Pact

Define a pact between service consumers and providers, enabling "consumer driven contract" testing.

Pact provides an RSpec DSL for service consumers to define the HTTP requests they will make to a service provider and the HTTP responses they expect back. These expectations are used in the consumers specs to provide a mock service provider. The interactions are recorded, and played back in the service provider specs to ensure the service provider actually does provide the response the consumer expects.

This allows testing of both sides of an integration point using fast unit tests.

This gem is inspired by the concept of "Consumer driven contracts". See http://martinfowler.com/articles/consumerDrivenContracts.html for more information.

Travis CI Status:

Features

• A service is mocked using an actual process running on a specified port, so javascript clients can be tested as easily as backend clients.
• "Provider states" (similar to fixtures) allow the same request to be made with a different expected response.
• Consumers specify only the fields they are interested in, allowing a provider to return more fields without breaking the pact. This allows a provider to have a different pact with a different consumer, and know which fields each cares about in a given response.
• Rake tasks allow pacts to be verified against a service provider codebase.
• Different versions of a consumer/provider pairs can be easily tested against each other, allowing confidence when deploying new versions of each (see the pact_broker and pact_broker-client gems).
• Autogenerated API documentation - need we say more?

How does it work?

1. In the specs for the provider facing code in the consumer project, expectations are set up on a mock service provider.
2. When the specs are run, the requests, and their expected responses, are written to a "pact" file.
3. The requests in the pact file are later replayed against the provider, and the actual responses are checked to make sure they match the expected responses.

Why is developing and testing with pacts better than using integration tests?

• Faster execution.
• Reliable responses from mock service provider reduce likelihood of flakey tests.
• Only one component is being tested at a time, making the causes of test failures easier to identify.
• Design of service provider is improved by considering first how the data is actually going to be used, rather than how it is most easily retrieved and serialised.
• No need to manage starting, stopping and fixture set up for multiple applications during a test run.

Contact

• Twitter: @pact_up
• Google users group: https://groups.google.com/forum/#!forum/pact-support

Installation

Put it in your Gemfile. You know how.

Usage

Service Consumer project

1. Start with you model

Imagine a model class that looks something like this. The attributes for a Something live on a remote server, and will need to be retrieved by an HTTP call.

class Something
  attr_reader :name

  def initialize name
    @name = name
  end

  def == other
    other.is_a?(Something) && other.name == name
  end
end

2. Create a skeleton client class

Imagine a service provider client class that looks something like this.

class MyServiceProviderClient
  include HTTParty
  base_uri 'http://my-service'

  def get_something
    # Yet to be implemented because we're doing Test First Development...
  end
end

3. Configure the mock server

The following code will create a mock service on localhost:1234 which will respond to your application's queries over HTTP as if it were the real "My Service Provider" app. It also creats a mock service provider object which you will use to set up your expectations. The method name to access the mock service provider will be what ever name you give as the service argument - in this case "my_service_provider"

# In /spec/service_providers/pact_helper.rb

require 'pact/consumer/rspec'

Pact.service_consumer "My Service Consumer" do
  has_pact_with "My Service Provider" do
    mock_service :my_service_provider do
      port 1234
    end
  end
end

4. Write a failing spec for the client

# In /spec/service_providers/my_service_provider_client_spec.rb

# Use the :pact => true describe metadata to include all the pact generation functionality in your spec.
describe MyServiceProviderClient, :pact => true do

  before do
    # Configure your client to point to the stub service on localhost using the port you have specified
    MyServiceProviderClient.base_uri 'localhost:1234'
  end

  subject { MyServiceProviderClient.new }

  describe "get_something" do

    before do
      my_service_provider.given("something exists").
        upon_receiving("a request for something").with(method: :get, path: '/something').
        will_respond_with(
          status: 200,
          headers: {'Content-Type' => 'application/json'},
          body: {name: 'A small something'} )
    end

    it "returns a Something" do
      expect(subject.get_something).to eq(Something.new('A small something'))
    end

  end

end

5. Run the specs

Running the consumer spec will generate a pact file in the configured pact dir (spec/pacts by default). Logs will be output to the configured log dir that can be useful when diagnosing problems.

Of course, the above specs will fail because the client method is not implemented, so next, implement your client methods.

6. Implement the client methods

class MyServiceProviderClient
  include HTTParty
  base_uri 'http://my-service'

  def get_something
    name = JSON.parse(self.class.get("/something").body)['name']
    Something.new(name)
  end
end

7. Run the specs again.

Green! You now have a pact file that can be used to verify your expectations in the provider project. Now, rinse and repeat for ALL the likely status codes that may be returned (we recommend 404, 500 and 401/403 if there is authorisation.)

