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Version Control with Subversion - Appendix B. WebDAV and Autoversioning
Appendix B. WebDAV and Autoversioning
WebDAV is an extension to HTTP, and is growing more and more
popular as a standard for file-sharing. Today's operating
systems are becoming extremely Web-aware, and many now have
built-in support for mounting “shares” exported by
WebDAV servers.
If you use Apache/mod_dav_svn as your Subversion network
server, then to some extent, you are also running a WebDAV
server. This appendix gives some background on the nature of
this protocol, how Subversion uses it, and how well Subversion
interoperates with other software that is WebDAV-aware.
This section provides a very brief, very general overview to
the ideas behind WebDAV. It should lay the foundation for
understanding WebDAV compatibility issues between clients and
servers.
RFC 2518 defines a set of concepts and accompanying
extension methods to HTTP 1.1 that make the web into a more
universal read/write medium. The basic idea is that a
WebDAV-compliant web server can act like a generic file
server; clients can mount shared folders that behave much like
NFS or SMB filesystems.
The tragedy, though, is that the RFC 2518 WebDAV
specification does not provide any sort of model for version
control, despite the “V” in DAV. Basic WebDAV
clients and servers assume only one version of each file or
directory exists, and can be repeatedly overwritten.
Here are the concepts and terms introduced in basic
WebDAV:
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Resources
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WebDAV lingo refers to any server-side object
(that can be described with a URI) as a
resource.
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New write methods
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Beyond the standard HTTP PUT
method (which creates or overwrites a web resource),
WebDAV defines new COPY and
MOVE methods for duplicating or
rearranging resources.
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Collections
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A collection is the WebDAV
term for a grouping of resources. In most cases, it
is analogous to a directory. Whereas file resources
can be written or created with a
PUT method, collection resources
are created with the new MKCOL
method.
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Properties
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This is the same idea present in
Subversion—metadata attached to files and
collections. A client can list or retrieve properties
attached to a resource with the new
PROPFIND method, and can change
them with the PROPPATCH method.
Some properties are wholly created and controlled by
users (e.g. a property called “color”),
and others are wholly created and controlled by the
WebDAV server (e.g. a property that contains the last
modification time of a file). The former kind are
called dead properties, and the
latter kind are called live
properties.
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Locking
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A WebDAV server may decide to offer a locking
feature to clients—this part of the
specification is optional, although most WebDAV
servers do offer the feature. If present, then
clients can use the new LOCK and
UNLOCK methods to mediate access to
a resource. In most cases these methods are used to
create exclusive write locks (as discussed in
the section called “The Lock-Modify-Unlock Solution”), although shared write
locks are also possible in some server
implementations.
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Access control
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A more recent specification (RFC 3744) defines a
system for defining access control lists (ACLs) on
WebDAV resources. Some clients and servers have begun
to implement this feature.
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