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

Question

I run a website where users can chat with each other through the browser (think Facebook chat). What is the best way to handle the live interaction? (Right now I have a poll going every 30 seconds to update online users and new incoming messages, and another poll going on chat pages every second to get new messages.)

Things I've considered:

• HTML5 Web Sockets: didn't use this because it doesn't work in all browsers (only chrome).
• Flash Sockets: didn't use this because I wanted to eventually support mobile web.

Right now, I am using short polling because I don't know how scalable AJAX long polling would be. I'm running a VPS server from servint right now (running apache). Should I use long polling or short polling? I don't need absolutely immediate response times (just "good enough" for a chat app). Is short polling this often with a few hundred-thousand users going to kill my server? How do I scale this, please help!

Answer 1

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.