Good questions in interview

🚀 During my recent backend interviews, I encountered some intriguing questions I came across. Here's some of them :

☕ Java + Spring Boot Q: How do you configure thread pools in Spring Boot to ensure efficient task execution under high load? ✅ Used ThreadPoolTaskExecutor to manage thread pools. ✅ Tuned corePoolSize and maxPoolSize based on whether tasks were CPU-bound or IO-bound. ✅ Adjusted keepAliveTime to recycle idle threads and reduce resource usage.

🔁 System Recovery with Backlog Processing Q: Your app handles 100 RPS and each task takes 100ms. A 5-minute outage means 30K pending tasks. Once restored, how do you process backlog + new incoming traffic in 5 minutes? ✅ Needed to support 200 RPS (100 backlog + 100 new). ✅ Designed executor with sufficient thread pool and queue capacity. ✅ Discussed scaling strategies: using separate executors to meet deadlines.

🧩 Singleton Injecting a Request-Scoped Bean Q: Why does injecting a @RequestScope bean into a singleton fail in Spring Boot? ✅ Request-scoped beans are instantiated per HTTP request, but singleton beans are created at app startup. 🔧 Fixes: Used ObjectFactory, Provider, or enabled proxying for lazy injection.

🔄 @RefreshScope in Spring Cloud Q: What is the role of @RefreshScope in dynamic configuration updates? ✅ Allows runtime refresh of beans when config values change in Spring Cloud Config. ✅ Works with Spring Cloud Bus to auto-propagate config changes—no app restart needed.

📩 Kafka Q: If a Kafka consumer fails to process a message, how do you retry it 5 times and eventually discard it? ✅ Built retry logic using separate retry topics with increasing backoff. ✅ After 5 attempts, failed messages are sent to a Dead Letter Queue (DLQ) for logging or manual handling. ✅ Ensures fault-tolerance without losing messages silently.

🧱 LLD – Canvas Fill Tool 🖌️ Design an MS Paint-like canvas fill backend: Q: Users can draw closed shapes. If they click inside, the entire shape should be filled. Clicking outside fills only that pixel. ✅ Tracked shape boundaries. ✅ Used Flood Fill Algorithm (DFS/BFS) to propagate fill from clicked pixel until boundaries are hit. ✅ Ensured scalability for large canvases and concurrent fill operations.

⚙️ Infra & System Design 📦 PostgreSQL Internals Q: How does PostgreSQL optimize query execution and data retrieval? ✅ Discussed B-Tree and GIN indexes.

🌐 Kubernetes Service Discovery Q: Pod IPs change frequently. How do services reliably discover and communicate with each other? ✅ Kubernetes assigns a stable ClusterIP to each service.

📬 Designing a Pub/Sub System Q: How do you implement a loosely-coupled publisher-subscriber architecture? ✅ Used Observer Pattern, Event Bus libraries, or message brokers like Kafka/RabbitMQ.