Intro to PACT for .NET Core: Events based systems

This is continuation of PACT for .NET series where I am going to cover usage of pact in event-based systems.

This article assumes the readers will have basic understanding of event-based systems and framework. In case not, then a good place to start can be RabbitMQ Tutorials | RabbitMQ

I will be using the same repo setup that I have used to demonstrate usage in API based systems. I have tried to cover in detail regarding the setup which can be found in previous blog in this series below

Contract testing for APIs: Intro to PACT for .NET Core | by Ajay kumar | Medium

For illustration purposes, we can imagine a scenario, where Result service (Consumer) wants to get informed whenever a student is created.

Image 1: Publisher/Broker/Listener

Now since both Result and Student are separate services, below is the infrastructure that has been implemented in order to make that work.

• I have incorporated RabbitMq library as a messaging service in both the APIs

Image 2: Publish event during student creation

• Whenever a new student is created using Student service’s (Provider) POST API, we are publishing an event called StudentCreatedEvent

Image 3: Properties inside StudentCreatedEvent

• Now the above event can be listened by any service which is interested in that event. In this scenario, that service is Result service
• There is a listener defined in the Result service which may perform any operation based on the information received in StudentCreatedEvent event. Maybe it wants to store some details of student in the result service which may be used while preparing result, so that they don’t have to depend on Student service every time.

Image 4: Student created event listener in Result service

Contract tests in action

Now the first part is where consumer side contract test project defines the contract that consumer service is expecting

Image 5: Consumer defining the contract

Once the contract is defined, a PACT JSON contract file will be generated that will look like below

Image 6: Contract snapshot

Now that this file has been created, we need to write test on provider side, which will honour the above created contract.

Provider side code will look something like below. We can see in the image below, line #50 shows the path to contract test file and line #78 shows the verification

Image 7: Provider side contract verification

One this setup is ready; we can quickly do a dotnet build to build the project and dotnet test to test the project. Alternatively, you can run test individually as well. For illustration, below the success result for pact verification for our scenario

Image 8: Success test

Failure case!

Now to test the failure case, we can make some changes in our provider. Lets rename the LastName field to LName and rerun the tests.

Image 9: LastName property renamed to LName

Lets run the tests and see the results.

Image 10: Provider tests hinting at the actual gap between consumer and provider contract

As we see in the results above, the error message suggests that LastName field is missing at provider side.

For reference, the code repository being discussed is available on github link: https://github.com/ajaysskumar/pact-net-example

Thanks for reading through. Please share feedback, if any, in comments or on my email ajay.a338@gmail.com.