Financial trading applications are a good example of this; pricing updates occur frequently and must be relayed to the client at a high rate, whereas user generated events such as buying and selling happen less often. Comet is also known as server push, HTTP push, or HTTP streaming.
How does it work?
How does Comet differ from traditional data transfer and Ajax?
Ajax was invented to overcome this overhead by doing away with the need to receive a new page and instead simply request a data response from the server. However, Ajax is still a request-response model, and since Ajax requires that the client initiates each interaction, data cannot be sent at will from the server. To get round this problem, Ajax applications often use polling to detect new information on the server, but this often results in unacceptable loads on the server and client.
Comet is a ‘push’ approach, greatly improving throughput and decreasing latency and server load. Once the client has made the initial connection, the server sends updates as they occur, without the client needing to issue further requests. See the figure above for a comparison of Ajax and Comet data transfer.