Command Query Responsibility Segregation (CQRS)
CQRS is an architectural pattern that separates the models for reading and writing data in an application. While traditional architectures use a single data model for both operations, CQRS advocates for two distinct paths: commands, which perform state changes (writes), and queries, which retrieve data (reads). This separation allows for each model to be optimized independently for its specific purpose, leading to improved performance, scalability, and security in complex systems, particularly those with asymmetrical read/write patterns.
Key Concepts
- Command: An operation that requests a change in the state of the application (e.g., “CreateUser,” “UpdateAddress”). Commands do not return data, only whether the operation succeeded.
- Query: An operation that requests data from the application without modifying its state. Queries return a data model optimized for the specific UI or client requirement.
- Task-Based UI: A UI design that focuses on specific user actions (commands) rather than direct manipulation of data models, which aligns naturally with the CQRS pattern.
- Eventual Consistency: In many CQRS implementations, the read model is updated asynchronously from the write model, meaning that the read model may briefly lag behind the write model.
- Read Model Optimization: The read database can be structured as a flattened, denormalized set of views (or even a different database type like Elasticsearch) to maximize query speed.
Common Patterns or Tools
- Event Sourcing: Frequently paired with CQRS, where the write model stores a sequence of events, and the read model is built by projecting those events into a query-optimized format.
- MediatR (or similar libraries): A library often used in .NET environments to implement the mediator pattern, facilitating the decoupling of commands and queries from their handlers.
- Projection: The process of taking events from the write side and using them to update the read database.
- Materialized Views: Pre-computed views of the data used on the query side to avoid complex joins and aggregations during read operations.