CQRS (Command Query Responsibility Segregation)

In most applications, the same data model is used for both reading and writing data. For example, an Order Service might use a single PostgreSQL database to create orders, update their statuses, and retrieve order details. This approach works well for many applications. It's simple, easy to maintain, and allows reads and writes to use the same transactional database.

However, as applications grow, read and write operations often develop very different requirements. Writing an order requires business validation, transactional consistency, and enforcing rules. Reading an order might require combining information from several services, applying filters, and returning data quickly to users. Trying to optimize one data model for both operations can become difficult.

This is where CQRS (Command Query Responsibility Segregation) comes in. CQRS is an architectural pattern that separates the parts of a system responsible for modifying data from those responsible for reading it. Let's understand how CQRS works, why it's useful, and when it makes sense in microservices.