Ubik: replicated servers made easy

While replication of administrative databases is important, quite often lazy propagation of updates does not suffice. This argument is couched in terms of experience with the Andrew file system (AFS) but the author believes that it generalizes to many environments. He discusses in some detail a replication library his group built, on top of which they built all of the AFS administrative servers. This library, named Ubik, is interesting for three reasons. First, it has a very simple programming interface, making it relatively easy to understand. Second, the library is quite easy to implement, while still being useful for a large class of server applications. Finally, the library uses an interesting nonblocking (in the database sense) commit protocol, providing good write as well as read availability. The author outlines Ubik's goals, gives an overview of it, and describes its quorum completion algorithm in some detail.<>