Implementing CQRS with Spring Modulith

Spring Modulith, a relatively new addition to the Spring ecosystem, provides excellent support for implementing CQRS in a clean, modular way.

In modern enterprise applications, separating read and write concerns often becomes essential as systems grow in complexity. The Command Query Responsibility Segregation (CQRS) pattern addresses this need. When implemented well, this separation can dramatically improve scalability, performance, and maintainability.

In this example, we are going to implement a simple application that handles a Product Catalog.

Let’s dive right in!

A canal in Amsterdam (2018)

1. Project Structure

We need two main modules, one for reads named query and the other for writes named command.

Each module will be connected to a different database. (Note: It’s not a prerequisite of CQRS using different databases for read and writes — we could have used a different schema or even just a different table for simplest cases)

The folder structure of our application will look like this:

gae.piaz.modulith.cqrs/  
├── command/  
│   ├── api/  
│   ├── config/  
│   ├── domain/  
│   ├── events/
│   │   ├── ProductCreatedEvent.java
│   │   ├── ProductUpdatedEvent.java
│   │   ├── ProductReviewEvent.java
│   │   └── package-info.java
│   ├── service/  
│   └── package-info.java  
├── query/  
│   ├── api/
│   ├── application/  
│   ├── config/  
│   ├── domain/  
│   └── package-info.java  
├── shared/  
│   ├── config/  
│   └── package-info.java  
└── CQRSApplication.java

The query folder contains a CLOSED module with reads business logic

• The command folder contains a CLOSED module with writes business logic
• The shared folder contains an OPEN module, with shared configuration

We need to consider CLOSED modules as completely decoupled applications. This means that it should not be possible for the command module to access any class of the query module and vice-versa.

The only point of contact is the classes inside the command/event folder.