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Version Control with Subversion
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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.

Basic WebDAV Concepts

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.

Original WebDAV

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:

Resources

WebDAV lingo refers to any server-side object (that can be described with a URI) as a resource.

New write methods

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.

Collections

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.

Properties

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.

Locking

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.

Access control

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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Version Control with Subversion
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