目录
WebDAV is an extension to HTTP, and it 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.
如果你使用Apache/mod_dav_svn作为你的Subversion网络服务器，某种程度上，你也是在运行一个WebDAV服务器。这个附录提供了这种协议一些背景知识，Subversion如何使用它，Subversion如何和认识WebDAV的软件交互工作。
DAV stands for “Distributed Authoring and Versioning.” 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 over HTTP that behave much like other network filesystems (such as NFS or SMB).
The tragedy, though, is that despite the acronym, the RFC specification doesn't actually describe any sort of version control. Basic WebDAV clients and servers assume that only one version of each file or directory exists, and that it can be repeatedly overwritten.
Because RFC 2518 left out versioning concepts, another committee was left with the responsibility of writing RFC 3253 a few years later. The new RFC adds versioning concepts to WebDAV, placing the “V” back in “DAV”—hence the term “DeltaV.” WebDAV/DeltaV clients and servers are often called just “DeltaV” programs, since DeltaV implies the existence of basic WebDAV.
The original WebDAV standard has been widely successful. Every modern computer operating system has a general WebDAV client built in (details to follow), and a number of popular standalone applications are also able to speak WebDAV—Microsoft Office, Dreamweaver, and Photoshop to name a few. On the server end, Apache HTTP Server has been able to provide WebDAV services since 1998 and is considered the de-facto open source standard. There are several other commercial WebDAV servers available, including Microsoft's own IIS.
DeltaV, unfortunately, has not been so successful. It's very difficult to find any DeltaV clients or servers. The few that do exist are relatively unknown commercial products, and thus it's very difficult to test interoperability. It's not entirely clear as to why DeltaV has remained stagnant. Some opine that the specification is just too complex. Others argue that while WebDAV's features have mass appeal (even the least technical users appreciate network file sharing), version control features just aren't interesting or necessary for most users. Finally, some believe that DeltaV remains unpopular because there's still no open source server product that implements it well.
When Subversion was still in its design phase, it seemed like a great idea to use Apache as a network server. It already had a module to provide WebDAV services. DeltaV was a relatively new specification. The hope was that the Subversion server module (mod_dav_svn) would eventually evolve into an open source DeltaV reference implementation. Unfortunately, DeltaV has a very specific versioning model that doesn't quite line up with Subversion's model. Some concepts were mappable; others were not.
这是什么意思呢？
First, the Subversion client is not a fully implemented
DeltaV client. It needs certain types of things from the server
that DeltaV itself cannot provide, and thus is largely dependent
on a number of Subversion-specific
HTTP REPORT requests that
only mod_dav_svn understands.
Second, mod_dav_svn is not a fully realized DeltaV server. Many portions of the DeltaV specification were irrelevant to Subversion, and thus left unimplemented.
在开发者社区一直有这样的讨论，是否值得弥补这种形势。改变Subversion的设计来匹配DeltaV看起来并不现实，所以可能没有办法让客户端从普通的DeltaV服务器上得到所有的东西。另一方面，mod_dav_svn可以继续开发来实现所有的DeltaV，但缺乏这样做的动力—几乎没有能与之交户的DeltaV客户端。