A Web-scale discovery service (discovery service, for short) is a new service that may realize the discovery and delivery of high-quality information in the library. A discovery service is composed of a unified index and a discovery layer. The unified index pre-harvests and pre-indexes a variety of information resources, including the MARC records created by the library, the metadata of the institutional repository or digital content management system of the library, the metadata and full-text (for indexing) of the databases and electronic journals subscribed by the library, and the metadata and full-text (for indexing) of open-access systems. A user can then search the contents in the unified index through the discovery layer. The discovery layer incorporates functionality such as relevance ranking, facet navigation, personalized service and social networking service. This article aims at explicating the features of a discovery service, and draw the functionality indicators that may be adopted and(or) amended by any library which wants to purchase a discovery service.