1. A computer including a content manager of a content management system that fills requests for requested content by accessing repositories storing the requested content when the repositories may not be compatible with one or more requesting systems, wherein the content management system stores, retrieves, indexes, searches, edits, approves, publishes, and otherwise manipulates content, the content manager comprising: a request for content stored in a repository employing a data access protocol inaccessible to an originator of the request; a metadata registry comprising a data structure and including metadata describing the requested content stored in the repository, where the metadata includes an identification of a particular external service configured to communicate with the repository using the data access protocol to acquire the requested content, wherein the particular external service comprises a repository interface, and wherein the metadata includes external reference metadata that contains parameter values that identify the requested content and the particular external service and location information as to the particular external service that enables requests for content to be filled; and an external service registry comprising a data structure and listing the particular external service among a plurality of external services, wherein the external service registry associates information for interacting with the data access protocol for the particular external service, and for each of the other external services, associates information for interacting with appropriate data access protocols for that other external service; and wherein the content manager is configured to: combine at least a portion of the metadata and at least a portion of information in the external service registry into a unique identifier that uniquely identifies the requested content to the particular external service; transmit the unique identifier to the particular external service, wherein the particular external service is configured to access the requested content based on the identifier; receive the requested content from the particular external service; and provide the requested data to one of a user, an application, and a process.