Progressive Delivery

Application-layer release control; Hodgson's four toggle types (release/experiment/ops/permission) with different lifecycles; flag lifecycle discipline; default behaviour and cohort routing; kill switches and load-shedding flags; flags as technical debt with a use-by date

Release in stages exposing change to controlled subsets; deployment vs release decoupling; canary/blue-green/ring/dark-launch/parallel-run/feature-flag spectrum; SRE order-of-fault model; "roll back first, diagnose second"; prerequisites and anti-patterns