Colm MacCarthaigh, Network Architect at Joost, gave this presentation at the UK Network Operators' Forum Meeting in Manchester on April 3rd, 2007.

Consistent hashing is one of those ideas that really puts the science in computer science and reminds us why all those really smart people spend years slaving over algorithms. Consistent hashing is "a scheme that provides hash table functionality in a way that the addition or removal of one slot does not significantly change the mapping of keys to slots" and was originally a way of distributing requests among a changing population of web servers. My first reaction to the idea was "wow, that's really smart" and I sadly realized I would never come up with something so elegant. I then immediately saw applications for it everywhere. And consistent hashing is used everywhere: distributed hash tables, overlay networks, P2P, IM, caching, and CDNs. Here's the abstract from the original paper and after the abstract are some links to a few very good articles with accessible explanations of consistent hashing and its applications in the real world.

Skype's 220 millions users lost service for a stunning two days. The primary cause for Skype's nightmare (can you imagine the beeper storm that went off?) was a massive global roll-out of a Window's patch triggering the simultaneous reboot of millions of machines across the globe. The secondary cause was a bug in Skype's software that prevented "self-healing" in the face of such attacks. The flood of log-in requests and a lack of "peer-to-peer resources" melted their system.

Who's fault is it? Is Skype to blame? Is Microsoft to blame? Or is the peer-to-peer model itself fundamentally flawed in some way?