Key Skills for Backend Developer Interviews

I interview backend engineers regularly … and this is my approach. What I do not focus on: - Libraries. - Frameworks. - Programming language. I focus on core backend development concepts: - System design. - HTTP concepts (REST, GraphQL, etc.) - API versioning and lifecycle management. - Authentication, authorization, and security. - Multithreading, concurrency, and scalability. - Database modeling and query optimization. - Logging and monitoring (key for maintainable systems). Bonus points: - Distributed systems. - Cloud technologies. - CI/CD pipelines. Why? Backend developers with strong fundamentals can quickly pick up new tools or technologies while contributing meaningfully from day one.

If I were to prepare for a Backend Engineer interview, here are the topics I would go over: (Working on enterprise products for over 5 years in Microsoft, I’ve spent significant time on backend, cloud infra, security, and live site. Sharing top topics from interviews — in priority order.) Master Your Data Layer: •⁠ ⁠SQL vs NoSQL •⁠ ⁠Indexing: B+ Trees, LSM Trees Scale Smarter: •⁠ ⁠Redis, Memcached •⁠ ⁠CDN-based caching •⁠ ⁠Autoscaling (AWS, GCP, Azure) Distribute the Load: •⁠ ⁠Techniques: Round-robin, Least Connections •⁠ ⁠Horizontal vs Vertical Scaling •⁠ ⁠Load balancers: HAProxy, Envoy Speak the Language of the Web: •⁠ ⁠TCP vs UDP •⁠ ⁠HTTP/1.1, HTTP/2, HTTP/3 •⁠ ⁠WebSockets, gRPC Build with correct infrastructure: •⁠ ⁠NGINX, Apache •⁠ ⁠Forward Proxy vs Reverse Proxy •⁠ ⁠CDN, API Gateway Message like a pro: •⁠ ⁠Schema: JSON, XML •⁠ ⁠Kafka, RabbitMQ •⁠ ⁠Pub/Sub model Secure by design: •⁠ ⁠TLS/SSL •⁠ ⁠XSS, SQL Injection •⁠ ⁠DDoS prevention Know your system’s pulse: •⁠ ⁠Logs: ELK, Splunk •⁠ ⁠Metrics: Prometheus, Grafana •⁠ ⁠Tracing: Jaeger, OpenTelemetry Tools: •⁠ ⁠Express, Django, Spring Boot •⁠ ⁠Docker, Kubernetes, Postman

You can memorize patterns and still build systems that fall apart. Because real system design comes in levels. ⬆️level 0 Fundamentals: • Clients send requests • Servers handle logic • Databases store data You learn HTTP methods, status codes, and what a REST API is. You pick between SQL and NoSQL without really knowing why. You're not a backend dev until you've panic-fixed a 500 error in production caused by a missing null check. ⬆️level 1 Master the building blocks: • Load balancers for traffic distribution • Caches (Redis, Memcached) to reduce DB pressure • Background workers for async jobs • Queues (RabbitMQ, SQS, Kafka) for decoupling • Relational vs Document DBs; use cases, not just syntax differences You realize reads and writes scale differently. You learn that consistency, availability, and partition tolerance don't always play nice. You stop asking "SQL or NoSQL?" and start asking “What are the access patterns?” ⬆️level 2 Architect for complexity: • Separate read and write paths • Use circuit breakers, retries, and timeouts • Add rate limiting and backpressure to avoid overload • Design idempotent endpoints You start drawing sequence diagrams before writing code. You stop thinking in services and start thinking in boundaries. ⬆️level 3 Design for reliability and observability: • Add structured logging, metrics, and traces • Implement health checks, dashboards, and alerts • Use SLOs to define what “good enough” means • Write chaos tests to simulate failure • Add correlation IDs to trace issues across services At this level, you care more about mean time to recovery than mean time between failures. You understand that invisible systems are the most dangerous ones. ⬆️level 4 Design for scale and evolution: • Break monoliths into services only when needed • Use event-driven patterns to reduce coupling • Support versioning in APIs and messages • Separate compute from storage • Think in terms of contracts, not code • Handle partial failures in distributed systems You design for change, not perfection. You embrace trade-offs. You know when to keep it simple and when to go all in. What’s one system design lesson you learned the hard way?

90% of backend engineering essentials are in these 10 topics... If you want to become a better backend engineer, focus your time and effort in learning on these areas: 1. Data Structures and Algorithms - Master key concepts like hash maps, trees, graphs, and linked lists. - Focus on sorting algorithms (like quicksort and mergesort) and search algorithms (binary search, DFS, BFS). - Understand time complexity (Big-O) to evaluate the efficiency of your code. 2. SQL and Normalization - Learn to write efficient SQL queries and understand joins, indexes, and views. - Focus on normalization to eliminate redundancy and optimize database performance. 3. SOLID Principles - These five principles help you build maintainable and scalable code. - Key examples include Single Responsibility (one class for one purpose) and Open-Closed Principle (code that can adapt without modification). 4. Authentication and Authorization - Understand OAuth2, JWT, session management, and role-based access control (RBAC). - Know how to implement secure login systems and protect APIs. 5. API Development - Focus on RESTful and GraphQL APIs. - Understand how to properly structure endpoints, handle HTTP methods, and ensure error handling. 6. Design Patterns - Get familiar with patterns like Singleton, Factory, Observer, and Repository. - These patterns help in writing reusable and modular code. 7. Testing and Debugging - Learn how to write unit tests, integration tests, and end-to-end tests. - Master debugging tools to quickly identify and fix bugs in production. 8. Server, Network, and Hosting Environments - Understand DNS, CDNs, HTTPS, firewalls, and how servers operate. - Get hands-on experience with cloud platforms like AWS, Azure, or GCP. 9. Load Balancing and Scalability - Learn how horizontal and vertical scaling works. - Understand load balancing algorithms to evenly distribute traffic (e.g., round-robin). 10. Caching and Optimization - Use Redis, Memcached, or in-memory caching to reduce load. - Optimize database queries, reduce API calls, and improve response times. Focusing on these areas will give you the foundation and tools to excel as a backend engineer and build systems that are scalable, secure, and maintainable.