> For the complete documentation index, see [llms.txt](https://docs.eventoframework.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.eventoframework.com/recq-patterns/recq-communication-pattern/component-to-component.md).

# Component to Component

Asynchronous Messaging and CQRS

In a RECQ architecture, components communicate with each other indirectly through asynchronous message exchange facilitated by the Message Gateway. This communication adheres to the Command Query Responsibility Segregation (CQRS) pattern, enforcing a clear separation between commands that modify the system state and queries that retrieve data without causing state changes.

**Message Types and Protocols:**

* **Commands:**
  * Commands typically follow a request/reply pattern:
    * The component sends a command message to the Message Gateway, specifying the desired action.
    * The Message Gateway routes the command to the appropriate Command Service.
    * The Command Service processes the command and performs the requested action.
    * The Command Service sends a response message back through the Message Gateway, indicating the success or failure of the operation.
    * The Message Gateway delivers the response message to the originating component.
  * Since the CQRS pattern is enforced, the response message for a command typically only includes confirmation of success or failure (e.g., boolean flag or error code) and not the updated data itself.
* **Queries:**
  * Queries typically follow a request/reply pattern:
    * The component sends a query message to the Message Gateway, specifying the requested data.
    * The Message Gateway routes the query to the appropriate Query Service.
    * The Query Service retrieves the data from the system state or relevant data store.
    * The Query Service sends a response message back through the Message Gateway, containing the requested data (the Views).
    * The Message Gateway delivers the response message containing the data (Views) to the originating component.

<figure><img src="https://3512123521-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTdWWm782uxBjp7snBdDU%2Fuploads%2FL2ZMErWlpeveIqZmhnsc%2Fimage.png?alt=media&amp;token=d083627c-b64b-4e37-bc61-c9bba170fa34" alt=""><figcaption><p>Component to Component Communication</p></figcaption></figure>

**Benefits of Asynchronous Messaging and CQRS:**

* **Improved Scalability and Performance:** Asynchronous message exchange allows components to send requests without waiting for immediate responses. This decouples components and enables independent scaling based on their workload.
* **Enhanced Maintainability:** Separating commands and queries simplifies the logic within each component and promotes code clarity.
* **Optimized Event Sourcing:** Separating commands from queries, updating the system state with commands and retrieving data through queries promotes a cleaner separation of concerns for event sourcing.

**Considerations:**

* **Increased Complexity:** Asynchronous messaging and CQRS introduce additional complexity compared to simpler synchronous communication patterns.
* **Potential for Data Inconsistency:** In rare cases, if events haven't been processed by the System State Store when a query is sent, the retrieved data might not reflect the latest state. Techniques like eventual consistency or materialized views can be used to mitigate this.

**Conclusion:**

Leveraging asynchronous messaging with the mandatory Message Gateway and adhering to the CQRS pattern provides a robust and scalable approach for component-to-component communication in a RECQ architecture. This approach promotes loose coupling, improves maintainability, and optimizes event sourcing for event-driven microservices.