The Cache That Was Lying to Us
The first time we ran two replicas of the same Symfony service behind a load balancer, everything looked fine. Health checks passed. Traffic split cleanly. Response times were good. Then someone noticed the rate limiter was acting strange. Hit the API five times, get blocked. Hit it five more times on the next request, get through. Depending on which pod answered, you were a different person. That was the cache talking. One config line, replicated across thirteen services, was blocking horizontal scaling entirely. ...