One of the most basic laws of a web application is that the client, not the server, must initiate any communication between the two. There are a number of common-use cases where, ideally, the server would like to talk to the client--dashboards and monitoring apps, chat rooms and other collaborations, and progress reports on long-running processes. Comet (a.k.a. Reverse Ajax) provides a mechanism for enabling this. Comet is moderately complex to implement. But this practical, hands-on book gets you going.

• In Part 1 of this book, we start by examining the use cases, and look at...