CQRS (Command Query Responsibility Segregation)

Split the model used to write data (commands) from the model used to read data (queries), instead of one model serving both.

• Command side — validates and applies state changes; optimized for consistency
• Query side — one or more read models (projections), denormalized and optimized for the specific queries the app needs

Why

• Read and write workloads often have very different shapes and scaling needs (e.g. 100x more reads than writes)
• Lets each side scale independently, and even use different storage technology

Trade-offs

• Eventual consistency — the read model lags behind the write model until it catches up
• More moving parts — two models (often two data stores) to keep in sync instead of one

Related

• Commonly paired with event sourcing — the write model appends events, and read models are projections built by consuming them