Service Provider project

1. Create the skeleton API classes

Create your API class using the framework of your choice - leave the methods unimplemented, we're doing Test First Develoment, remember?

2. Tell your provider that it needs to honour the pact file you made earlier

Require "pact/tasks" in your Rakefile.

# In Rakefile
require 'pact/tasks'

Create a pact_helper.rb in your service provider project. The file must be called pact_helper.rb, however there is some flexibility in where it can be stored. The recommended place is specs/service_consumers/pact_helper.rb.

See the Provider section of the Configuration documentation for more information.

# In specs/service_consumers/pact_helper.rb

require 'pact/provider/rspec'

Pact.service_provider "My Service Provider" do

  app { MyApp.new } # Optional, loads app from config.ru by default

  honours_pact_with 'My Service Consumer' do

    # This example points to a local file, however, on a real project with a continuous
    # integration box, you would publish your pacts as artifacts,
    # and point the pact_uri to the pact published by the last successful build.

    pact_uri '../path-to-your-consumer-project/specs/pacts/my_consumer-my_provider.json'
  end
end

3. Run your failing specs

$ rake pact:verify

Congratulations! You now have a failing spec to develop against.

At this stage, you'll want to be able to run your specs one at a time while you implement each feature. At the bottom of the failed pact:verify output you will see the commands to rerun each failed interaction individually. A command to run just one interaction will look like this:

$ rake pact:verify PACT_DESCRIPTION="a request for something" PACT_PROVIDER_STATE="something exists"

4. Implement enough to make your first interaction spec pass

Rinse and repeat.

5. Keep going til you're green

Yay! Your provider now honours the pact it has with your consumer. You can now have confidence that your consumer and provider will play nicely together.

Using provider states

Provider states allow you to set up data on the provider before the interaction is run, so that it can make a response that matches what the consumer expects. It also allows the consumer to make the same request with different expected responses.

Read more about provider states here.

Verifying pacts

You can verify a pact at an arbitrary local or remote URL

$ rake pact:verify:at[../path-to-your-consumer-project/specs/pacts/my_consumer-my_provider.json]
$ rake pact:verify:at[http://build-box/MyConsumerBuild/latestSuccessful/artifacts/my_consumer-my_provider.json]

To make a shortcut task for pact at an arbitrary URL, add the following to your Rakefile. The pact.uri may be a local file system path or a remote URL.

# In Rakefile or /tasks/pact.rake

# This creates a rake task that can be executed by running
# $ rake pact:verify:dev

Pact::VerificationTask.new(:dev) do | pact |
  pact.uri '../path-to-your-consumer-project/specs/pacts/my_consumer-my_provider.json'
end

Configuration

See the Configuration section of the documentation for options relating to thing like logging, diff formatting, and documentation generation.

Pact best practices

As in all things, there are good ways to implement Pacts, and there are not so good ways. Check out the Best practices section of the documentation to make sure you're not Pacting it Wrong.

Docs

• Example
• Frequently Asked Questions
• Rarely Asked Questions
• Terminology
• Configuration

Related libraries

Pact Provider Proxy - Verify a pact against a running server, allowing you to use pacts with a provider of any language.

Pact Broker - A pact repository. Provides endpoints to access published pacts, meaning you don't need to use messy CI URLs in your codebase. Enables cross testing of prod/head versions of your consumer and provider, allowing you to determine whether the head version of one is compatible with the production version of the other. Helps you to answer that ever so important question, "can I deploy without breaking all the things?"

Pact Broker Client - Contains rake tasks for publishing pacts to the pact_broker.

Pact JVM - A Pact implementation for the JVM (Java and Scala). It generates pact files that are compatible with the Ruby implementation.

Shokkenki - Another Consumer Driven Contract gem written by one of Pact's original authors, Brent Snook. Shokkenki allows matchers to be composed using jsonpath expressions and allows auto-generation of mock response values based on regular expressions.

Links

Integrated tests are a scam - J.B. Rainsberger

Consumer Driven Contracts - Ian Robinson

Integration Contract Tests - Martin Fowler

TODO

Short term:

• Support hash of query params

Long term:

• Provide more flexible matching (eg the keys should match, and the classes of the values should match, but the values of each key do not need to be equal). This is to make the pact verification less brittle.
• Add XML support
• Decouple Rspec from Pact and make rspec-pact gem for easy integration

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request