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🧩 CQRS (Command Query Responsibility Segregation)

βœ… Overview​

Architecture style separating Read (Query) and Write (Command).

βœ… Problems Addressed​

  • Balancing read optimization vs write consistency.
  • Query performance problem in large-scale systems.
  • General-purpose model cannot satisfy specific data access patterns.

βœ… Basic Philosophy & Rules​

  • Design Command Model and Query Model separately.
  • Often separate DB for reading (Cache / Search specific) and DB for writing.
  • Often used in combination with Event Sourcing.

Conceptual Diagram​

CQRS Architecture diagram

Source: Greg Young, β€œCQRS Documents”, 2010, Figure 12.
https://cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf

βœ… Suitable Applications​

  • Systems where load balance of write and read differs significantly.
  • API with high traffic and large scale requirements.
  • Domains with complex search or aggregation requirements.

❌ Unsuitable Cases​

  • Small-scale CRUD apps (Complexity outweighs benefits).
  • Situations where separation increases consistency and operational load.

βœ… History​

  • Proposed in DDD context, but widely adopted also from scalability perspective.

βœ… Related Styles​

  • Event Sourcing: Good compatibility with style holding state by events.
  • EDA: Utilized for asynchronous update.

βœ… Representative Frameworks​

  • EventStoreDB
    Widely adopted in CQRS implementation combined with Event Sourcing.

  • Axon Framework (Java)
    Possible to implement integrating Command/Query separation and Event Sourcing.

  • Lagom (Scala / Java)
    Microservices framework premised on CQRS + ES.

  • Kafka + Custom Command Processor
    CQRS implementation exchanging Command/Event via Kafka Topic is also common.

βœ… Design Patterns Supporting This Style​

  • Command
    Central pattern treating write operation as "operation with intent".

  • Memento
    Used when handling snapshot at specific point as auxiliary of state management.

  • Strategy
    Switching different data fetching methods in Read / Write models.

  • Observer
    Used when subscribing to write result events and updating read model.

  • Mediator
    Appears in structure where Command Handler / Query Handler cooperate loosely.

βœ… Summary​

CQRS is a powerful method realizing scale and flexible model design with the simple concept of separation of reading and writing.