Delayed is a multi-threaded, SQL-driven ActiveJob backend used at Betterment to process millions of background jobs per day. It supports postgres, mysql, and sqlite, and is designed to be Reliable (with co-transactional job enqueues and guaranteed, at-least-once execution), Scalable (with an optimized pickup query and concurrent job execution), Resilient (with built-in retry mechanisms, exponential backoff, and failed job preservation), and Maintainable (with robust instrumentation, continuous monitoring, and priority-based alerting).