Access control is very hard to get right and at the same time to keep simple. Notes have currently a very flexible but also very complex model that has evolved over the years. It was not clear whether the Notes solution would port to the Web. Flash crowds really do happen. A document often gets highly popular very quickly - the hot spot of the day which results in flash crowds.
Different data is hot at different places and at different times.
The hot spots of the net load are highly dynamic in that they move around- typically from day to day. This was subject for a detailed discussion in the afternoon whether this is due to the fact that content providers deliberately short circuits caches in order to get hit counts. Van Jacobsen notes that almost all dynamic data is to force hit count. Margo's model exploits the fact that Web servers have a lot of information about which documents are popular and where they are requested. Cooperative caches flattens out a more hierarchical cache structure and in stead generates a dynamic tree structure.