18/30 Days System Design Questions!

Your Redis cache just expired on a key that 8,000 users hit every second. Every single one of those requests is now flying straight at your database. This is the thundering herd. You didn't have a traffic problem — you had a cache problem. Now you have both. Here's the setup: Service → Node.js API, 8,000 req/sec on the /feed endpoint Cache → Redis, TTL = 60s on the feed key DB → Postgres, comfortable at ~200 req/sec sustained What happened → TTL expired at peak traffic, all 8,000 req/sec hit Pos