35 Interview questions to ask while hiring a backend developer 

A reliable and scalable program requires a suitable backend developer who must be hired. According to a recent survey by Stack Overflow, 57% of developers identify as backend developers, making it a highly competitive field.

With rapid technological advancements, HR professionals and CXOs must ensure they ask the right questions to gauge a candidate’s technical proficiency, problem-solving skills, and cultural fit. Effective interview questions help identify top talent and reduce turnover rates, which can cost companies up to 33% of an employee’s annual salary.

This guide will provide key questions to ask during the interview process to ensure your next hire is well-equipped to drive your organization’s backend development projects to success.

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Why use skills assessments for assessing backend developer candidates?

When assessing applicants for backend development positions, skill evaluations are crucial. They provide objective data on a candidate’s coding proficiency, problem-solving abilities, and technical knowledge, ensuring that only the most qualified individuals advance in the hiring process. Testlify offers comprehensive assessments tailored for backend developers, covering a range of skills from coding to database management. Incorporating these assessments into your hiring strategy can reduce the risk of bad hires and improve team efficiency. Our platform’s tests help you identify candidates who not only have the necessary technical skills but also fit well within your company’s culture, ultimately contributing to the overall success of your projects.

When should you ask these questions in the hiring process? 

The ideal approach to hiring a backend developer begins with inviting applicants to complete a skills assessment. This first stage ensures that only eligible candidates proceed through the process by screening out applicants who do not meet the necessary technical proficiency. Utilizing platforms like Testlify can streamline this stage, providing objective data on each candidate’s coding skills and technical knowledge.

Once the skills assessment is complete, you can use targeted interview questions to delve deeper into the candidates’ problem-solving abilities, experience, and cultural fit. Conduct these interviews in the subsequent stages of the hiring process, focusing on their ability to handle real-world challenges and their alignment with your team’s dynamics. This structured approach not only saves time but also enhances the quality of your hires, ultimately contributing to the success of your projects.

Keep Learning: How do you hire the best? Achieve pinpoint accuracy in hiring with our Top blog on how to screen candidates for backend developer?

General backend developer interview questions to ask applicants

General backend developer interview questions focus on assessing candidates’ understanding of server-side programming, database management, and API development. Key questions might explore their experience with languages like Java, Python, or Node.js, knowledge of database systems such as SQL and NoSQL, and familiarity with RESTful and GraphQL APIs. Additionally, questions may delve into their problem-solving skills, ability to optimize backend performance, and experience with version control systems like Git. These questions help determine the candidate’s technical proficiency, problem-solving abilities, and readiness to handle backend development tasks effectively.

1. How do you handle concurrent requests in Python?

Look for: Understanding of concurrency, Python’s limitations with threads, and practical experience with async programming.

What to Expect: The candidate should explain Python’s Global Interpreter Lock (GIL), threading, and asynchronous processing with libraries like asyncio or frameworks like Tornado and FastAPI.

2. How does Node.js handle asynchronous operations?

Look for: Clear understanding of the non-blocking nature of Node.js and how asynchronous operations are managed efficiently.

What to Expect: The candidate should discuss the event loop, callbacks, promises, and async/await syntax in Node.js.

3. Can you explain garbage collection in Java?

Look for: Deep understanding of Java’s memory management and practical experience with tuning garbage collection.

What to Expect: The candidate should explain Java’s automatic memory management, the garbage collection process, and different GC algorithms like Serial, Parallel, CMS, and G1.

4. How do you manage sessions in PHP?

Look for: Practical knowledge of session handling, security measures like session hijacking prevention, and managing session data in a scalable way.

What to Expect: The candidate should explain PHP session management using $_SESSION superglobal, session_start(), and session configuration settings.

5. What are the common pitfalls of using Active Record in Ruby on Rails?

Look for: Awareness of Active Record’s limitations and strategies for mitigating them, such as using eager loading and service objects.

What to Expect: The candidate should discuss issues like N+1 queries, complex callbacks, and object-relational impedance mismatch.

6. Describe how you would implement a RESTful API in Python.

Look for: Experience with API design, understanding of RESTful principles, and practical coding examples.

