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What is CQRS - Command Query Responsibility Segregator Pattern in Java? Example Tutorial

Hello guys, if you are wondering what is CQRS pattern and when and how to use it in your Microservices then you have come to the right place. CQRS is one of the 10 essential Microservice pattern and can be used when your application is either read heavy or write heavy and reading and writing requirement are very different. It aims to separate your app into two parts, command part which writes data and query part which read data and that's how it allows you to develop, scale and optimize them separately. It's also one of the popular Microservices Questions and if you are preparing for interviews, you should prepare this pattern as well. Earlier, I have explained SAGA Pattern and Database Per Microservice pattern and in this article I will explain CQRS pattern, when to use it and what problem does it solve. 



What exactly is the Command Query Responsibility Segregator (CQRS) Pattern?

The Command Query Responsibility Segregator (CQRS) pattern is a software design pattern that separates the responsibilities of reading and writing data. It separates the read model, which is responsible for retrieving data, from the write model, which is responsible for modifying data.

The advantage of using the CQRS pattern is that it allows you to optimize the read and write operations independently, resulting in a more scalable and performant application. It also helps to improve the maintainability of the code by separating the concerns of reading and writing data.


Command Query Responsibility Segregator With Examples

To implement the CQRS pattern in Java, you'll need to create separate classes for the read model and the write model, and define the interfaces for the read and write operations. You'll also need to create a service layer to orchestrate the read and write operations.

There are several reasons why you might want to use the CQRS pattern in your application:
  • Improved scalability
    By separating the responsibilities of reading and writing data, you can optimize the read and write operations independently, resulting in a more scalable application.

  • Improved performance
    By optimizing the read and write operations separately, you can improve the performance of your application.

  • Improved maintainability
    The CQRS pattern helps to improve the maintainability of the code by separating the concerns of reading and writing data.

  • Enhanced security
    The CQRS pattern can also help to improve security by separating the read and write models. This allows you to apply different security measures to the read and write operations, such as limiting access to the write model to a select group of users
The CQRS pattern is particularly useful in situations where the read and write operations have different performance requirements. For example, if you have an application that requires frequent read operations but infrequent write operations, the CQRS pattern can help to optimize the performance of the read operations.

The CQRS pattern is also useful in situations where the read-and-write models have different data structures. For example, if the read model requires a different data structure than the write model, the CQRS pattern can help to simplify the implementation by separating the concerns of reading and writing data.

It's worth noting that the CQRS pattern is not a one-size-fits-all solution, and it may not be appropriate for all applications. Before implementing the CQRS pattern, you should carefully consider your application's specific requirements and whether the CQRS pattern is the right fit.



1. The Read Model and the Write Model

The read model and write model represent the two sides of the CQRS pattern. The read model is responsible for retrieving data, and the write model is responsible for modifying data.

Here's an example of how to define the read model and write model in Java:

public class UserReadModel {
  private final long id;
  private final String username;
  private final String email;

  public UserReadModel(long id, String username, String email) {
    this.id = id;
    this.username = username;
    this.email = email;
  }

  public long getId() {
    return id;
  }

  public String getUsername() {
    return username;
  }

  public String getEmail() {
    return email;
  }
}

public class UserWriteModel {
  private final long id;
  private final String username;
  private final String email;

  public UserWriteModel(long id, String username, String email) {
    this.id = id;
    this.username = username;
    this.email = email;
  }

  public long getId() {
    return id;
  }

  public String getUsername() {
    return username;
  }

  public String getEmail() {
    return email;
  }
}


2. Read and Write Interfaces

Next, you'll need to define the interfaces for the read and write operations. The read interface should define the methods for retrieving data, and the write interface should define the methods for modifying data.

