This diagram depicts the main WebCenter Sites servlets. The WebCenter Sites operating system consists of several servlets that run on top of an application server. Each servlet is invoked when necessary to perform a discrete set of tasks. Each servlet has a corresponding Java API with Java methods and JSP tags that you use to invoke the functions that you need to use. The main WebCenter Sites servlets are the ContentServer, CatalogManager, TreeManager, BlobServer, DebugServer, CookieServer, and HelloCS servlets.