What to Expect: The candidate should outline steps including setting up a framework (e.g., Flask, Django), defining endpoints, handling requests/responses, and ensuring REST principles.

7. How does Node.js handle scaling for high traffic applications?

Look for: Understanding of scaling strategies and experience with real-world implementation for handling high traffic.

What to Expect: The candidate should explain clustering, load balancing, and using external tools like Nginx or PM2.

8. Explain the concept of multithreading in Java.

Look for: Deep understanding of multithreading, concurrency issues, and experience with concurrent programming in Java.

What to Expect: The candidate should explain creating threads, the Runnable interface, thread lifecycle, and synchronization.

9. How do you secure a PHP application against common vulnerabilities?

Look for: Strong knowledge of web security practices and practical experience in securing PHP applications.

What to Expect: The candidate should discuss SQL injection, cross-site scripting (XSS), cross-site request forgery (CSRF), and using functions like ‘htmlspecialchars’ and ‘mysqli_real_escape_string’.

10. How do you manage dependencies in a Ruby project?

Look for: Proficiency with Bundler and experience managing project dependencies effectively.

What to Expect: The candidate should explain the use of Bundler, Gemfile, and managing different gem versions.

11. Describe how you would optimize a slow SQL query in Python.

Look for: Analytical skills in identifying performance issues and practical experience with query optimization.

What to Expect: The candidate should discuss identifying bottlenecks, using indexes, optimizing queries, and using ORM features.

12. How does Node.js handle file I/O operations?

Look for: Practical knowledge of file handling in Node.js and understanding of asynchronous operations.

What to Expect: The candidate should explain the asynchronous nature of file I/O, using the fs module, and handling callbacks or promises.

13. Explain the differences between HTTP and HTTPS.

Look for: Strong understanding of web protocols and security implications.

What to Expect: The candidate should explain SSL/TLS, encryption, and how HTTPS ensures secure communication.

14. What are the benefits of using microservices architecture?

Look for: Understanding of microservices benefits and challenges, and experience with implementation.

What to Expect: The candidate should discuss scalability, independent deployment, fault isolation, and technology diversity.

15. How do you handle background jobs in Python?

Look for: Practical experience with background job processing and familiarity with relevant tools.

What to Expect: The candidate should discuss using libraries like Celery, RQ, or integrating with external services like AWS SQS.

16. Explain the event-driven architecture in Node.js.

Look for: Clear understanding of event-driven programming and experience with Node.js events.

What to Expect: The candidate should discuss event emitters, the role of the event loop, and practical use cases.

17. How do you manage database migrations in a Java application?

Look for: Practical experience with database migrations and understanding of best practices.

What to Expect: The candidate should explain using tools like Flyway or Liquibase, version control for database schema, and handling schema changes.

18. Describe the lifecycle of a PHP request.

Look for: Deep understanding of PHP execution model and request handling.

What to Expect: The candidate should explain the process from request initiation to server processing, script execution, and response generation.

19. How do you test a Ruby on Rails application?

Look for: Proficiency with testing tools and methodologies for ensuring code quality.

What to Expect: The candidate should discuss using testing frameworks like RSpec or Minitest, and writing unit, integration, and functional tests.

20. Explain the concept of middleware in Express.js.

Look for: Understanding of Express.js middleware and practical examples of implementation.

What to Expect: The candidate should explain middleware functions, their execution order, and use cases for authentication, logging, and error handling.

21. How do you handle exceptions in a Python web application?

Look for: Strong error handling practices and experience with debugging and logging in web applications.

What to Expect: The candidate should discuss using try-except blocks, custom exception classes, and logging errors.

22. What are the advantages of using Java over other backend languages?

Look for: Balanced view of Java’s strengths and practical experience with its ecosystem.

What to Expect: The candidate should discuss performance, robust libraries, strong typing, and enterprise support.

23. How do you perform input validation in PHP?

Look for: Practical knowledge of input validation techniques and security best practices.

What to Expect: The candidate should explain using built-in functions like filter_var(), regular expressions, and custom validation functions.

