Hosting a Web site at a single server creates performance and reliability issues when request load increases, availability is at stake, and, in general, when quality-of-service demands rise. A common approach to these problems is making use of a content delivery network (CDN) that supports distribution and replication of (parts of) a Web site. The nodes of such networks are dispersed across the Internet, allowing clients to be redirected to a nearest copy of a requested document, or to balance access loads among several servers. Also, if documents are replicated, availability of a site increases. The design space for constructing a CDN is large and involves decisions concerning replica placement, client redirection policies, but also decentralization. We discuss the principles of various types of distributed Web hosting platforms and show where tradeoffs need to be made when it comes to supporting robustness, flexibility, and performance.
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