> Head-Heart-Hands Model in CQRS and Event Sourcing
In the architecture of CQRS (Command Query Responsibility Segregation) and Event Sourcing, we can map the components onto a simple, human-centered metaphor: the head, the heart, and the hands.
# Head: The Realm of Knowledge and Queries The "head" represents the query side of the CQRS model. This is where we consult the immutable event log to retrieve information. It's all about asking questions and gaining understanding without changing the state of the system. Like a head that reasons and knows, this part of the system deals with descriptive queries and knowledge retrieval.
# Heart: The Realm of Commands and Intent The "heart" corresponds to the command side of the model. Here we issue commands that reflect intent and purpose. Commands are the directives that change the state of the system. This part is performative and mission-driven, just like the heart that drives action and purpose in a human being.
# Hands: The Realm of Integration and Action The "hands" are the integration side. After commands are issued and events are recorded, the hands integrate these changes back into the system. They update read models and projections, ensuring that all parts of the architecture are in sync. Much like hands that carry out tasks, this part of the system applies the knowledge and intent into real-world changes.
# Relating Hands to the Controller In traditional MVC architectures, the controller handles both user input and the coordination of model and view. In the CQRS and Event Sourcing model, the "hands" can be thought of as a specialized kind of controller focused on integration. Instead of combining all responsibilities, the hands ensure that the results of commands (heart) are properly integrated into the read side (head), maintaining consistency and ensuring that all queries reflect the latest state.
# Conclusion By framing the CQRS and Event Sourcing architecture in terms of head, heart, and hands, we align the technical components with a human-centered metaphor. The head is for knowledge and queries, the heart is for intent and commands, and the hands are for integrating and applying these changes. This makes it easier to understand how each part of the architecture contributes to a coherent, well-governed system.
# See
- CQRS
- Event Sourcing
- martinfowler.com 
- DSLs and HyperTalk