While Comet may be all the rage, some of us are still stuck in web 2.0. And those of us that are use Ajax polling to see if there’s anything new on the server.
Here at GitHub we normally do this with memcached. The web browser polls a URL which checks a memcached key. If there’s no data cached, the request returns and polls again in a few seconds. If there is data, the request returns with it and the browser merrily goes about its business. On the other end our background workers stick the goods in memcached when they’re ready.
In this way we use memcached as a poor man’s message bus.
Yet there’s a problem with this: if after a few Ajax polls there’s no data, there probably won’t be for a while. Maybe the site is overloaded or the queue is backed up. In those circumstances the continued polling adds additional unwanted strain to the site. What to do?
The solution is to increment the amount of time you wait in between each poll. Really, it’s that simple. We wrote a little jQuery plugin to make this pattern even easier in our own JS. Here it is, from us to you:
Any time you see “Loading commit data…” or “Hardcore Archiving Action,” you’re seeing smart polling. Enjoy!
I did this for a project a while back, but used the Fibonacci sequence instead of a *1.5 curve. You know ... just because.
I've heard that exponential backoff works much better with web 3.0
linear increase, exponential backoff
I dig it.
See also http://www.prototypejs.org/api/ajax/periodicalUpdater
Yeah, PeriodicalUpdater does this for Prototype.
I called them stepped intervals back in 2005.
http://www.robertames.com/blog.cgi/entries/js-stepped-intervals.html
Code is old-ish and depends on a global variable, feel free to steal or clean it up under the BSD license of your choice.
I preferred being able to explicitly set timeouts so it's easier to explain and reason about:
“20 seconds for 6 times (2 minutes)”, then “60 seconds for 5 times (5 minutes)”, then “2 minutes for 30 times (1 hour)”.
--Robert
http://github.com/visionmedia/jquery-smart-poll
haha
While this is great for good ol' regular AJAX, it could also be used with a few modifications for reconnecting to your long-polling Comet server when it goes down. ("Oops, looks like you lost your connection. Reconnecting in 5 seconds..." and so on)
Wow - this poller addresses several things I want - which prototype and scriptaculous lack - (you'd think they'd never heard of etags or if-modified-since !!) Look forward to playing with it..