Command Query Responsibility Segregation (CQRS) is an architectural pattern that separates the responsibility of handling commands (requests that change the state of the system) from the responsibility of handling queries (requests that return information from the system).
This separation of concerns can offer several benefits, such as improved performance, scalability, and maintainability.
In a CQRS architecture, commands are typically handled by a separate service from the one that handles queries. This separation can be implemented in different ways, but a common approach is to use different database tables for storing data that is accessed by commands and data that is accessed by queries.
Spring Boot is a popular Java framework for building web applications. It offers a convenient way to create stand-alone, production-grade Spring-based applications that can be "just run".
In this article, we'll take a look at how to implement a CQRS architecture in a Spring Boot application. We'll start by looking at the different components that are involved in such an application. Then, we'll implement a simple example application that uses CQRS.
A CQRS application typically has the following components:
Now that we've looked at the different components of a CQRS application, let's implement a simple example application that uses CQRS.
Our example application will be a task manager. Users will be able to create tasks and assign them to themselves or to other users. They'll also be able to mark tasks as complete.
We'll start by implementing the command service. This service will be responsible for validating and executing commands.
The command service will have the following methods:
We'll implement these methods in a class called TaskCommandService. This class will be annotated with @Service, which is a Spring annotation that marks a class as a service component.
@Service
public class TaskCommandService {
public void createTask(Task task) {
// validate the task
// save the task
}
public void updateTask(Task task) {
// validate the task
// save the task
}
public void markTaskAsComplete(String taskId) {
// validate the task
// mark the task as complete
// save the task
}
}
As you can see, the createTask and updateTask methods accept a Task object as a parameter. This object will be used to hold the data for the task that is being created or updated.
The markTaskAsComplete method accepts a task ID as a parameter. This task ID will be used to look up the task that is to be marked as complete.
Each of these methods starts with a call to a validate method. This validate method will be responsible for validating the data that is passed in. We'll implement this method in the next section.
The validate method will be responsible for validating the data that is passed in to the command service methods. We'll implement this method in a class called TaskCommandValidator. This class will be annotated with @Component, which is a Spring annotation that marks a class as a component.
@Component
public class TaskCommandValidator {
public void validate(Task task) {
// validate the task
}
public void validate(String taskId) {
// validate the task ID
}
}
As you can see, this class has two validate methods. One of these methods accepts a Task object as a parameter. This method will be used to validate the data for creating and updating tasks.
The other validate method accepts a task ID as a parameter. This method will be used to validate the task ID for marking tasks as complete.
In a real application, these validate methods would contain logic for validating the data. For the purposes of this example, we'll just leave them empty.
The query service will have the following methods:
We'll implement these methods in a class called TaskQueryService. This class will be annotated with @Service.
@Service
public class TaskQueryService {
public List<Task> getAllTasks() {
// execute query
// return results
}
public Task getTaskById(String taskId) {
// execute query
// return results
}
}
As you can see, the getAllTasks method returns a list of Task objects. This list will contain the data for all of the tasks in the system.
The getTaskById method returns a single Task object. This object will contain the data for the task with the specified ID.
The repository is a data access layer that provides an interface for accessing the data store. In our example application, we'll use a repository for accessing the tasks data.
We'll implement the repository in a class called TaskRepository. This class will be annotated with @Repository, which is a Spring annotation that marks a class as a repository component.
@Repository
public class TaskRepository {
public void save(Task task) {
// save the task
}
public Task findById(String id) {
// find the task
// return the task
}
public List<Task> findAll() {
// find all tasks
// return the tasks
}
}
As you can see, this repository has methods for saving, finding, and deleting tasks.
The model is the domain model for the application. In our example application, the task will be the only model object.
We'll implement the task model in a class called Task. This class will be annotated with @Entity, which is a JPA annotation that marks a class as an entity.
@Entity
public class Task {
@Id
private String id;
private String name;
private String description;
private boolean complete;
// getters and setters
}
As you can see, this task model has four fields: id, name, description, and complete. The id field is annotated with @Id, which is a JPA annotation that marks a field as the primary key of an entity.
Now that we've implemented the different components of our CQRS application, let's put it all together and see how it works.
We'll start by creating a task. To do this, we'll first create a Task object and populate it with the data for the task. Then, we'll pass this object to the createTask method of the TaskCommandService.
Task task = new Task();
task.setName("Write example");
task.setDescription("Write an example of the CQRS pattern");
taskCommandService.createTask(task);
This will create a new task in the system.
Now, let's say we want to get a list of all tasks. To do this, we'll call the getAllTasks method of the TaskQueryService.
List<Task> tasks = taskQueryService.getAllTasks();
This will return a list of all tasks in the system.
Finally, let's say we want to mark a task as complete. To do this, we'll first get the task by ID. Then, we'll call the markTaskAsComplete method of the TaskCommandService.
Task task = taskQueryService.getTaskById("1");
taskCommandService.markTaskAsComplete(task.getId());
This will mark the task with ID "1" as complete.
In this article, we've looked at the Command Query Responsibility Segregation (CQRS) pattern and how it can be implemented in a Spring Boot application.
We've also seen how this pattern can offer several benefits, such as improved performance, scalability, and maintainability.
If you're interested in learning more about CQRS, I recommend the following resources: