When building a web application, you'll quickly run into the challenge of getting different parts or components within your page to talk to each other. At any single time, your application should be representing the exact same set of data. If something changes, either via user action or some other mechanism, that change needs to be reflected in all of the appropriate places.
This problem occurs at different scales. You may have a chat application where an entered message needs to be propagated to all participants as fast as possible. Or you may have a piece of data that needs to be represented several times within the same application view and hence all of these representations need to be kept in sync. For example, ...
