CQRS – Command Query Responsibility Segregation – Comprehensive Guide

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In CQRS, the write model publishes events after changes, and event handlers (projections) process these events to update the read model. This can be synchronous or asynchronous; use an event bus or message queue for delivery. It's crucial to handle out-of-order events and idempotency for projections.

CQRS naturally fits with Event Sourcing: the write side uses Event Sourcing (events as the source of truth), and the read side utilizes projections to create read models from these events. This provides a complete audit trail, time travel, and replay capabilities; events bridge the gap between the write and read sides. Many systems combine CQRS + Event Sourcing.

Testing in CQRS requires unit tests for command handlers, unit tests for query handlers, integration tests for the command → event → projection flow, testing eventual consistency scenarios, and end-to-end tests. It’s vital to test that events correctly synchronize the read model, that read model updates are accurate, and that consistency guarantees function as expected.

Validation can occur at various levels: client-side validation for UX, command validation before handler execution, domain validation within aggregates, and database constraints. In CQRS, validation is often handled at the command level prior to execution. It’s important to verify business rules, uniqueness constraints, and permissions; read models can even use validation (e.g., checking email uniqueness).