'] }
  config.ignore_localhost = true
  config.default_cassette_options = {
    record: :new_episodes
  }
end

• allow_http_connections_when_no_cassette - allows actual full request and response cycle, when no cassette found.
• cassette_library_dir - directory where cassettes are stored.
• hook_into - specifies a request stubbing library
• ignore_request - default is false when true VCR makes full request without using cassettes even if they exists.
• ignore-localhost - prevent VCR from having any affect on localhost requests
• default_cassette_options - takes a hash that provides defaults for each cassette you use

Testing API calls

We are now ready to start testing APIs calls. For demo, purpose lets use JSONplaceholder a fake API for developers.

class VCRTest < Minitest::Test
  def test_fetch_post
    VCR.use_cassette("get_post") do
      response = Net::HTTP.get_response(URI('https://jsonplaceholder.typicode.com/posts/1'))
      assert_equal(200,response.status)
    end
  end
end

Run the test once and VRC will record in cassette_library_dir you defined in configure block, for our case cassettes/get_post.yml. When you run the test again no real connection is made making the process very fast. You may realize the speed when you have very many API endpoints to test.

Tips

If you are having issues with your cassettes or you need new data you can deletes the cassettes and a new version will be created.

If you have sensitive information in your responses like API keys. You can filter the keys in your configuration block.

VCR.configure do |config|
 config.filter_sensitive_data('<API_KEY>',ENV('API_KEY'))
end

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