CQRS: (Command Query And Responsibility Segregation)

Right way of doing DDD (Domain Driven Development)

• CQRS enforce segregation of read and write data model.
• CQRS is derived from CQS (Command Query Separation), the term mentioned for the first time in the book by Bertrand Meyer (Object Oriented Software Construction).
• It's main idea is split application majorly in two parts:
  • Query
    • These are the read models that are going to fetch data, without modifying the state.
  • Command
    • These are the write models that modify the state.
• The domain object has to implement functions for fetching and modifying the state. Each function can either read or write but not both.

Components and system requirements

Command Handling Component

• Validate the command.
• Provide a place holder to implement logic based on which the event has to trigger.

Event Handling Component

Event Sourcing Handler

• Asynchronously receives event.
• Persist event in Write DB

Event Handler

• Asynchronously receives event.
• Can publish the projected state in Read DB.

Query Handling Component

• Query projected results from Read DB.

These components are wired using

• Spring Boot
• Axon Framework
• Kotlin

Storage

Event Store / Write DB: Axon Server Docker Container

docker run -p 8024:8024 -p 8124:8124 axoniq/axonserver

• 8024: Http Port
• 8124: gRPC Port

Read DB: Postgres Docker Container

docker run -e POSTGRES_PASSWORD=admin123 -e POSTGRES_USER=admin -p 5432:5432 postgres