Over the past decade, rapidly growing Internet-based services such as e-mail, blogging, social networking, search, and e-commerce have substantially redened the way consumers communicate, access contents, share information, and purchase products. In particular, the recent advances in the web technology have made it easy for any user to provide and consume content of any form. For example, building a personal web page (e.g., Google Sites*), starting a blog (e.g., WordPress,† Blogger,‡ LiveJournal§), and making both searchable for the public have become a commodity that is available for users all over the world. Arguably, the main goal of the next wave is to facilitate the job of implementing every application as a distributed, scalable, and widely accessible service on the web. Services such as Facebook,¶ Flickr,** YouTube,†† Zoho,‡‡ and LinkedIn§§ are currently leading this approach. Such applications are both data-intensive and very interactive. For example, the Facebook social network has announced that it has more than 800 millions of monthly active users.¶¶ Each user has an average of 130 friendship relations. Moreover, there are about 900 million objects that registered users interact with, such as pages, groups, events, and community pages. Other smaller scale social networks such as LinkedIn, which is mainly used for professionals, has more than 120 million registered users. Twitter has also claimed to have over 100 million active monthly users. Therefore, it becomes an ultimate goal to make it easy for every application to achieve such high scalability and availability goals with minimum efforts.