Command Query Responsibility Segregation (CQRS) separates the model that changes state (commands) from the model that reads state (queries). This lets each side be optimized, scaled, and stored independently.

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Commands vs. queries

CommandsQueries
IntentChange stateRead state
ReturnsAcknowledgement / resultData
ModelRich domain model, enforces invariantsRead-optimized projections
ScalingScales with write loadScales with read load

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Read models

Query-side projections are built from events emitted by the write side, often kept in a store shaped for fast reads. They are eventually consistent with the write model.
Apply CQRS where read and write workloads differ significantly. For simple CRUD capabilities, a single model is usually the better trade-off.