CQRS (Command Query Responsibility Segregation)

1. Introduction

CQRS, or Command Query Responsibility Segregation, is an architectural pattern that separates the responsibilities of reading and writing data. In traditional architectures, commands (writes) and queries (reads) are often handled by the same model, which can lead to complications, especially as applications scale.

2. Key Concepts

• Commands: Actions that change the state of the system.
• Queries: Requests to read data without side effects.
• Event Sourcing: Storing state changes as a sequence of events rather than just the current state.
• Read and Write Models: Separate models for reading and writing data, often leading to optimized performance.

3. Architecture

In a CQRS architecture, the application is typically divided into two parts:

1. Command Side: Handles commands and updates the write model.
2. Query Side: Handles queries and retrieves data from the read model.

flowchart TD
    A[Command] -->|update| B[Write Model]
    B --> C[Event Store]
    C -->|event| D[Read Model]
    D -->|query| E[Query]
        

4. Implementation

Below is a simple example of how to implement CQRS in a Node.js application:

class Command {
    execute() {
        // Handle command
    }
}

class Query {
    fetch() {
        // Fetch data from read model
    }
}

class UserCommand extends Command {
    execute(user) {
        // Logic to create or update user
    }
}

class UserQuery extends Query {
    fetch(userId) {
        // Logic to fetch user data
        return { id: userId, name: "John Doe" };
    }
}

// Usage
const userCommand = new UserCommand();
userCommand.execute({ id: 1, name: "Jane Doe" });

const userQuery = new UserQuery();
const user = userQuery.fetch(1);
console.log(user);
        

5. Best Practices

• Keep commands and queries separate to maintain clarity.
• Consider using event sourcing to track changes in state.
• Design read models tailored to specific query needs for optimal performance.
• Use eventual consistency where appropriate to enhance system responsiveness.

6. FAQ