# Testing Event-Driven Systems — Event-Driven Architecture & CQRS

Source: https://www.skillbyai.com/en/event-driven-architecture/o-testing

> Test handlers, contracts and whole flows with the right mix of tests.

## Tests at several levels

Most event-driven logic is ordinary code, so test it as such. **Unit tests**: call handlers and aggregates directly with events and check resulting state or emitted events; event-sourced aggregates read naturally as *given these past events, when this command, then these new events*. **Idempotency tests**: deliver the same event twice and assert the effect happened once. **Contract tests**: check that producers still emit events matching the schema consumers rely on, using a schema registry's compatibility checks or consumer-driven contract tools such as Pact's message support. **Integration tests**: run a real broker and database in containers (Testcontainers) to check serialisation, outbox relays and consumer configuration. **End-to-end tests** of whole flows are slow and flaky; keep a few for critical paths and use polling with timeouts (`eventually`) rather than fixed sleeps.

## Given-when-then for an event-sourced aggregate

History in, command applied, new events out.

```python
def test_cannot_withdraw_more_than_balance():
    # given
    acct = Account("a-7")
    for e in [{"type": "MoneyDeposited", "amount": 1000}]:
        acct.apply(e)
    # when / then
    with pytest.raises(ValueError):
        acct.withdraw(1500)
    assert acct.pending == []

def test_payment_captured_is_idempotent(db):
    event = make_event("PaymentCaptured", order_id="o-1001")
    handle_payment_captured(event, db)
    handle_payment_captured(event, db)          # duplicate delivery
    assert db.count("payments_applied", order_id="o-1001") == 1
```

## Testing a relay race

You time each runner alone (unit tests), check that the baton fits every hand (contract tests), and run a few full races before the competition (end-to-end tests). Running only full races tells you something is slow, but not who.

**Quiz:** Why are contract tests important for events?

- [ ] They make brokers faster
- [ ] They replace unit tests
- [ ] They test the network
- [x] They catch producer changes that would break consumers before deployment

*Answer:* They catch producer changes that would break consumers before deployment. Contract or schema checks detect incompatible event changes early.