Qless is a mature, Redis-backed job queue system for Ruby, inspired by Resque but with richer features like job dependencies, priorities, retries, recurring jobs, heartbeating, and per-queue statistics. Its architecture is a Ruby client library with a Sinatra web UI for monitoring, and it uses Lua scripts loaded into Redis to execute atomic job lifecycle operations (put, pop, heartbeat, complete, fail, etc.). This language-agnostic core makes the queue engine reusable from any language that can call Redis EVALSHA. The Ruby gem provides two worker types (Forking and Serial), pluggable job reservers (Ordered, RoundRobin, ShuffledRoundRobin), and a middleware stack for cross-cutting concerns like timeout enforcement, retry logic, exception notifications (Sentry), and memory monitoring. The codebase demonstrates good engineering practices: clear separation of concerns, well-structured middleware pattern, comprehensive test coverage (RSpec with integration and unit tests), and a focus on operational metrics (histograms, fail/retry counts). However, the frontend assets (Bootstrap 2.0.2, outdated jQuery plugins) and some dependencies (Thin, legacy Redis compatibility shims) show technical drift. The project receives active maintenance (recent compatibility updates for Redis 6+ and Valkey 8) but retains some technical debt.