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Handle Server: Overview

The Handle Server System is the mechanism developed by the Corporation for National Research Initiatives (CNRI) to support a system of global identifiers for resources on the Internet. Two or three such schemes for Uniform Resource Names (URNs) have been proposed through the Internet Engineering Task Force. In December 1995, the active participants in this effort agreed to deploy their proposed systems in a compatible fashion that will allow practical evaluation and continuing development.

A primary task of the handle server is to provide an address lookup service. Each item in the digital library is given a handle, an identifier that is globally unique, valid for the long term (persistent), and independent of its current location in a particular file attached to a particular computer. If presented with a handle, the handle server will supply data that allows a user (or, more likely, a computer program on behalf of a user) to access the item identified by the handle.

Here is a simple example:

• A particular photograph from the Detroit Publishing collection has the control identifier 4a32371. (This corresponds to frame 32371 on side A of the 4th videodisk made by the Prints & Photographs division.) A possible handle for a thumbnail version of this image is hdl:loc.pp/detroit/4a32371t, where "hdl:" indicates that the identifier is a handle within the CNRI scheme and "loc.pp" identifies the naming authority that assigned this name.
• The image can currently be retrieved by the World Wide Web's HTTP (HyperText Transfer Protocol) using the URL (Uniform Resource Locator): http://lcweb2.loc.gov/image/4a/4a30000/4a32000/4a32300/4a32371t.gif

The handle server has a two-part record for each item, associating a handle with the data necessary to access the item identified by the handle. In this simple example, the record associates the handle with the URL for the digitized image.

When a program presents the handle server with the handle, the handle server resolves the handle and supplies the program with the URL. The program can then use the URL to retrieve the image.

All external references, whether from LC's MARC records or finding aids or from online research papers or teaching materials developed elsewhere, should use the handle, not the URL, so that when LC moves this file, the only change that would have to be made is to the record in the handle server. (This digital archive is being built for the long term. LC will not move files often, but developments in the computing industry over the coming decades are unpredictable and not under LC's control. It would be short-sighted to adopt a design that did not allow filenames to be changed.)

Important note:

Details of conventions for LC handles will be developed over the next few months. The handle suggested above has characteristics that LC handles will probably have.

• The handle follows the recommendation by the committee that has been considering naming conventions for the digital archive that LC names incorporate an identifier for an aggregate (e.g. collection) and an identifier for the item within that aggregate.
• The handle has a hierarchical form, essential for any naming scheme that guarantees uniqueness without requiring centralized pre-registration of each item.
• The top level of the hierarchical identifier ("loc") specifies the Library of Congress as the naming authority within a global system. Within "loc," authority for naming items could be further delegated, in this example to the Prints and Photographs Division.

For more details, continue through this section on the Handle Server:

• More on Handles and Naming Authorities
• A Global System of Handle Servers
• How the Handle Server Works with the Repository

Delve deeper for further discussion of:

• Naming conventions for Digital Resources (Rebecca Guenther of LC's Network Development and MARC Standards Office. January 3, 1996) reviews the state of URN standardization discussions and naming efforts at some libraries and library organizations.
• The relationship between URNs, Handles, and PURLs.

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Handle Server

National Digital Library Program
Comments: caar@loc.gov (05/08/97)