Phusion's Ruby application server used in production at Chattermill. Spawns and manages worker processes; exposes a 'passenger queue length' metric (visualized in Grafana) that grows rapidly when workers are saturated. Has a deployment-error / 'reses' mode that stops starting new processes when startup repeatedly times out. In the third war story, Passenger's log showing 'startup timeout' on spawn was the key clue that the problem was in worker boot (bootsnap cache size), not in request handling.