Datatypes for WebDAV properties

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This specification extends the Web Distributed Authoring Protocol (WebDAV) to support datatyping. Protocol elements are defined to let clients and servers specify the datatype, and to instruct the WebDAV method PROPFIND to return datatype information.¶

This specification builds on the infrastructure provided by the WebDAV Distributed Authoring Protocol, adding support for data-typed properties.¶

Although servers must support XML content in property values, it may be desirable to persist values as scalar values when possible, and to expose the data's type when the property value is returned to the client. The client is free to ignore this information, but it may be able to take advantage of it when modifying a property.¶

On the other hand, when setting new properties, it can be desirable to pass data type information along with the value. A server can take advantage of this information to optimize storage and to perform additional parsing (for instance of dates). Servers that support searching can also take advantage of known data types when doing comparisons and sorting.¶

The following potential datatyping related features were deliberately considered out of scope: ¶

• getting "schema" information for classes of resources (set of "required" properties, their types, display information),
• definition of a set of mandatory property types,
• discovery of supported property types,
• extensions to PROPPATCH that would allow updates to parts of a (structured) property.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

The term "property element" refers to the XML element that identifies a particular property, for instance¶

The term "prop element" is used for the WebDAV "prop" element as defined in section 12.11 of [RFC2518].¶

The XML representation of schema components uses a vocabulary identified by the namespace name "http://www.w3.org/2001/XMLSchema". For brevity, the text and examples in this specification use the prefix "xs:" to stand for this namespace; in practice, any prefix can be used. "XML Schema: Structures" ([XS1]) also defines several attributes for direct use in any XML documents. These attributes are in a different namespace named "http://www.w3.org/2001/XMLSchema-instance". For brevity, the text and examples in this specification use the prefix "xsi:" to stand for this latter namespace; in practice, any prefix can be used.¶

Although WebDAV property types can be anything that can be marshaled as content of an XML element, in many cases they actually are simple types like integers, booleans or dates. "XML Schema Part 2: Datatypes" [XS2] defines a set of simple types which can be used as a basis for supplying type information to attributes.¶

Data type information is represented using the attribute "type" from the XML Schema namespace "http://www.w3.org/2001/XMLSchema-instance". In XML Schema, data types are qualified names, and the XML Schema recommendation defines a set of built-in datatypes (section 3 of [XS2]), defined in the namespace "http://www.w3.org/2001/XMLSchema".¶

To avoid unnecessary verbosity, data type information should only be supplied if it adds usable information to the protocol. In particular, type information is not required for live properties defined in WebDAV [RFC2518] and for properties of type "xs:string".¶

A server may implement any combination of datatypes, both from the XML Schema recommendation and possibly from other namespaces.¶

Note that a particular property can be typed for a number of reasons: ¶

• The property is a live property with server-defined semantics and value space.
• The property may have been set using a non-WebDAV protocol that the server understands in addition to WebDAV.
• The type may have been specified in an extended PROPPATCH method as defined in Section 4.

If the property element has an XML attribute named "xsi:type", the server may use this information to select an optimized representation for storing the property value. For instance, by specifying a type as "xs:boolean", the client declares the property value to be of type boolean (as defined in [XS2]). The server may choose any suitable internal format for persisting this property, and in particular is allowed to fail the request if the format given does not fit the format defined for this type.¶

The server should indicate successful detection and parsing of the typed value by setting the xsi:type attribute on the property element in the response body (this implies that it should return a MULTISTATUS status code and a <multistatus> response body).¶

PROPFIND is extended to return the data type information for properties by adding "xsi:type" attributes to the property elements unless one of the following conditions is met: ¶

• The data type MUST be different from "xs:string" (because this can be considered the default data type).
• The property's data type MUST NOT be defined in [RFC2518] (because these types are already well-defined).

Servers that support other methods using the DAV:multistatus response format (such as the REPORT method defined in [RFC3253], section 3.6) SHOULD apply the same extensions as defined in Section 5.¶

This part of this specification does not introduce any new protocol elements, nor does it change the informal WebDAV DTD. It merely specifies additional server semantics for the case where clients submit additional data type information in an attribute on the property element (previously undefined), and adds an additional attribute on property elements upon PROPFIND.¶

Clients not aware of datatype handling should not supply the "xsi:type" attribute on property elements (after all, this attribute belongs to the XML Schema-Instance namespace which has been defined for exactly this purpose). Old clients should also ignore additional attributes on property elements returned by PROPFIND (and similar methods), although the WebDAV specification only defines this behaviour for unknown elements (and is silent about unknown attributes).¶

Servers not aware of datatype handling either drop the "xsi:type" attribute, or persist it along with the property value. However, they will never indicate successful parsing of the data type by returning back the type in the response to PROPPATCH. Thus, clients can supply type information without having to poll for server support in advance.¶

This proposal builds on [RFC2518], and inherits its internationalizability.¶

This proposal does not introduce any new IANA considerations, since it does not specify any new namespaces (in the general sense), but merely uses existing ones.¶

This draft has benefited from thoughtful discussion by Lisa Dusseault, Stefan Eissing, Eric Sedlar and Kevin Wiggen.¶

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