A computing system capable of associating annotations with millions of content sources is described. An annotation is any content associated with a document space. The document space is any document identified by a document identifier. The document space provides the context for the annotation. An annotation is represented as an object having a plurality of properties. The annotation is associated with a content source using a document identifier property. The document identifier property identifies the content source with which the annotation is associated. A scalable computing system for managing annotations responds to requests for presenting annotations to millions of documents a day. The computing system consists of multiple tiers of servers. A tier I server indicates whether there are annotations associated with a content source. A tier II server provides an index to the body of the annotations. A tier III server provides the body of the annotation.