Cracking the Redis Interview: Tips and Must-Know Questions

Cracking the Redis Interview: Tips and Must-Know Questions Work through these 12 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.

1. Your Spring Boot app works locally but fails after deployment. What are the first 3 things you check?

   First: confirm `spring.profiles.active` and required env vars exist in prod (`kubectl describe pod`, deployment manifest). Second: read startup logs and `/actuator/health` —...

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2. You deployed a new version but users still hit the old behavior. Why?

   Check K8s rollout status — old ReplicaSet still receiving traffic. Bust cache on static assets; API GET with `Cache-Control`. Confirm image tag in running pod matches CI build....

   Full answer

3. How do you implement rate limiting for APIs?

   Apply at API Gateway; in-app use Redis-backed limiter for cluster-wide counts. ```java @RateLimiter(name = "apiLimiter") @GetMapping("/users") public List<User> users() {...

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4. What is the difference between ArrayList and LinkedList?

   ArrayList: dynamic array, great for get(i) and iteration, amortized append. LinkedList: node chain, costly random access, only wins for frequent insert/remove in middle with...

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5. What is Spring Boot Docker Compose dev services support?

   Add compose.yaml with postgres and redis services. Include spring-boot-docker-compose on runtimeOnly. Boot starts Postgres, sets JDBC URL — zero local install. CI uses...

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6. How would you design a rate-limited REST API?

   API gateway or filter applies token bucket per API key — Redis stores tokens with TTL. Exceeding limit returns 429 + Retry-After. Different tiers per plan. Log rejections for...

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7. What is Hibernate second-level cache and when enable it?

   Enable for ProductCategory, Currency — READ_ONLY, rarely changed. spring.jpa.properties.hibernate.cache.use_second_level_cache=true with Caffeine region factory. Avoid on...

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8. What are idempotency keys and how implement them in REST APIs?

   Client sends Idempotency-Key: uuid on POST /payments. Server Redis SETNX key, process, store response JSON. Retry with same key returns 201 + original body without double...

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9. Your application memory usage keeps increasing over time. What do you suspect?

   Suspect memory leak if heap used grows after GC without traffic drop. Capture heap dump on OOM (`-XX:+HeapDumpOnOutOfMemoryError`), compare dominator tree. Common in Spring:...

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10. What is an ExecutorService and why use it?

    Instead of new Thread per task, submit Runnable to fixed pool sized to CPU/IO profile. Handle Future results and exceptions. On shutdown hook call shutdown() and...

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11. What are weak references and WeakHashMap use cases?

    Attach metadata to objects without extending their lifetime — canonical mapping while object lives. When key GCs, entry drops. Do not rely on timing. For API response caching...

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12. Redis caching patterns: cache-aside vs write-through?

    @Cacheable("users") on findById — Redis cache-aside via Spring Data Redis. Update user: @CacheEvict. TTL 15m + jitter. Hot product page: lock on miss with Redisson. Do not...

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