What is CQRS - Command Query Responsibility Segregator Pattern in Java? Example Tutorial



Here's an example of how to define the read and write interfaces in Java:

public interface UserReadRepository {
  UserReadModel getById(long id);
  List<UserReadModel> getAll();
}


public interface UserWriteRepository {
  void save(UserWriteModel user);
  void delete(long id);
}


3. Service Layer

Finally, you'll need to create a service layer to orchestrate the read and write operations. The service layer should delegate the read and write operations to the appropriate repository.

Here's an example of how to define the service layer in Java:

public class UserService {
  private final UserReadRepository readRepository;
  private final UserWriteRepository writeRepository;

  public UserService(UserReadRepository readRepository, UserWriteRepository writeRepository) {
    this.readRepository = readRepository;
    this.writeRepository = writeRepository;
  }

  public UserReadModel getById(long id) {
    return readRepository.getById(id);
  }

  public List<UserReadModel> getAll() {
    return readRepository.getAll();
  }

  public void save(UserWriteModel user) {
    writeRepository.save(user);
  }

  public void delete(long id) {
    writeRepository.delete(id);
  }
}


Now let's put everything together and see how the CQRS pattern works in action.

UserReadRepository readRepository = new DatabaseUserReadRepository(connection);
UserWriteRepository writeRepository = new DatabaseUserWriteRepository(connection);
UserService userService = new UserService(readRepository, writeRepository);

// create a user
UserWriteModel user = new UserWriteModel(1, "john.doe", "john.doe@example.com");
userService.save(user);

// retrieve the user
UserReadModel user = userService.getById(1);


CQRS Frequently Asked Questions

Now, let's see a couple of frequently asked questions about CQRS design pattern which many Java programmer ask:


1. What is the CQRS pattern?

The CQRS pattern is a software design pattern that separates the responsibilities of reading and writing data. It separates the read model, which is responsible for retrieving data, from the write model, which is responsible for modifying data.

2. What are the advantages of using the CQRS pattern?

The main advantage of the CQRS pattern is that it allows you to optimize the read and write operations independently, resulting in a more scalable and performant application. It also helps to improve the maintainability of the code by separating the concerns of reading and writing data.

3. What problem is solved by CQRS Pattern?
CQRS pattern aims to segregate read and write operation into different Microservices and with this approach it solve the problem of scalability and complexity especially if the read and write model is different. CQRS allows you independency develop read and write system and adapt and optimize them based upon your use cases. 

But it also worth noting that CQRS pattern also introduce more complexity as you may need to synchronize the read and write operation, so make sure you use it judiciously. 


4. When to use the CQRS Pattern?
You can use CQRS pattern if your system has high read-write ratio and also read and write requirements are significantly different. For example, if your system performs read operation 90% of time and only 10% of time write data then you can separate these two operation and optimize it accordingly. So whenever you have a situation where your application is read heavy or write heavy, CQRS pattern can improve flexibility and performance. 

Conclusion

The Command Query Responsibility Segregator (CQRS) pattern is a useful tool for improving the scalability and performance of applications by separating the responsibilities of reading and writing data. By creating separate classes for the read model and write model and defining the interfaces for the read and write operations, you can easily implement the CQRS pattern in your Java applications.

The CQRS pattern can be a powerful tool for improving the scalability, performance, and maintainability of your applications. By separating the concerns of reading and writing data, you can optimize the read and write operations independently and create a more scalable, performant, and maintainable application.

I hope this article on the CQRS pattern was helpful and gave you a good understanding of how to implement it in Java. As always, if you have any questions or need further clarification, don't hesitate to ask!

Other Java Microservices articles and tutorials you may like:


Thanks for reading this article so far. If you like this CQRS or Command Query Responsibility design pattern and when and how to use it then please share them with your friends and colleagues. If you have any questions, feedback, or other fee courses to add to this list, please feel free to suggest.

P. S. - If you are new to Microservice architecture and want to learn more about Microservice Architecture and solutions from scratch and looking for free resources then I highly recommend you to check out my post about 7 free Microservice courses. It contains free Udemy and Coursera and courses to learn Microservice architecture from scratch.  


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