24. Explain the Model-View-Controller (MVC) architecture in Ruby on Rails.

Look for: Clear understanding of MVC principles and practical experience with Rails.

What to Expect: The candidate should describe how MVC separates concerns, how each component interacts, and the flow of data.

25. How do you ensure the scalability and performance of a backend system?

Look for: Holistic understanding of system performance and scalability, and experience with implementing these strategies.

What to Expect: The candidate should discuss strategies like load balancing, caching, database optimization, and using efficient algorithms.

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Code-based backend developer interview questions to ask applicants

Code-based interview questions are essential for evaluating a candidate’s practical coding skills and problem-solving abilities. These questions help assess their proficiency in writing efficient, clean, and functional code. You can gauge their familiarity with programming concepts, algorithms, and database management by asking candidates to complete tasks such as reversing a string, writing SQL queries, or merging arrays. These exercises are designed to be completed within a few minutes, providing a quick yet effective way to determine a candidate’s technical capabilities and readiness for backend development roles.

26. Write a Python function to reverse a string.

Look for: Understanding of string manipulation, the use of Python slicing, and code simplicity.

def reverse_string(s):
    return s[::-1]

# Example usage:
# reverse_string("hello") -> "olleh"

27. Write a SQL query to find the second highest salary from an employee’s table.

Look for: Proficiency with SQL subqueries and aggregate functions, and understanding of database querying.

SELECT MAX(salary) AS second_highest_salary
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);

28. Write a Node.js function to read a JSON file and return its contents.

Look for: Familiarity with Node.js filesystem module, asynchronous programming, and JSON parsing.

const fs = require('fs');

function readJSONFile(filePath) {
    return new Promise((resolve, reject) => {
        fs.readFile(filePath, 'utf8', (err, data) => {
            if (err) {
                reject(err);
            } else {
                resolve(JSON.parse(data));
            }
        });
    });
}

// Example usage:
// readJSONFile('data.json').then(console.log).catch(console.error);

29. Write a Java method to check if a given number is a prime number.

Look for: Knowledge of basic algorithms, loop control structures, and mathematical logic in Java.

public boolean isPrime(int num) {
    if (num <= 1) return false;
    for (int i = 2; i <= Math.sqrt(num); i++) {
        if (num % i == 0) return false;
    }
    return true;
}

// Example usage:
// isPrime(11) -> true
// isPrime(4) -> false

30. Write a Ruby method to convert an array of strings to a hash, where the keys are the strings and the values are their lengths.

Look for: Proficiency with Ruby’s enumerable methods, understanding of hash creation, and code readability.

def array_to_hash(arr)
  arr.each_with_object({}) do |str, hash|
    hash[str] = str.length
  end
end

# Example usage:
# array_to_hash(["apple", "banana", "cherry"]) -> {"apple" => 5, "banana" => 6, "cherry" => 6}

Interview questions to gauge a candidate’s experience level

31. Can you describe a challenging project you worked on and how you overcame the obstacles?

32. How do you prioritize your tasks when working on multiple projects with tight deadlines?

33. Tell me about a time when you had to collaborate with cross-functional teams. How did you ensure effective communication and coordination?

34. How do you handle feedback on your code, and can you give an example of how you used feedback to improve your work?

35. Describe a situation where you identified a performance issue in an application. How did you diagnose and resolve it?

Key takeaways

When interviewing backend developers, assessing technical and soft skills is crucial. Technical questions should cover topics like RESTful APIs, database migrations, and coding tasks such as reversing a string or writing SQL queries. These help gauge a candidate’s coding proficiency and problem-solving abilities. Utilizing platforms like Testlify.com for skills assessments ensures an objective evaluation of technical competencies.

Equally important are questions that assess soft skills and past experiences. Inquiring about challenging projects, task prioritization, cross-functional collaboration, feedback handling, and performance issue resolution provides insight into their working style and team dynamics. Combining technical and soft skill assessments offers a comprehensive evaluation, helping you identify technically adept candidates and effective team players.

Explore More: Explore more strategies for successful hiring with our Backend Java Developer test.