DistOS 2014W Lecture 20
Cassandra
Cassandra is essentially running a BigTable interface on top of a Dynamo infrastructure. BigTable uses GFS' built-in replication and Chubby for locking. Cassandra uses gossip algorithms: Scuttlebutt. Apache Zookeeper is used for distributed configuration.
Comet
The major idea behind Comet is triggers/callbacks. There is an extensive literature in extensible operating systems, basically adding code to the operating system to better suit my application.