Meteor has a simple dependency tracking system which allows it to automatically rerun templates and other computations whenever Session variables, database queries, and other data sources change.
Unlike most other systems, you don’t have to manually declare these dependencies — it “just works”. The mechanism is simple and efficient. When you call a function that supports reactive updates (such as a database query), it automatically saves the current Computation object, if any (representing, for example, the current template being rendered). Later, when the data changes, the function can “invalidate” the Computation, causing it to rerun (rerendering the template).... More Info »
Applications will find Tracker.autorun useful, while more advanced facilities such as Tracker.Dependency and onInvalidate callbacks are intended primarily for package authors implementing new reactive data sources.