Consumer-Driven Contract Testing - Part I

Posted by nmosf on June 3, 2020


This is the first of a series of blog posts about Contract Testing which cover the minimum set of theory and practice necessary for an effective adoption in your team, from design to code integration.

Contract Testing is a category of testing activity where the data formats and conventions defined by two systems (services) which communicate a business value, is tested against a Mock called “Contract”. A service provides a callable API that can be consumed by another (or many) service which create an interaction between parties that needs to be satisfied during the evolution and developing of services which are now coupled. The interaction between services probably relies on a communication layer which can be slow or not reachable which can affect the test results. For this reason, sometimes the best solution is to verify the interaction with a TestDouble which describe the expectations between the parties.

For the sake of clarity, I have strong opinion.


Let’s suppose that we have a Product Service which has an HTTP method that provides product information in a JSON format upon receiving a product id:

HTTP GET: api/v1/product?id=123

  "name": "nexus",
  "type": "smartphone",
  "price": "21.03"

The product information are consumed by Customer Service.


Conventions are defined in this way:

  • Customer Service is the Consumer
  • Product Service is the Provider

During developing, the Product service can evolve the API in a way that the HTTP response change the data format. For example, the type field from a string value change to object:

  "name": "nexus",
  "type": {
    "category": "smartphone",
    "weight": 0.2,
    "color": "blue"
  "price": "21.03"

this is defined as a breaking change in which Provider doesn’t respect the defined interaction between Consumer.

In the same way, the Customer service can evolve the request call. For example, instead of call the API with a query parameter id, it can use a field in the header to query the Product Service. Also, this example is a sort of breaking change in which Consumer doesn’t respect the defined interaction between Provider.

Therefore, the interaction between services can be broken in many ways, and the cause can be triggered by both sides which makes necessary some checks to avoid regression during the development.

e2e Testing

The simplest thing to do, is to create an end-to-end test that cover the entire calls flow between Consumer Service and Provider Service.


Despite what I previously said about simplicity, e2e testing give the best confidence on software behaviour but with some problems. They are flaky tests and often fail with false positive furthermore e2e tests are hard do maintain, slow, and it’s necessary to span multiple services in testing environment with terrible slowness caused by deploy flow and environment nightmares. There are some solutions (1, 2) that can provide the right balance between simplicity and maintainability which are a step back e2e tests but often you need a staging environment to execute tests verification or add a level of complexity to remove the environment need, which personally I found with these solutions a tedious process. Moreover, all these solutions are reactive approaches which in the simplest case, doesn’t avoid the integration of code that breaks the services interactions, but they provide a sort of alerting like open a Jira defect or github issue. It’s possible to avoid code integration in case of breaks but imply an overcomplicated CI or some experimental branch.


Fake news

Developers (I) usually dislike maintaining things which depend on environments or that needs hours to have a result. If something is faulty and slow, it’s often untrusted and consequently useless. Furthermore, works with other services means deal with other teams which are often too much busy doing new bugs features. So then, to avoid this annoying stuff, it’s necessary to adopt a strategy which imply a level of complexity which usually developers are used to see: Mocks.

Mocks are a type of TestDouble that define a sort of specification based on expectations and, in this particular case, mocks can substitute APIs or clients reducing in this way, parties to set up and run.

services with mocks

Even if the image show deployed services, there are many solutions to load part of an application which doesn’t imply a real execution. however, you can simulate an interaction through a mock or stub based on Contracts. As showed before, we can have two categories of breaking change based on the side that doesn’t maintain expectations and in the same way, we can identify two categories of contracts:

  • Provider Contracts
  • Consumer Contracts

Contracts characteristics

A Provider service exposes a set of business functionalities which can be used or not by one or many consumers and ingested in different ways. A change in the provider interface can break interactions with many consumers, but it is not the same for the consumer. In the same way, a Provider contract cover completely the functionalities exposed by the service with only one definition. On the contrary, a Consumer contract can cover only a set of functionalities that are interesting for the service.

consumer contracts

We can say that a provider contract is a closed and complete expression of business functionalities. Instead, consumer contract is an open and incomplete definition of business expectations. When a provider accepts the expectations defined by the consumer contract, confirms that the expectation is a functionality that supported for a period of time.

Consumer-driven Contracts

google search true story

Ok, so now? Well, If you take a look on internet, you will not find any trace of provider driven contract testing or question about it. On the contrary, the web is full of posts about Consumer-driven contract testing. This should be enough for you and me to choose a consumer-driven solution but, as I said at the beginning, the idea of this post is to give the minimum set of knowledge about Contract testing, and to have an answer in case of your colleagues are free mind enough to ask if exists an alternative meanwhile you are exposing why Contract tests are so cool.

In short terms, it’s all about business. Consumer contracts point the finger on current supported business value exposed by the provider. In a consumer-driven approach, the sum of all contracts generated by the consumers and asserted by the provider is closed and complete respect the functionalities requested.

Anyway Consumer-driven Contracts drive the evolution of the services keeping the focus on what really matter with a feedback loop on the changes that will arrive during the life and death of the services which cannot be achievable with a Provider-driven approach which not take in account the feedback from the consumer.

In my experience, it’s easier and natural to think in a Provider-First manner than Consumer-first, and the reason behind this mindset, it might be connected to the fact that a breaking change made by the Provider is more detectable than the one made by the Consumer. In a real world example where your team isn’t the owner of both services, you have to deal with meetings, longer meetings and extravagant, informal, hermetic, long-winded design docs and other mythical beasts. A contract is a formal expression of needs and duties which can be another tool in your pocket that can be used in order to reduce the background noise during the process of evolution of services. Probably you know the sensation of powerlessness when a design doc or swagger definition reaches the mailbox or a shared folder and it’s necessary another meeting or mail tread to have a change in the definition or worse, get an HTTP response status error because someone made a little change in the API which isn’t versioned.

Consumer Driven Process

To summarize, Consumer-Driven contracts can give you a process that presents an iterative way of proceeding with a formalised format which can help large organization with services which are owned by different teams that can be in different locations.

Contract Testing with Pact is an implementation of Consumer-driven contract testing which actually support many languages. At the moment of writing this post, pact foundation released v3 specification which cover the following features:

  • pact format for message queues
  • regular expression and type matching
  • specification shared between Ruby, JVM and .Net versions

Pact ecosystem

Furthermore, it’s available a broker which can be used to publish and share contracts between services.

What’s Next

We have covered the minimum set of principles and motivations that are necessary in my opinion to work with contract testing. In the next posts we will take a look to how:

  • create consumer-driven contracts tests with Pact and Java
  • setup broker server
  • integration with Continuous Integration