Top Backend Developer Interview Questions & Answers (2026)

💬 Can you explain the difference between REST and GraphQL?

Why they ask: To assess your understanding of API design paradigms and your ability to choose the right tool for a given problem.

Sample answer: REST is an architectural style that uses standard HTTP methods and exposes multiple endpoints for different resources, which can sometimes lead to over-fetching or under-fetching of data. GraphQL, on the other hand, is a query language that exposes a single endpoint, allowing clients to request exactly the data they need. In my previous role, we migrated from REST to GraphQL to reduce the payload size for our mobile app, resulting in a 30% improvement in load times.

💬 How do you handle database migrations in a production environment with zero downtime?

Why they ask: To evaluate your experience with live systems and your strategies for minimizing disruption during updates.

Sample answer: I typically approach zero-downtime migrations by decoupling the database schema changes from the application code deployment. First, I apply non-breaking database changes, such as adding new columns or tables. Then, I deploy the application code that writes to both the old and new schema. Finally, after backfilling existing data, I deploy code that only uses the new schema and remove the old columns. This phased approach ensures the application remains available throughout the process.

💬 Describe a time you had to optimize a slow-performing API endpoint.

Why they ask: To gauge your problem-solving skills and your methodology for identifying and resolving performance bottlenecks.

Sample answer: We had an endpoint generating complex reports that took over 10 seconds to respond. I started by profiling the database queries and found that we were missing several crucial indexes and suffering from N+1 query problems. After optimizing the SQL queries, adding the appropriate indexes, and implementing a Redis caching layer for frequently accessed, rarely changing data, we reduced the response time to under 200 milliseconds.

💬 What is your approach to writing unit and integration tests for backend services?

Why they ask: To check your commitment to code quality and your understanding of testing methodologies.

Sample answer: I believe in a test-driven approach where unit tests cover individual functions and edge cases, mocking out external dependencies to ensure fast execution. For integration tests, I test the interaction between components, such as the API layer and the database, often using containerized databases like Docker to ensure a consistent testing environment. In my last project, maintaining a minimum of 80% test coverage helped us catch several critical bugs before they reached production.

💬 How do you ensure the security of the APIs you develop?

Why they ask: To ensure you understand common security threats and how to implement best practices to protect sensitive data.

Sample answer: Security is a multi-layered approach. I always ensure APIs are served over HTTPS to encrypt data in transit and implement robust authentication and authorization, typically using OAuth2 or JWT. Additionally, I protect against common vulnerabilities like SQL injection and cross-site scripting by using prepared statements and validating and sanitizing all user input. Rate limiting is also crucial to mitigate DDoS attacks and brute-force attempts.

🧠 Tell me about a time you disagreed with a senior engineer on an architectural decision.

Tip: Focus on your communication skills, how you presented data to back up your viewpoint, and your willingness to compromise for the team's benefit.

🧠 Describe a situation where a deployment caused a critical issue in production. How did you handle it?

Tip: Emphasize your ability to remain calm under pressure, your troubleshooting methodology, and the post-mortem steps taken to prevent recurrence.

🧠 How do you balance the need to deliver features quickly with the need to write clean, maintainable code?

Tip: Discuss your understanding of technical debt and how you communicate trade-offs with product managers to find the right balance.

🧠 Tell me about a time you had to learn a new technology or framework on the fly to complete a project.

Tip: Highlight your adaptability, your learning process, and how quickly you were able to contribute using the new tool.

🧠 Describe a project where you had to collaborate closely with frontend developers.

Tip: Showcase your empathy for frontend challenges, how you established clear API contracts, and your communication style during integration.

🔧 Explain the concept of eventual consistency and when you would use it.

Tip: Discuss distributed systems, the CAP theorem, and scenarios where high availability is more critical than immediate consistency.

🔧 How does a message broker like RabbitMQ or Kafka work, and what problems do they solve?

Tip: Explain asynchronous processing, decoupling of services, and handling high-throughput data streams.

🔧 What are the differences between horizontal and vertical scaling?

Tip: Compare adding more power to a single machine versus adding more machines to a pool, including the architectural implications of each.

🔧 Explain how a reverse proxy works and what benefits it provides.

Tip: Mention load balancing, SSL termination, caching, and adding an extra layer of security to backend servers.

🔧 What is the N+1 query problem, and how can it be avoided in an ORM?

Tip: Describe how lazy loading causes multiple database queries and how using eager loading or batching can resolve the issue.