A Novel Approach to Managing Consistency in Content Distribution Networks

Content distribution network (CDN) is a technique deployed to push content from the origin server to geographically distributed replicas, usually located at the edge of the network where the clients are attached. One of the important problems in CDNs is how to manage the consistency of content at replicas with that at the origin server, especially for those documents changing dynamically. In the traditional propagation approach the updated version of a document is delivered to all replicas whenever a change is made to the document at the origin server. It may generate significant levels of unnecessary traffic if documents are updated more frequently than accessed. Another approach is invalidation, in which an invalidation message is sent to all replicas when a document is changed at the origin server. This approach does not make full use of the distribution network for content delivery and each replica needs to fetch an updated version individually at a later time. This can also lead to inefficiency in managing consistency at replicas. In this paper, we propose a novel hybrid approach that will generate less traffic than the propagation approach, and the invalidation approach. The origin server makes the decision of using either propagation or invalidation method for each document, based on the statistics about the update frequency at the origin server and the request rates collected by replicas. We develop a technique that can reduce the burden of request rate collection at replicas and get rid of the implosion problem when replicas send the statistics to the origin server. Extensive simulations are performed to examine how the traffic generated and freshness rate at replicas are affected by various parameters. We experiment with a wide range of request rate, update frequency, and the number of replicas. The results show that our approach can take advantage of content distribution network and significantly reduce the traffic generated.