Measuring and Evaluating Live Content Consistency in a Large-Scale CDN

Content Delivery Networks (CDNs) play a central role of today's Internet infrastructure, and have seen a sharp increment in scale. More and more internet sites are armed with dynamic (or live) content (such as live sports game statistics, e-commerce and online auction), and there is a need to deliver dynamic content freshly in scale. To achieve high scalability, the consistency maintenance problem for dynamic content (contents with frequent updates) served by CDNs is non-trivial. The large number of widely scattered replicas guarantee the service QoS of end-users, meanwhile largely increase the complexity of consistency maintenance. Current consistency maintenance infrastructures and methods cannot simultaneously satisfy the two requirements: scalability and consistency. In this paper, we first analyze our crawled trace data of a cached sports game content on thousands of content servers of a major CDN. We analyze the content consistency from different perspectives, from which we try to break down the reasons for inconsistency among content servers. Finally, we further evaluate the performance in consistency, scalability and overhead for different infrastructures with different update methods. We itemize the advantages and disadvantages of different methods and infrastructures in different scenarios through the evaluation. We aim to give guidance for appropriate selections of consistency maintenance infrastructures and methods for a CDN, and for choosing a CDN service with different considerations.