CQRS (Split Reads From Writes)

Think of a restaurant kitchen and its menu board. The kitchen — the 'write side' — is built to correctly cook complicated orders, one at a time. The menu board — the 'read side' — is a simple, quick-to-scan summary customers glance at. Nobody walks into the kitchen to check what's available. Whenever the kitchen changes something, it also updates the board. CQRS splits a system the same way. CQRS stands for Command Query Responsibility Segregation — a fancy way of saying 'writing and reading are two different jobs.' Commands (do this — place an order) go through a model built for correctness. Queries (tell me this — show my order history) go through a separate, simpler model built for fast reading.

1. 1Keep two models. The 'write model' handles commands — create, update, delete — and enforces the real rules. The 'read model' is a simplified, ready-to-read copy, shaped exactly for what queries need.
2. 2Every time a command changes the write model, also update the read model — this step is called 'projecting' the change — so it reflects the update.
3. 3Queries only ever read from the read model. They never touch the write model. So reads stay fast and simple, even as the write-side rules grow more complicated.

Use CQRS once your app is popular enough that how you write data doesn't match how you read it. Writing is careful, one record at a time, with lots of checks. Reading needs fast summaries across many records. For example, in an order system, placing an order is complex, but showing 'orders per customer' needs to be instant. CQRS lets you optimize each side on its own, instead of forcing one model to do both jobs well.

• ⚠Letting a query read straight from the write model 'just this once.' That brings back the tight link CQRS is meant to remove, and the fast read model quietly drifts out of sync.
• ⚠Forgetting to update the read model after a command. The command succeeds on the write side, but users never see the change, because nothing projected it into what they actually read.