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Api Contract Testing

API Contract Testing#

Contract testing - a type of integration testing that is well-suited to microservice architectures

  • Consumer - application that uses the API
  • Provider - application that publishes the API
  • Contract - a file describing the expected consumer/provider behaviour (in the form of an OpenAPI spec)
  • Consumer-driven test - A test where the consumer side of a contract is used to create a simulated consumer, used to test a provider.
  • Provider-driven test - A test where the provider side of a contract is used to create a simulated provider, used to test a consumer.

Traditional Integration Testing#

  • Test using mock or simulated services
  • Test using the real dependencies

API Contract#

An API contract can be created as an OpenAPI spec by both the provider and consumer teams - a single file.

Simulated consumer or provider is created from the spec


  • Serves data and functionality to the consumer
  • API publisher

Provider’s contract should include:

  • Available endpoints
  • Possible responses
  • Expected inputs


  • Accepts funtionality and responses from the provider

A Consumer’s contract should include:

  • What endpoints it needs
  • The type of data it can send
  • The responses it can accept

Usually the providers will give the contract - it is however more useful to find out how a calling service will use the api.

Let the consumers call the shots

The onus is on the provider to give the functionality to the consumer.

Advantages of Rest API Contract Testing#

  • The provider only supplies what is necessary - “You aren’t going to need it”
  • Consumer doesn’t need to worry about fucntionality it doesn’t need
  • Provider is notified quickly if they are implementing unnecessary - quick feedback loop
  • Ensures feedback for all teams involved

When should we use Contract Testing#

  • When we need to test quickly
  • When we don’t want to spend too much time setting up environments
  • On mission critical systems that use APIs
  • To make sure implementation matches the specification

Who benefits from Contract Testing#

  • Integration testers
  • Automation testers
  • Manual testers
  • Anyone who needs to make sure their microservices are doing what they are supposed to be doing

How do we do it#

  • Write your contract - OpenAPI Spec or Swaggerhub
  • Take the specification into SoapUI Pro to generate your consumer / provider tests
  • Use ServiceV to mock the consumer / provider


You would have an OpenAPI spec now and then use ServiceV pro - that creates a virtual service from your spec.

You can then use SoapUI Pro to send requests to that virtual / simulated service