Home → NOTES → CQRS (Command Query Responsibility Segregation)

CQRS (Command Query Responsibility Segregation)

category: Architecture
tags: cqrs, java, axon, eventsourcing, kubernetes

Main Topic 1

CQRS separates the responsibility for reading and writing data into different models, enhancing scalability and performance for complex systems.

Subtopic A: What It Is

• Command model handles updates (create/update/delete)
• Query model handles reads
• Often combined with Event Sourcing

Subtopic B: Why Use It

• Optimized read/write performance
• Better separation of concerns
• Enables eventual consistency and complex audit trails

Subtopic C: When To Use It

• High-traffic applications with different read/write loads
• Event-driven systems
• Systems requiring auditability or history tracking

Main Topic 2

Code Example (Java + Axon)

@Aggregate
public class Account {
    @CommandHandler
    public Account(CreateAccountCommand cmd) {
        AggregateLifecycle.apply(new AccountCreatedEvent(cmd.getId()));
    }

    @EventSourcingHandler
    public void on(AccountCreatedEvent evt) {
        // handle event
    }
}

Infra Example (Kubernetes)

apiVersion: apps/v1
kind: Deployment
metadata:
  name: cqrs-app
spec:
  replicas: 1
  selector:
    matchLabels:
      app: cqrs-app
  template:
    metadata:
      labels:
        app: cqrs-app
    spec:
      containers:
      - name: cqrs-container
        image: myregistry/cqrs-app:latest

Key Concepts Summary

• Separate read/write models
• Often used with Event Sourcing
• Enables eventual consistency

Best Practices / Tips

1. Keep command and query models independent
2. Use messaging/event systems for syncing models
3. Monitor latency for eventual consistency

Common Issues / Troubleshooting

Problem 1: Increased complexity

• Cause: Two data models and sync logic
• Solution: Only use CQRS where justified

Problem 2: Stale reads

• Cause: Eventual consistency delay
• Solution: Use polling or notifications to update UI

References / Further Reading

• https://martinfowler.com/bliki/CQRS.html
• https://docs.axoniq.io/

Last updated: 2025-08-26 20:00 UTC