Chapter 1. What Are Comet and Reverse Ajax?

The term "Comet" was coined by Alex Russell of the Dojo project to describe exchanges between a client and a server in which the server, rather than the client, initiates the contact. Joe Walker of the Direct Web Remoting (DWR) project refers to a similar mechanism as "Reverse Ajax." Much like when the term "Ajax" was coined in 2005, the name "Comet" has served as a rallying point around a number of previously disconnected technological projects, such as the nonblocking I/O introduced into Java in 2002, message queue technologies, and, further back, HTTP 1.1's persistent connections and the push technologies of the late 1990s.

These technologies have in common an interest in initiating communication between a client and a server from the server's end. Conventional web-based applications are all about client-led communication, but there has been a repeated need to discuss server-led communication within the web development community and to provide a name for it. To understand the phenomenon of Comet and Reverse Ajax, we need to consider why there is a need for it and why it is so out of the ordinary as to require a label of its own.