19.1. WebDAV Server

Virtuoso supports the WebDAV protocol, which is an extension of HTTP for cooperative work on the Internet. DAV resources can be of any mime type, including binary types. The DAV resources are stored in the Virtuoso database as large objects, they are not in the file system and can only be accessed through the DAV protocol. Direct SQL access to the DAV tables is also possible, so there's a set of Virtuoso/PL procedures that acts as DAV API to let server-side applications access DAV. Virtuoso DAV can be extended by virtual collections. Instead of accessing DAV tables, DAV server can retrieve data from applications, thus an application can generate documents on demand and these documents will be available via DAV as well as plain DAV resources. Moreover, resources can be submitted directly to the application via DAV. Virtuoso DAV provides settable access rights and ownership of resources. Access rights as such are not covered by the DAV specification but Virtuoso implements both ACLs (access control lists) and a Unix file system like scheme for ownership and permissions. SQL accounts enabled for DAV are valid owners of DAV resources. A resource has both a user owner and a group owner, plus an optional access control list that can grant privileges to users and roles alike. A user can have DAV-only access, SQL/ODBC-only access, both of them or neither. User account information is stored in relational tables and can be manipulated from SQL or through a Web UI.

Virtuoso DAV can store metadata about resources. These metadata are extracted from resources automatically, and can be edited by users. In addition, users can place public and personal 'tags' on resources to categorize data according for personal needs without interference between users. Virtuoso DAV has a powerful and scalable search engine that let application locate resources that match given list of criteria. The search can process both plain DAV resources and data published by applications in virtual collections. Search criteria can check for resource properties, content, metadata and tags.

Certain special processing is provided for the following types of DAV resources:

Any textual content types are automatically indexed in a free text index.
Any XML content types are indexed in the same free text index as other text plus can be queried with the xcontains SQL predicate.
Some well-known types of documents are parsed in order to extract metadata, such as author of MS Office document or musical genre of MP3 audio file or resolution of an image.
VSP/VSPX pages - DAV resources can be dynamic web pages written in VSP or VSPX. Requesting such a page with GET or POST will execute the logic on the page. This is very convenient way of building web applications.

DAV resources are stored in an ordinary relational table. The text and XML query features used with DAV are separately usable from SQL on any table with the appropriate indexes and are thus not intrinsically related to DAV.

WebDAV (Web Distributed Authoring and Versioning) is a communication protocol for the Internet implemented as an extension to HTTP 1.1. The WebDAV specification was published by the Internet Engineering Task Force (IETF) in February 1999.

Most operating systems have support for accessing data hosted on DAV servers.

DAV was designed to provide more methods for handling server resources. In addition to the usual HTTP methods such as GET, POST, HEAD, PUT, DELETE, OPTIONS are methods for making directories (or collections), a lock mechanism, copying of resources and collections, predefined and userd-defined properties of resources and collections.

DAV consists of HTTP extensions, often with a message body containing XML. DAV also provides a basic schema for resource metadata by allowing arbitrary XML properties to be attached to resources.

Virtuoso supports the following HTTP methods:

Table 19.1. Features List - Virtuoso Web DAV HTTP Method Support

HTTP Method Description
HEAD Meta method for examining server properties or network behavior.
GET Retrieves documents from the server
POST, PUT Posts or puts documents on the server
MKCOL Creates a collection
MOVE For moving resources and/or collections within the server
DELETE Removes resources or collections of resources from the server
LOCK, UNLOCK Locks or unlocks a resource or collection to protect from modification by others
PROPFIND, PROPPATCH Retrieves and sets properties for collections and/or resources. In addition to arbitrary user-defined properties, Virtuoso supports two sets of predefined properties: standard DAV properties (type, creation time, modification time, size etc.) and Virtuoso-specific DAV properties (access control, tags, extensions for virtual collections.)
MGET (experimental) Retrieves RDF metadata of a document according to URIQA rules
MPUT/MDELETE (experimental) Updates RDF metadata of a document according to URIQA rules and appropriate RDF Schemas