Chord

This paper describes in detail the design and implementation of Chord DHT system. It is a dynamic system that allows efficient content retrieval based on key value pairs and without centralized coordination.

The central idea to CHord is consistent hashing to allow some ways of balancing the load on different nodes in the chord ring. To further enhance the performance, Chord uses finger tables to allow O(log n) look up time of a particular key in the ring.

There are a couple of issues in this paper that were not clearly addressed. Issues such as fault recovery and migration of state between nodes on a planned take down of a node seems central to the correct functioning and operation of chord ring but received very little attention.

Another problem with this design is the latency in the lookup. Although Log(n) in terms of number of nodes in the hash space, the delay is still significant. Also each hop in the chord ring is an overlay and might translate into multiple hops in IP.

Overall, I still like this DHT compared to others in that it is simple to follow and works reasonably well. As system designers, in many cases, we tend to over design a system and makes it difficult to get implementation right. At some point, law of diminishing return kicks in and it is better to use the simpler design. I think Chord strikes a balance between simplicity and functionality, which is why it is one of the more popular DHTs.