Ephemeral Response

Save HTTP responses to give your tests a hint of reality.


Web responses are volatile. Servers go down, API's change, responses change and every time something changes, your tests should fail. Mocking out web responses may speed up your test suite but the tests essentially become lies. Ephemeral Response encourages you to run your tests against real web services while keeping your test suite snappy by caching the responses and reusing them until they expire.

  1. run test suite
  2. all responses are saved to fixtures
  3. disconnect from the network
  4. run test suite


require 'benchmark'
require 'ephemeral_response'


5.times do
  puts Benchmark.realtime { Net::HTTP.get "example.com", "/" }

1.44242906570435     # First request caches the response as a fixture

With Rspec

require 'ephemeral_response'

Spec::Runner.configure do |config|

  config.before(:suite) do

  config.after(:suite) do


All responses are cached in yaml files within spec/fixtures/ephemeral_response.

I'd recommend git ignoring this directory to ensure your tests always hit the remote service at least once and to prevent credentials (like API keys) from being stored in your repo.

Customize how requests get matched by the cache

Every request gets a unique key that gets added to the cache. Additional requests attempt to generate this same key so that their responses can be fetched from the cache.

The default key is a combination of the URI, request method, and request body. Occasionally, these properties contain variations which cannot be consistently reproduced. Time is a good example. If your query string or post data references the current time then every request will generate a different key therefore no fixtures will be loaded. You can overcome this issue by registering a custom key generation block per host.

An example may help clear this up.

EphemeralResponse.configure do |config|
  config.register('example.com') do |request|

# This will get cached
Net::HTTP.start('example.com') do |http|
  get = Net::HTTP::Get.new('/')
  get['Date'] = Time.now.to_s

# This is read from the cache even though the date is different
Net::HTTP.start('example.com') do |http|
  get = Net::HTTP::Get.new('/')
  get['Date'] = "Wed Dec 31 19:00:00 -0500 1969"

Take a look in examples/custom_cache_key.rb to see this in action.

Grouping fixtures to isolate responses

Occasionally you are in a situation where you are making the same request but you are expecting a different result, for example, a list of all resources, create a new resource, and then list them again. For this scenario, you can use a 'newly_created' fixture set. In your specs, change the fixture set to 'newly_created' for the create and second list requests. The default fixture set is named :default (or nil).

# pseudo code
get('http://example.com/books').should_not contain('The Illiad')
EphemeralResponse.fixture_set = :newly_created
post('http://example.com/books', {:title => 'The Illiad'})
get('http://example.com/books').should contain('The Illiad')
EphemeralResponse.fixture_set = :default


Change the fixture directory; defaults to "spec/fixtures/ephemeral_response"

EphemeralResponse::Configuration.fixture_directory = "test/fixtures/ephemeral_response"

Change the elapsed time for when a fixture will expire; defaults to 24 hours

EphemeralResponse::Configuration.expiration = 86400 # 24 hours in seconds

Pass a block when setting expiration to gain access to the awesome helper method one_day

EphemeralResponse::Configuration.expiration = lambda do
  one_day * 30 # Expire in thirty days: 60 * 60 * 24 * 30

Never let fixtures expire by setting skip_expiration to true.

EphemeralResponse::Configuration.skip_expiration = true

Print debugging information

EphemeralResponse::Configuration.debug_output = $stderr

Selenium Tip

Always allow requests to be made to a host by adding it to the white list. Helpful when running ephemeral response with selenium which makes requests to the local server.

EphemeralResponse::Configuration.white_list = "localhost", ""

All together now!

EphemeralResponse.configure do |config|
  config.fixture_directory = "test/fixtures/ephemeral_response"
  config.expiration = lambda { one_day * 30 }
  config.skip_expiration = true
  config.white_list = 'localhost'
  config.debug_output = $stderr

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Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.

Copyright (c) 2010 Sandro Turriate. See MIT_LICENSE for details.