CORAL

CORAL

According to Wikipedia, a Content Delivery Network (CDN) is ?a term coined in the late 1990's to describe a system of computers networked together across the Internet that cooperate transparently to deliver content (especially large media content) to end users?.

Coral is a CDN with two basic aims. First, it seeks to prevent flash crowds on Web sites furnishing static content, whose bandwidth may be modest. Second, it seeks to minimise (RTT) latency for clients.

The CDN is composed of a number of volunteer machines running Coral software that incorporates a DNS server and a Web proxy. The DNS server converts URLs for a target Web server into URLs for a Web proxy, and tries to ensure that the proxy is close to the requesting client. Web proxies cache content from target Web sites. There is no centralised management of proxies; the network is self-organising and a modified traceroot tool is used by nodes to maintain a performance map of the network.

Flash crowds are avoided by limiting the hits on each Web proxy cache. This is achieved using two techniques. The first involves a weaker form of DHT known as Distributed Sloppy Hash Tables (DSHT). The technique is based on the observation that DHTs perform poorly when there is high demand for a key. In DSHT, each node is annotated with information relating to recent hits on that node. During insertion, if the load on a node is high, then the content is stored on a node at a greater distance from the key. The second technique for combating flash crowds is that the set of volunteer sites are organised in a hierarchy of clusters. The nodes of a cluster are related in that recent measures confirm that the RTT between then is inferior to a certain value. Clusters towards the bottom of the hierarchy have greater RTT times, and consequently are more populated. The basic ideas is that, in a lookup or publish operation, the system restricts itself to a top-level hierarchy. If the DSHT operation fails, then the operation is repeated in a more densely populated, though slower, cluster.

Though the Coral system has interesting proposals for flash crowds, it does not treat some points that are important for the EDOS General Architecture, notably the links between units of content (dependencies) and the large size of the content base.