The virtual link manager

The virtual link manager is designed to permit readers and (perhaps most important) authors of web pages to use link names arranged in a conceptual and coherent manner while the pages are actually spread out between different servers and at different locations within the servers.

Instead of using the "real" links to the pages, "virtual" links are used which are then mapped to real links by virtual link managers at the servers. The vlm:s are programs that are installed at the servers and are executed whenever a page is accessed, usually via a cgi script.

There are two advantages with this schema:

Virtual links can be named in a mnemonic way, like http://site1/lac/logics/modal, independently of the "real" path names. Furthermore, any page can be accessed at any server, so you don't have to remember where a page is located. This is especially useful for people using the links in their own documents.
Individual pages and whole substructures can easily be moved to other locations within the same server or to other servers. No references to the pages moved will have to be updated. It suffices to update the maps used by the virtual link managers.

For instance, assume we have set up the following virtual path structure.

In reality, the different files and directories might be distributed over several different servers, like in the following picture.

Moreover, the paths do not even have to correspond to real paths within the servers. For instance, there need not be any real path lac/logics/semantics/ anywhere at site no. 1.

Now assume somebody wants to read a document about modal logics and sends the request http://site1/lac/logics/modal/index.html as in step (1) below.

The lac component in the path is actually a call to a vlm script that invokes a vlm at site 1 with the remainder logics/modal/index.html of the path and approriate maps. The vlm recognizes that logics/modal is located at site 2, and therefore redirects the request to site 2 in steps (2) and (3).

Next, the vlm at site 2 recognices that logics/modal is actually located at site 2. The link is transformed to the corresponding real file path in step (4), e.g. /info/www/ext/lac-dir/modal-logics/index.html, and the file is fetched and sent to the client in step (5).

For the technically interested, detailed instructions for setting up a virtual link manager are available (however, the software is currently not publically available).

The virtual link manager and this page was created by:
Lars Karlsson
Linköping University