20 Command Query Responsibility Segregation Interview Questions and Answers

Command Query Responsibility Segregation (CQRS) is a software design pattern that separates the responsibility of issuing commands from the responsibility of querying data. This separation of concerns can improve the performance and scalability of a software system. If you are interviewing for a position that involves CQRS, you should be prepared to answer questions about this design pattern. In this article, we review some common CQRS interview questions and provide tips on how to answer them.

Command Query Responsibility Segregation Interview Questions and Answers

Here are 20 commonly asked Command Query Responsibility Segregation interview questions and answers to prepare you for your interview:

1. What is the Command Query Responsibility Segregation (CQRS) principle?

The CQRS principle states that you should keep your database queries separate from your database commands. This means that you should have two different sets of code, one for each type of operation. This can help to improve performance and reduce the risk of database corruption.

2. Can you give me an example of a command and a query in CQRS?

A command is an instruction to change something, while a query is a request for information. In CQRS, these two operations are separated so that they can be handled by different parts of the system. For example, a command might be used to update a database, while a query might be used to retrieve information from that database. This separation of concerns can help to improve performance and scalability.

3. How do commands differ from queries?

Commands are used to make changes to the data store, whereas queries are used to retrieve data from the store. Commands are typically used to insert, update, or delete data, while queries are used to select data.

4. Why is it critical to separate read-only actions from write-based actions when designing APIs or architectures?

By separating read-only actions from write-based actions, you can ensure that your API or architecture is more scalable. This is because read-only actions can be easily cached or replicated, while write-based actions will always require some level of communication with the central data store. By segregating these two types of actions, you can make your overall system more scalable and efficient.

5. What are some common use cases for CQRS?

CQRS is often used in cases where there is a need to have separate read and write models. This can be helpful in cases where reads are much more common than writes, or where reads and writes need to be handled by different parts of the system. CQRS can also be helpful in cases where it is important to maintain a high degree of consistency between the read and write models.

6. What are the advantages of using CQRS?

CQRS provides a number of advantages, including improved performance, scalability, and flexibility. Additionally, CQRS can help to simplify complex applications by separating the data model into two separate models: the command model and the query model. This separation of concerns can make it easier to understand and maintain complex applications.

7. When should I not use CQRS?

CQRS is not a silver bullet and should not be used in every situation. You should not use CQRS if you are not prepared to invest the time and effort required to properly implement it. Additionally, CQRS may not be appropriate if your system is not complex enough to warrant the added complexity that CQRS brings.

8. Is CQRS related to event sourcing? If yes, then how?

CQRS is not directly related to event sourcing, but the two concepts are often used together. Event sourcing is a way of storing data that keeps track of every change made to that data, in the form of an event. CQRS is a way of structuring an application so that there are separate components for handling commands (which change data) and queries (which retrieve data). When used together, event sourcing can provide the history of changes made to data, and CQRS can ensure that those changes are made in a safe and consistent manner.

9. In what way does CQRS help with scalability?

CQRS helps with scalability by allowing for the separation of reads and writes. This means that reads can be done from a separate database than writes, which can help to improve performance. Additionally, it can also help to reduce the amount of data that needs to be read from the database, as only the relevant data will be read.

10. Does CQRS require two databases for reads and writes? If yes, then why?

No, CQRS does not require two databases for reads and writes. However, it is often recommended to have two databases in order to keep the reads and writes separate. This separation can help to improve performance and prevent data corruption.

11. What’s your opinion on the disadvantages of using CQRS?

While there are many advantages to using CQRS, there are also a few disadvantages that should be considered. One disadvantage is that CQRS can add complexity to an application, since it requires two separate models for querying and updating data. Additionally, CQRS can make it more difficult to track changes made to data, since those changes will be spread across two different models.

12. Can you explain what the eventual consistency property means in context with CQRS?

Eventual consistency is a property of some distributed systems where the system will eventually converge on a consistent state, even if individual nodes in the system are not perfectly synchronized. In a CQRS system, this means that eventually the read and write models will contain the same data, even if they are not updated at the same time.

13. Does CQRS work well with microservices? If yes, then why?

CQRS works well with microservices because each microservice can be responsible for a specific task or command, and then can be queried for information about that task. This segregation of responsibility can help to keep microservices small and focused, which can make them more manageable and easier to understand.

14. What are the main goals of CQRS?

The main goals of CQRS are to improve performance and scalability by separating reads and writes, and to improve maintainability by decoupling the data model from the business logic.

15. What is the purpose of aggregates in CQRS?

The purpose of aggregates in CQRS is to provide a way to group related data together so that it can be easily retrieved and updated as a unit. This is especially important in a distributed system, where data may be spread across multiple different databases. By grouping related data together into an aggregate, you can make sure that all of the data remains consistent and can be easily updated.

16. Can you explain what Domain Eventing is?

Domain Eventing is a technique used in Domain-Driven Design that helps to keep the domain model consistent by tracking changes to the domain objects and then propagating those changes throughout the system. This allows for a more reactive and responsive system, as changes can be made and then immediately reflected in the UI or other parts of the system.

17. Do you think CQRS will ever become obsolete?

I don’t think so. CQRS provides a lot of benefits in terms of performance and scalability, and I think it will continue to be a popular approach for building software applications.

18. What type of applications benefit most from the use of CQRS?

CQRS is most commonly used in applications that require high performance and scalability, such as e-commerce or social media applications. These types of applications often have a large number of users making concurrent requests, which can put a strain on the traditional request-response model. CQRS allows for a more efficient use of resources by segregating the responsibility for handling requests and responses, which can improve performance and scalability.

19. What are some examples of real-world apps that used CQRS successfully?

There are many examples of successful CQRS implementations, but some notable ones include Evernote, Trello, and Google Calendar. CQRS can be used in a variety of different ways, so it is really up to the developers to decide how best to utilize it for their particular application.

20. What is the difference between an API and a service bus in context with CQRS?

An API is a set of programming instructions and standards for accessing a web-based software application. A service bus is a software architecture pattern that allows communication between software applications or components to be decoupled for improved scalability and flexibility. In the context of CQRS, a service bus can be used to provide a communication layer between the command and query sides of the application, allowing them to be scaled independently.