README.md

Exercise 15 - Multi-Stream Projections

In exercises 12-14 you built projections from a single event stream. Now you'll combine events from multiple streams into a single read model.

Scenario: Payment Verification

A payment verification requires data from three independent checks, each producing events on its own stream:

1. Payment recorded — from the payment service (amount, order reference)
2. Merchant limits checked — from the merchant service (within daily limits?)
3. Fraud score calculated — from the fraud detection service (risk score, acceptable?)
4. Verification completed — final decision event (approved or rejected)

All events share a PaymentId that ties them to the same payment verification read model.

What to implement

With the Database interface representing the sample database, implement a PaymentVerification read model and projection:

1. Define the PaymentVerification read model properties — the test assertions tell you what shape it needs.
2. Create a PaymentVerificationProjection class with typed Handle methods for each event.
3. Register handlers in the test using eventStore.Register.

The key difference from single-stream projections: each event arrives on a different stream ID, but they all reference the same PaymentId. Your projection must use PaymentId (not the stream ID) as the read model key.

Reference

Read more about multi-stream projections and handling events from multiple sources:

• Handling Events Coming in an Unknown Order