38 SEPTEMBER • OCTOBER 2002 http://computer.org/internet/ 1089-7801/02/$17.00©2002 IEEE IEEE INTERNET COMPUTING Guest Editor’s Introduction Global Deployment of Data Centers Pankaj Mehra Hewlett-Packard NonStop Labs D ata centers first arose from IT efforts to consolidate the server and storage resources of one or more enterprises, so they could be central- ly managed and shared among various departments and functions. IT architec- tures involving multiple data centers dis- tributed around the globe exist in part due to the evolutionary nature of enterprises and, in part, by design. By effectively utilizing distributed re- sources, enterprises seek to improve their services’ scalability, availability, and responsiveness. Scalability here denotes the ability to extend the resources avail- able to a service beyond a single data cen- ter’s boundaries in order to address grow- ing user demand. High availability means ensuring that users have continuous access to a service despite faults in sys- tems, networks, hardware, or software — even when caused by natural disasters or human error. Responsiveness refers to short and predictable turnaround times on service requests; it is usually improved using affinity. A site in Singapore, for instance, might field all service requests originating in Southeast Asia. Architects of global-scale services often achieve all three objectives using load bal- ancing to distribute the task of servicing Internet requests among servers at multi- ple sites. Load-balancing algorithms dynamically adjust the number of sites, and the number of servers used at each site, in response to changes in demand. They are programmed to work around unreachable sites, and they account for affinities between Internet requests and the sites available to service them. With large enterprises continuing to develop and deploy globally distributed content, it is important to understand the ecosystem for global-scale applications. At the base, a well-understood and widely adopted tiered architecture determines how to arrange servers and routers within a data center. At the top of the ecosystem are the applications, which today primarily include Web content and first-generation Internet services that follow the client-server para- digm, such as mail and chat. The articles in this issue discuss the two broad approach- es generally used to deploy services glob- ally across multiple data centers. In the future, most applications will likely be Web services, which involve wide-area server- to-server peering. The Internet community