Event sourcing is a powerful architectural pattern used in building robust and scalable systems. It allows capturing and storing a sequence of events that represent changes to an application’s state over time. Testing event sourcing systems is crucial to ensure correctness and reliability. In this blog post, we will explore how to test event sourcing systems using the popular Java Spock testing framework.

Setting Up the Environment

To begin, we need to set up our testing environment. Make sure you have the following dependencies in your project:

dependencies {
    testCompile 'org.spockframework:spock-core:2.0-groovy-3.0'
    testCompile 'org.codehaus.groovy:groovy-all:3.0.9'
}

Writing Test Cases

Mocking Dependencies

When testing event sourcing systems, it’s common to have dependencies such as repositories, event buses, or external services. We can mock these dependencies using Spock’s Mock() annotation. Let’s say we have a AccountRepository and an EventBus as dependencies. Here’s an example of how we can mock them in our test:

def "should publish events when creating a new account"() {
    given:
    def accountRepository = Mock(AccountRepository)
    def eventBus = Mock(EventBus)
    def eventHandler = new AccountEventHandler(accountRepository, eventBus)

    when:
    eventHandler.handle(new CreateAccountEvent())

    then:
    1 * accountRepository.save(_)
    1 * eventBus.publish(_)
}

Testing Event Handlers

Event handlers are responsible for processing incoming events and updating the application’s state accordingly. We can write test cases to ensure that the event handlers correctly handle events and produce the desired outcome. Here’s an example of testing an event handler:

def "should update account balance when processing a money deposit"() {
    given:
    def accountRepository = Mock(AccountRepository)
    def eventBus = Mock(EventBus)
    def eventHandler = new AccountEventHandler(accountRepository, eventBus)
    def account = new Account()
    account.deposit(100) // Initial balance

    when:
    eventHandler.handle(new DepositMoneyEvent(50))

    then:
    account.balance == 150
    1 * accountRepository.save(_)
    1 * eventBus.publish(_)
}

Testing Event Store

The event store is responsible for storing and retrieving events. We can write test cases to ensure that events are correctly stored and retrieved from the event store. Here’s an example of testing an event store:

def "should store and retrieve events from event store"() {
    given:
    def eventStore = new InMemoryEventStore()
    def event1 = new Event("1")
    def event2 = new Event("2")
    def event3 = new Event("3")

    when:
    eventStore.store(event1)
    eventStore.store(event2)
    eventStore.store(event3)

    then:
    eventStore.getEvents() == [event1, event2, event3]
    eventStore.getEvents("2") == [event2]
}

Conclusion

Testing event sourcing systems is essential to ensure their correctness and reliability. In this blog post, we explored how to test event sourcing systems using the Java Spock testing framework. We discussed mocking dependencies, testing event handlers, and testing the event store. By following these testing practices, you can build robust and bug-free event sourcing systems.

#testing #eventsourcing