How to unit test a service which output is an event with data?

Question

Technologies : NodeJs, Kafka, Mocha

The issue : I have a nodeJS service which given a String returns events with a many data.

How can I test this using Mocha ?

In a simple scenario the service returns one event with some data attached. In a more complex scenario the service publishes multiple events.

Is there a way I can test the services output at unit test level?

Possible solutions : A solution I can think of is to create a consumer inside the mocha test. Is there another practice/method I can follow ? Should I test this at a different level aka integration level ?

Answer 1

I haven't done extensive unit testing in java script. But the principles remain fairly same in all the languages.

Properties of a good unit test would be

• Runs quickly and provides nearly immediate feedback
• Doesn't have side effects, like charging a credit card, or modifying a database or event publishing an event

What needs to be tested in your case

• Is it publishing the event in the needed scenario
• Is it publishing the event with the correct values

Suggested design

• Design a "Event Publisher" object that is publishes the given event (string) to the broker
• Design the service to use the "Event Publisher"
• Inject the event publisher in the service when creating the service
• Create a "Mock Event Publisher" that is having the same method as "Event Publisher". But instead of publishing the message, it records the message published using it. Add a method to verify the messages published in the "Mock Event Publisher".
• While testing inject the mock event publisher in to the service instead of event publisher, and verify if the correct message is published at the correct time.

Integration Tests

I wouldn't test the actual insertion of the event to the kafka broker in unit test. I would test this in integration test.