From Phil Windley - this a
very good summary of the web-native event notification system that has evolved from Ajax - Comet. It hits all the points that KnowNow dealt with five or six years ago.
One nitpicking point is this "The problem is that each connection takes a process or thread and there might be thousands of them. Comet can reduce load but not on your current Web infrastructure." A server doesn't necessarily have to allocate a thread or process for each connection - if you look at IRC servers, they handle many thousands of concurrent connections. What is needed is non-blocking IO and Phil's post later points to SEDA - staged event driven architecture. Which brings this full circle from my work at KnowNow that used persistent connections to the browser, to my team at Amazon that used SEDA in a new real-time publishing system we built over the past few months.
It's great to see everybody up to speed on internet-scale event notifications - the power of event driven browser applications is now clear, XML messaging via Atom/RSS is widespread, even Microsoft has contributed algorithms for supporting messages criss crossing a cross-linked web of subscriptions. What's next? I think the 2-connection limit needs to be solved and that will be the final step in the next wave of live Web apps.