My store throws a 503 right after deploy, how do I fix it?

A 503 immediately after deploy is nearly always one of two things: the store is still in maintenance mode (var/.maintenance.flag wasn’t removed), or the generated/ directory is empty or stale so the autoloader throws Class … does not exist. The fix is a clean setup:di:compile followed by setup:static-content:deploy, then clearing the maintenance flag. We verify the generated classes resolve and watch the logs go quiet before we call it done.