Understanding CQRS#

Command Query Responsibility Segregation (CQRS) is a pattern that separates read and write operations into distinct models. Instead of using the same model for both updates and queries, CQRS uses different models optimized for their specific purposes.

Separating write and read sides#

Write side (Commands):

• Processes commands that express intent to change state
• Uses models that enforce business rules
• Records events when operations succeed

Read side (Queries):

• Provides optimized views of current state
• Uses projections denormalized for specific queries
• Updated by reacting to events from the write side

In Backslash:

• Commands → Handlers → Models → Events
• Events → Event Handlers → Projections

Benefits of CQRS#

Independent optimization. Write and read models can be optimized separately; complex business rules don’t complicate queries, and query optimization doesn’t constrain domain logic.

Denormalized views. Projections can be structured however queries need them, with precomputed values and joined data for fast reads.

Clear separation of concerns. Business logic lives in models; query logic lives in projections. Each has a single, focused responsibility.