stored on a disk.  For this simple case, the URI at which a resource
   is accessed is identical to the URI at which the source (the
   persistent state) of the resource is accessed.  This is also the case
   for HTML source files that are not processed by the server prior to
   transmission.

   However, the server can sometimes process HTML resources before they
   are transmitted as a return entity body.  For example, a server-
   side-include directive within an HTML file might instruct a server to
   replace the directive with another value, such as the current date.
   In this case, what is returned by GET (HTML plus date) differs from
   the persistent state of the resource (HTML plus directive).
   Typically there is no way to access the HTML resource containing the
   unprocessed directive.

   Sometimes the entity returned by GET is the output of a data-
   producing process that is described by one or more source resources
   (that may not even have a location in the URI namespace).  A single
   data-producing process may dynamically generate the state of a
   potentially large number of output resources.  An example of this is
   a CGI script that describes a "finger" gateway process that maps part
   of the namespace of a server into finger requests, such as
   http://www.foo.bar.org/finger_gateway/user@host.






 
RFC 2518                         WEBDAV                    February 1999


   In the absence of distributed authoring capabilities, it is
   acceptable to have no mapping of source resource(s) to the URI
   namespace. In fact, preventing access to the source resource(s) has
   desirable security benefits.  However, if remote editing of the
   source resource(s) is desired, the source resource(s) should be given
   a location in the URI namespace.  This source location should not be
   one of the locations at which the generated output is retrievable,
   since in general it is impossible for the server to differentiate
   requests for source resources from requests for process output
   resources.  There is often a many-to-many relationship between source
   resources and output resources.

   On WebDAV compliant servers the URI of the source resource(s) may be
   stored in a link on the output resource with type DAV:source (see
   section 13.10 for a description of the source link property).
   Storing the source URIs in links on the output resources places the
   burden of discovering the source on the authoring client.  Note that
   the value of a source link is not guaranteed to point to the correct
   source.  Source links may break or incorrect values may be entered.
   Also note that not all servers will allow the client to set the
   source link value.  For example a server which generates source links
   on the fly for its CGI files will most likely not allow a client to
   set the source link value.

6  Locking

   The ability to lock a resource provides a mechanism for serializing
   access to that resource.  Using a lock, an authoring client can
   provide a reasonable guarantee that another principal will not modify
   a resource while it is being edited.  In this way, a client can
   prevent the "lost update" problem.

   This specification allows locks to vary over two client-specified
   parameters, the number of principals involved (exclusive vs. shared)
   and the type of access to be granted. This document defines locking
   for only one access type, write. However, the syntax is extensible,
   and permits the eventual specification of locking for other access
   types.

6.1 Exclusive Vs. Shared Locks

   The most basic form of lock is an exclusive lock.  This is a lock
   where the access right in question is only granted to a single
   principal.  The need for this arbitration results from a desire to
   avoid having to merge results.







 
RFC 2518                         WEBDAV                    February 1999


   However, there are times when the goal of a lock is not to exclude
   others from exercising an access right but rather to provide a
   mechanism for principals to indicate that they intend to exercise
   their access rights.  Shared locks are provided for this case.  A
   shared lock allows multiple principals to receive a lock.  Hence any
   principal with appropriate access can get the lock.

   With shared locks there are two trust sets that affect a resource.
   The first trust set is created by access permissions.  Principals who
   are trusted, for example, may have permission to write to the
   resource.  Among those who have access permission to write to the