Interview questions

Backend Developer Interview Questions

Assess Backend Developer candidates with interview questions covering API design, database architecture, system scalability, security, and server-side best.

What's here

11 Backend Developer interview questions, grouped into 3 areas: api design & architecture questions, database design & data management questions and scalability & system design questions. Each one comes with why it is worth asking and what a strong answer contains, so the same question can be scored the same way by different interviewers.

Include a live coding exercise focused on API design or data modeling rather than algorithm puzzles. Practical backend skills are better assessed through realistic scenarios.
Ask candidates to review a code snippet with security vulnerabilities and identify the issues. This reveals security awareness in a practical context.
Test their understanding of operational concerns by discussing monitoring, logging, and alerting. Backend developers own production reliability for their services.

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Hiring a Backend Developer requires evaluating expertise in server-side architecture, database design, API development, and system scalability. The best candidates write reliable, secure, and performant code while thinking holistically about system design and operational concerns. Use these questions to assess architectural thinking, database proficiency, security awareness, and the ability to build systems that scale gracefully under increasing load.

API Design & Architecture Questions

These questions evaluate how candidates design and build robust server-side APIs and services.

1
How do you design a RESTful API for a new feature? Walk me through your process from requirements to implementation.
Why ask it
Tests API design methodology and ability to think about developer experience.
What to look for
Starts with understanding use cases, designs resource-oriented endpoints with proper HTTP methods, considers versioning strategy, implements consistent error handling, documents the API, and thinks about backward compatibility.
2
Compare REST, GraphQL, and gRPC. When would you choose each, and what are the tradeoffs?
Why ask it
Evaluates breadth of knowledge and ability to select the right communication protocol for different scenarios.
What to look for
Understands that REST is simple and widely adopted, GraphQL reduces over-fetching for complex frontends, and gRPC is efficient for service-to-service communication. Should discuss caching, tooling ecosystem, and team familiarity as selection factors.
3
How do you handle authentication and authorization in a backend system? Describe the patterns you have implemented.
Why ask it
Tests security-critical knowledge that every backend developer needs.
What to look for
Understands the difference between authentication and authorization, has implemented JWT or OAuth2 flows, uses role-based or attribute-based access control, and considers security best practices like token expiry, refresh mechanisms, and protection against common attacks.
4
Describe your approach to error handling in a backend application. How do you design for both expected and unexpected failures?
Why ask it
Evaluates defensive programming skills and operational awareness.
What to look for
Distinguishes between client errors and server errors, implements consistent error response formats, uses structured logging, handles edge cases gracefully, and designs retry and circuit breaker patterns for external dependencies.

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Database Design & Data Management Questions

These questions assess database architecture skills and data modeling proficiency.

5
How do you design a database schema for a new application? What factors influence your decisions about normalization, indexing, and data types?
Why ask it
Tests fundamental database design skills.
What to look for
Considers access patterns before designing schema, understands normalization tradeoffs, selects appropriate data types, plans indexes based on query patterns, and thinks about data growth and migration strategies.
6
Explain the CAP theorem and how it influences your database technology choices for different use cases.
Why ask it
Evaluates distributed systems knowledge and technology selection judgment.
What to look for
Accurately explains consistency, availability, and partition tolerance tradeoffs. Can give practical examples of when to choose CP systems (financial transactions) versus AP systems (social media feeds) and understands that the tradeoffs are nuanced.
7
A database query that used to run in milliseconds is now taking several seconds. How do you diagnose and resolve the performance issue?
Why ask it
Tests practical database performance troubleshooting skills.
What to look for
Uses EXPLAIN or query plan analysis, checks for missing or unused indexes, identifies table growth or data distribution changes, considers query optimization techniques, evaluates caching opportunities, and thinks about read replicas or partitioning for longer-term solutions.

Scalability & System Design Questions

These questions evaluate the ability to design systems that handle growing load and complexity.

8
How would you design a system to handle a sudden ten-times increase in traffic? What architectural patterns would you employ?
Why ask it
Tests scalability thinking and knowledge of horizontal scaling patterns.
What to look for
Discusses horizontal scaling, load balancing, caching layers (CDN, application cache, database cache), asynchronous processing with message queues, database read replicas, and auto-scaling. Should mention monitoring and alerting to detect scaling issues.
9
Explain the event-driven architecture pattern. When is it appropriate and what challenges does it introduce?
Why ask it
Evaluates knowledge of modern architectural patterns and their tradeoffs.
What to look for
Understands event sourcing, message brokers (Kafka, RabbitMQ), eventual consistency, and the benefits of decoupling services. Should also discuss challenges like event ordering, idempotency, debugging complexity, and schema evolution.
10
How do you approach implementing caching in a backend system? What strategies do you use and what pitfalls do you watch for?
Why ask it
Tests understanding of a critical performance optimization technique and its complexities.
What to look for
Discusses cache invalidation strategies, TTL settings, cache-aside vs. write-through patterns, cache stampede prevention, and the importance of monitoring cache hit rates. Should acknowledge that caching introduces consistency challenges.
11
Describe how you would break down a monolithic application into microservices. What criteria drive your service boundaries?
Why ask it
Tests architectural decomposition judgment and practical migration skills.
What to look for
Uses domain-driven design to identify bounded contexts, applies the strangler fig pattern for incremental migration, addresses data ownership and consistency, establishes service communication patterns, and considers operational complexity tradeoffs.

Running the interview well

1Include a live coding exercise focused on API design or data modeling rather than algorithm puzzles. Practical backend skills are better assessed through realistic scenarios.
2Ask candidates to review a code snippet with security vulnerabilities and identify the issues. This reveals security awareness in a practical context.
3Test their understanding of operational concerns by discussing monitoring, logging, and alerting. Backend developers own production reliability for their services.
4Evaluate how they approach testing. Strong backend developers have opinions about test strategy, including unit tests, integration tests, and contract tests.
5Ask about their experience with production incidents. How they debug and resolve issues under pressure reveals both technical depth and composure.

Common questions

Should I test language-specific knowledge or general backend concepts?

Prioritize general backend concepts like API design, database fundamentals, security, and scalability patterns. These transfer across languages. Test language-specific knowledge only if your stack requires deep expertise in a particular language. A strong backend developer with solid fundamentals can learn a new language efficiently.

How do I evaluate system design skills for mid-level backend candidates?

Start with smaller-scope design problems rather than full system designs. Ask candidates to design an API for a specific feature, model a database schema, or propose a caching strategy. Mid-level candidates should demonstrate sound principles even if they lack experience designing at massive scale.

What security knowledge should a Backend Developer demonstrate?

At minimum, they should understand input validation, SQL injection prevention, authentication and authorization patterns, HTTPS and data encryption, secure secret management, and OWASP Top Ten vulnerabilities. Security should be integrated into their development process, not treated as a separate phase.

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