2.2 Determining URLs from Resource Specification

Given a file or directory on the server, it is straighforward to convert this into a relative URL. This allows for translation of the internal information about a file into its external view. Note that this will be a one-to-one relationship, but that if the target resource can take user input, that there may be more than one returned object for each given server resource.

2.2.1 Determining Resources From URLs

Given a URL that references a resource, it is relatively straightforward to convert this into an internal resource specification. However in some cases URLs are not pointed at filesystem files or directories. Instead, the URLs reference server redirection commands, usually written into the server configuration files. Alternatively, there may be executable modules compiled into the server that are mapped to particular URLs.

2.2.2 Server Executable Mapped URLs

In some cases the server itself may contain compiled in modules that map certain URLs, or portions of URLs, to particular built-in processing. Examples are compiled-in imagemap programs invoked through URL root paths such as "www.xxx.org/imap/...". In such cases the type of the returned data cannot be known until the process has run. In practice, however, the type of the returned data may be well defined, but there is no mechanism for exctracting or verifying this information.

Note that, as far as I know, there is no server configuration file that explains which URL fragments are related to built-in modules.

2.2.3 Server Redirections

A server can implement redirections, which map map classes of URLs to new locations on the server or on another server. This does not affect the local filesystem, but does mean that there are "virtual" locations in the filesystem that map to elsewhere. These are not the same as symbolic links, since they are configured solely in the server configuration files. Note that redirecting a directory redirects every request to an object lying below this point in the tree -- this can create problems, and is a useful condition to check for.

There are two types of redirections:

• Type 302 -- these reference resources that have permanently moved to another location. This is useful if you have moved documents to a new location.
• Type 301 -- these reference resources that have temporarily moved to another location. This is useful if the server (or gateway program) software has created a temporary file which will be soon deleted. A user-agent will not cache this type of redirected URL.