Good overview of reverse proxy, LB, and caching in one place. In production the failure mode we keep seeing is treating proxy_next_upstream as a free retry: without limiting non-idempotent methods you can double-POST on a slow 504. Pair next_upstream with a tight proxy_read_timeout, only retry safe methods, and keep max_fails/fail_timeout honest so a half-dead peer leaves the pool instead of collecting every retry.

For caching, start with a tiny proxy_cache zone and explicit bypass on anything that sets cookies; microcaching HTML for one to two seconds often buys more than a large stale zone that serves yesterday's auth header. After a rolling restart, watch 502 rate and upstream response time together — spikes usually mean health checks still pointed at draining workers while the cache kept sending traffic their way.