Understanding CQRS: Command Query Responsibility Segregation
Command Query Responsibility Segregation (CQRS) is an architectural pattern that separates read and write operations into distinct models. This separation allows for optimization of each model independently, addressing different requirements and scaling needs.
Core Components of CQRS
flowchart TD
Client[Client Application] --> Commands[Commands]
Client --> Queries[Queries]
Commands --> CommandHandler[Command Handler]
Queries --> QueryHandler[Query Handler]
CommandHandler --> WriteModel[Write Model/Domain Model]
WriteModel --> EventStore[Event Store]
EventStore --> ReadModelProjection[Read Model Projection]
ReadModelProjection --> ReadModel[Read Model]
QueryHandler --> ReadModel
ReadModel --> QueryResults[Query Results]
QueryResults --> Client
Key Components and Their Responsibilities:
- Commands: Instructions to change state (e.g., CreateOrder, UpdateCustomer).
- Queries: Requests for information without state changes.
- Command Handler: Processes commands and applies them to the write model.
- Write Model: The domain model with rich business logic.
- Event Store: Records all state-changing events as the source of truth.
- Read Model Projection: Processes events to update the read model.
- Read Model: Optimized for querying, often denormalized for performance.
- Query Handler: Retrieves data from the Read Model in response to queries.
The separation of write and read models allows for independent optimization, scalability, and security for both operations.
Benefits of CQRS
- Scalability: Read and write sides can scale independently
- Performance: Read models can be optimized for specific query needs
- Security: Access control can be applied differently to commands vs. queries
- Complexity Management: Simplifies complex domain models by separating concerns
When to Use CQRS
CQRS is particularly valuable in:
- Systems with high-performance query requirements
- Complex domains with sophisticated business rules
- Applications with significant disparity between read and write operations
- Event-sourced systems that need to maintain a complete history
While powerful, CQRS introduces complexity and should be applied selectively to parts of your system where the benefits outweigh the implementation costs.