Design load balancing, caching, and idempotent APIs

You are designing a backend service that will be deployed across multiple instances behind a load balancer. Answer the following: ## 1) Load balancing - What is a load balancer and why is it used? - Compare common routing policies: - round-robin / weighted round-robin - least connections - consistent hashing - latency/health-based routing - sticky sessions (session affinity) - Discuss L4 vs L7 load balancing and when you’d choose each. ## 2) Caching - Where can caching be applied (client/CDN, edge, reverse proxy, application, database)? - Explain cache read/write patterns (cache-aside, read-through, write-through, write-back). - How would you handle invalidation, TTLs, and staleness? - Describe how to prevent or mitigate cache stampede/thundering herd. ## 3) Idempotency support Design an API/system that supports **idempotent requests** (e.g., payment charge, order creation). - Define idempotency and why it matters. - Propose an idempotency-key design (where the key comes from, how long it lives). - Explain how the service stores and checks idempotency state. - Address concurrency (same key arrives simultaneously), retries, and partial failures. - Mention how idempotency interacts with load balancing and distributed deployments.