In WebDAV: Next-Generation Collaborative Web Authoring, Lisa Dusseault thoroughly describes the WebDAV protocol and the rationale behind the current version (see Y. Goland et al., HTTP Extensions for Distributed Authoring—WebDAV (http://asg.web.cmu.edu/rfc/rfc2518.html), RFC 2518, Feb. 1999). Dusseault explains the WebDAV protocol's strengths and weaknesses with remarkable clarity.

WebDAV stands for Web-based distributed authoring and versioning, although the standard doesn't include versioning support itself. The DeltaV standard, which is defined in a different RFC document (G. Clemm et al., Versioning Extensions to WebDAV (http://www.ietf.org/rfc/rfc3253.txt), RFC 3253, Mar. 2002), addresses versioning issues separately.

The introduction asserts that "the Web was intended from the start to be an editable medium as well as a browsable one." However, the standard HTTP used by Web servers and browsers only partially supports an editable Web. HTTP lacks important authoring features. On the one hand, it includes a reduced set of file management services (just compare HTTP with the standard FTP to discover its limitations). On the other hand, HTTP completely lacks support for multiple authors working on—and producing different versions of—the same document. You can find these features in any configuration management system or version control system, but they aren't standardized (or at least, they weren't until the advent of WebDAV and its DeltaV extension).

Basically, WebDAV extends the standard HTTP with seven new methods that intend to make it the successor to FTP. It provides enough functionality to become a de facto standard for network file systems over the Internet. In fact, it allows the typical operations you might expect from any file system, such as hierarchically organizing files or providing a locking mechanism to ensure exclusive access to a shared resource.

Apart from those predictable features, it provides support for attaching metadata to any resource. You can attach customizable WebDAV properties, which are expressed in XML, to any resource available from a WebDAV server. In other words, you can define as many properties as you might need. These properties tag your resources and help you organize them.

Separate standards are addressing the hairiest WebDAV-related issues as "WebDAV extensions," including the aforementioned DeltaV standard for WebDAV versioning and the WebDAV access control list proposal for access control settings (G. Clemm et al., Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol (http://www.ietf.org/rfc/rfc3744.txt), RFC 3744, May 2004).

The DeltaV standard is crucial for configuration management and content management vendors. Consequently, it exhibits some of the worst design-by-committee standards features in its lengthy 118 pages. For instance, some reports defined for the DeltaV report method should have been left out of the core standard. Just take a look at Figure 12-12 on page 333 of Dusseault's book, and you'll understand what I mean by "design by committee"—five different recommended features packages instead of a unified, simple conceptual model.

With respect to the ACL proposal, which addresses a particularly hard area to standardize, WebDAV access control aims for both Network File System and NT File System compatibility. Such compatibility expectations require a fine-grained design that lets users create custom permission mappings to a known set of standard permissions. These standard permissions include the ability to read a particular resource, change its properties, modify its content, read its access control list, or remove its associated lock without being the lock owner. The proposed solution is elegant and powerful enough to deserve more exposure and use in practice, even outside WebDAV's scope. In sharp contrast to the somewhat Byzantine DeltaV standard, the ACL proposal provides an integrated, well-defined access control approach.

Although it's too early to say whether WebDAV will become a cornerstone for the Web of the future, it extends HTTP in useful and interesting ways. It's yet to be seen if Microsoft's "embrace and extend strategy" will help the mainstream adoption of WebDAV or their proprietary extensions will cause insurmountable compatibility problems. In any case, Dusseault has written a comprehensive WebDAV guide that discusses protocol details, discloses potential interoperability problems, cursorily overviews products and tools, and even explains where—and where not—to use WebDAV. Her book is valuable even if you're interested only in the standard HTTP protocol's innards.

Fernando Berzal is an assistant professor in the Dept. of Computer Science and Artificial Intelligence at the University of Granada and a cofounder of iKor Consulting, an IT services and consulting firm. Contact him at berzal@acm.org.