The Chain of Responsibility pattern

Checks before a payment: why a ladder of ifs stops scaling, how a chain gives each check its own handler that decides or passes on, what every chain must settle (what if nobody answers, who decides the order, who builds the chain), the middleware shape where a link wraps the rest, how a chain differs from Decorator, Observer, Strategy and a pipeline, how the logger's chain varies it, and when not to use it.