Dynamic Web Development with Seaside

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22.1Inside Comet

Before we dive into a comet application, let us have a look at precisely how Comet works. The figure depicts the basic interaction between the web browser and the Seaside server in a Comet setup. The first round-trip between the web browser and Seaside is a normal HTTP request that returns a full XHTML page. This first page includes a small JavaScript snippet, that is executed once the page has finished loading. This starts a new asynchronous connection to the server. This connection now persists for a much longer time, essentially as long as the web browser is listening. This connection can then be used by the server to send data to the web browser at any time.

Comet enables the server to push data to the client, without that the client has to poll the server for fresh content.

When using Comet, keep in mind that HTTP was not designed to push data from the server to the client. Comet is a hack, even if the details are hidden in Seaside. Luckily the implementation works on all modern web browsers, but there is no guarantee that these tricks will continue to work with the next generation of web browsers. It should be noted that upcoming generations of web browsers will very likely support this type of inverse communication, and there is work on an emerging Comet standard.

Enough ranting, let’s give it a quick try.

Copyright © 26 May 2017 Stéphane Ducasse, Lukas Renggli, C. David Shaffer, Rick Zaccone
This book is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license.

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