A popular technique for organizing the execution of complex
processing flows is the "Chain of Responsibility" pattern, as
described (among many other places) in the classic "Gang of Four"
design patterns book. Although the fundamental API contracts
required to implement this design pattern are extremely simple,
it is useful to have a base API that facilitates using the pattern,
and (more importantly) encouraging composition of command
implementations from multiple diverse sources.
Towards that end, the Chain API models a computation as a series
of "commands" that can be combined into a "chain". The API for a
command consists of a single method (execute()), which is passed
a "context" parameter containing the dynamic state of the
computation, and whose return value is a boolean that determines
whether or not processing for the current chain has been completed
(true), or whether processing should be delegated to the next
command in the chain (false).