Thoughts on Chord

A peer-to-peer system departs from a client-server system in that each peer has equal privileges and similar responsibilities. Most such systems lack a centralized source of control. Peer-to-peer systems have been built to accomplish a variety of tasks: job partitioning, content distribution, content lookup, file systems, and media streaming.

In 2001, a significant peer-to-peer system known as Chord was invented for the purpose of scalable file lookup. With Chord, a number of peers are organized in a large ring. Each peer stores a number of files of interest. A peer does not choose which files it stores; the files it stores are determined by its position in the ring, and on the name of the files.

When someone wants to access a particular file, they use a lookup service through Chord that can identify where a file is. Because of Chord's design, this lookup can be done in a number of steps logarithmic to the number of peers in the ring. This lookup capability provided great improvement on contemporary file lookup services, which usually required querying a significant percent of the group. With Chord, the group of peers could easily scale.

Chord's architecture is well-designed, but there are several things that Chord does not address. First is the case that one peer in the ring may have less resources (e.g., bandwidth, disk space) than another; therefore that peer will slow down file transfer for some of the files, or not be capable of storing a file. While Chord distributes files evenly among peers, it still would be preferable to keep most of the resources with the most capable peers. It is not likely that a simple change to Chord could accomplish this.

Also, peers may desire to minimize the shuffle of files between each other. That is, a peer may want to maximize on the number of its own files it stores, and to minimize on storing files it previously did not have. To make this possible, Chord could be modified to store references to files instead of the files themselves. With this modification, it remains to figure out how peers announce which files they have to begin with, and what to do when all peers which have a particular file leave the group.