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

摘要

Content Delivery Networks (CDNs) play a central role in today's Internet infrastructure and have seen a sharp increase in scale. More and more internet sites are armed with live contents, such as live sports game statistics, e-commerce, and online auctions, and they rely on CDNs to deliver such contents freshly at scale. However, the problem of maintaining consistency for live (dynamic) contents while achieving high scalability is non-trivial in CDNs. The large number of widely scattered replicas guarantees the QoS of end-users while substantially increasing the complexity of consistency maintenance under frequent updates. Current consistency maintenance infrastructures and methods cannot simultaneously satisfy both scalability and consistency. In this paper, we first analyze our crawled trace data of cached sports game content on thousands of content servers of a major CDN. We analyze the content consistency from different perspectives, from which we break down the reasons for inconsistency among content servers. We verify that the CDN uses unicast instead of multicast trees as the update infrastructure, which may not scale effectively. Then, 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. Based on this evaluation, we propose our hybrid and self-adaptive update method to reduce network load and improve scalability under the conditions recorded in the trace and prove its effectiveness through trace-driven experiments. 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.