CQRS

CQRS stands for Command Query Responsibility Segregation. It is an architectural pattern that separates the models used to update information from the models used to read information. In a CQRS-based system, commands represent intent to change state and are processed by the write model, while queries read from a read model. The read and write models may reside in separate data stores and can be scaled independently. CQRS does not require a separate database, but the separation often motivates different storage and indexing strategies.

In the write path, a command is validated by a domain model (often using aggregates in domain-driven design), and upon success emits domain events. An event handler or message bus routes these events to update one or more read models. The read model is designed for queries and can be denormalized, cached, or materialized for fast reads. The system may use asynchronous messaging, leading to eventual consistency between write and read sides. Event sourcing is a related pattern where events are the source of truth; CQRS can be used without event sourcing.

Benefits and trade-offs: CQRS can improve scalability and performance by allowing independent optimization of reads and writes and enabling separate teams to own different parts. It also simplifies complex business rules by separating concerns. The downsides include added complexity, operational overhead, and potential data consistency challenges due to eventual consistency. For simple CRUD applications, the added complexity often outweighs benefits.

Relation to other concepts: CQRS is often discussed in conjunction with Domain-Driven Design and bounded contexts. It is frequently used in microservices architectures and event-driven systems. It is not a universal solution; in many cases a straightforward CRUD model suffices.