Scaling a chat app - short polling vs. long polling (AJAX, PHP)

A few notes:

• Polling every second is overkill. The app will still feel very responsive with a few seconds of delay between checks.
• To save your db's traffic and speed responses, consider using an in memory cache to store undelivered messages. You could still persist messages to the db, the in memory cache would simply be used for queries for new messages to avoid queries to the db every x seconds by each user.
• Timeout the user's chat after x seconds of inactivity to stop polling to your server. This assures someone leaving a window open won't continue to generate traffic. Offer a simple "Still there? Continue chatting." link for sessions that timeout and warn the user before the timeout so they can extend the timeout.
• I'd suggest starting out with polling rather than comet/long polling/sockets. Polling is simple to build and support and will likely scale just fine in the short-term. If you get a lot of traffic you can throw hardware and a load balancer at the problem to scale. The entire web is based on polling - polling most certainly scales. There's a point where the complexity of alternatives like comet/long polling/etc make sense, but you need a lot of traffic before the extra development time/complexity are justified.

Some project links -

http://code.google.com/p/phpwebsocket/

http://code.google.com/p/php-websocket-server/

With APE (based on C), you could real time communication between server and client.

http://www.ape-project.org/

The difference is this: long polling allows for some kind of event-driven notifying, so the server is able to actively send data to the client. Normal polling is a periodical checking for data to fetch, so to say. Wikipedia is quite detailed about that:

With long polling, the client requests information from the server in a way similar to a normal polling; however, if the server does not have any information available for the client, then instead of sending an empty response, the server holds the request and waits for information to become available (or for a suitable timeout event), after which a complete response is finally sent to the client.

Long polling reduces the amount of data that needs to be sent because the server only sends data when there really IS data, hence the client does not need to check at every interval x.

If you need a more performant (and imho more elegant) way of full duplex client/server communication, consider using the WebSocket protocol, it's great!