Our ECS service started, registered, failed health checks and got replaced, over and over. Two separate problems stacked: the container moved to port 8080 while the target group still probed 80, and a slow JVM start ran out the clock with no grace period.
During a flash sale our checkout API on ECS Fargate slowed to multi-second responses, yet the service stayed at six tasks. Target tracking on average CPU at 70% never fired: tasks were blocked on I/O, not CPU, and the max capacity was already reached on one service. How we chose better scaling signals.
Our ECS Fargate workers scaled on SQS queue depth. After each burst, scale-in stopped tasks in the middle of 10-minute PDF jobs; messages reappeared and the jobs ran again, sending customers duplicate documents. How we fixed it with task scale-in protection, SIGTERM handling and a visibility timeout that fits the work.