WebDAV Ordered Collections Protocol

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Copyright © The Internet Society (1999). All Rights Reserved.¶

This specification extends the WebDAV Distributed Authoring Protocol to support server-side ordering of collection members. Of particular interest are orderings that are not based on property values, and so cannot be achieved using a search protocol's ordering option and cannot be maintained automatically by the server. Protocol elements are defined to let clients specify the position in the ordering of each collection member, as well as the semantics governing the ordering.¶

Distribution of this document is unlimited. Please send comments to the Distributed Authoring and Versioning (WebDAV) working group at w3c-dist-auth@w3.org, which may be joined by sending a message with subject "subscribe" to w3c-dist-auth-request@w3.org.¶

Discussions of the WEBDAV working group are archived at URL: http://lists.w3.org/Archives/Public/w3c-dist-auth/.¶

Since this document describes a set of extensions to the WebDAV Distributed Authoring Protocol [6], itself an extension to the HTTP/1.1 protocol, the augmented BNF used here to describe protocol elements is exactly the same as described in Section 2.1 of HTTP [5]. Since this augmented BNF uses the basic production rules provided in Section 2.2 of HTTP [5], these rules apply to this document as well.¶

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 [3].¶

This specification builds on the collection infrastructure provided by the WebDAV Distributed Authoring Protocol, adding support for the server-side ordering of collection members.¶

There are many scenarios where it is useful to impose an ordering on a collection at the server, such as expressing a recommended access order, or a revision history order. The members of a collection might represent the pages of a book, which need to be presented in order if they are to make sense. Or an instructor might create a collection of course readings, which she wants to be displayed in the order they are to be read.¶

Orderings may be based on property values, but this is not always the case. The resources in the collection may not have properties that can be used to support the desired ordering. Orderings based on properties can be obtained using a search protocol's ordering option, but orderings not based on properties cannot. These orderings generally need to be maintained by a human user.¶

The ordering protocol defined here focuses on support for such human-maintained orderings. Its protocol elements allow clients to specify the position of each collection member in the collection's ordering, as well as the semantics governing the ordering. The protocol is designed to allow support to be added in the future for orderings that are maintained automatically by the server.¶

The remainder of this document is structured as follows: Section 3 defines terminology that will be used throughout the specification. Section 4 provides an overview of ordered collections. Section 5 describes how to create an ordered collection, and Section 6 discusses how to set a member's position in the ordering of a collection. Section 7 explains how to change a collection ordering. Section 8 discusses listing the members of an ordered collection. Sections 9 through 11 define the headers, properties, and XML elements needed to support ordered collections. Section 12 describes capability discovery. Sections 13 through 15 discuss security, internationalization, and IANA considerations. The remaining sections provide supporting information.¶

The terminology used here follows that in the WebDAV Authoring Protocol specification [6]. Definitions of the terms resource, Uniform Resource Identifier (URI), and Uniform Resource Locator (URL) are provided in URI [2].¶

Ordered Collection

A collection for which the results from a PROPFIND request are guaranteed to be in the order specified for that collection

Unordered Collection

A collection for which the client cannot depend on the repeatability of the ordering of results from a PROPFIND request

Client-Maintained Ordering

An ordering of collection members that is maintained on the server based on client requests specifying the position of each collection member in the ordering

Server-Maintained Ordering

An ordering of collection members that is maintained automatically by the server, based on a client's choice of ordering semantics

If a collection is unordered, the client cannot depend on the repeatability of the ordering of results from a PROPFIND request. By specifying an ordering for a collection, a client requires the server to follow that ordering whenever it responds to a PROPFIND request on that collection.¶

Server-side orderings may be client-maintained or server-maintained. For client-maintained orderings, a client must specify the ordering position of each of the collection's members, either when the member is added to the collection (using the Position header (Section 9.2)) or later (using the ORDERPATCH (Section 7.1) method). For server-maintained orderings, the server automatically positions each of the collection's members according to the ordering semantics. This specification supports only client-maintained orderings, but is designed to allow future extension to server-maintained orderings.¶

A collection that supports ordering is not required to be ordered. It is up to the client to decide whether a given collection is ordered and, if so, to specify the semantics to be used for ordering its members.¶

If a collection is ordered, each of its internal member URIs MUST be in the ordering exactly once, and the ordering MUST NOT include any URI that is not an internal member of the collection. The server is responsible for enforcing these constraints on orderings. The server MUST remove an internal member URI from the ordering when it is removed from the collection. The server MUST an internal member URI to the ordering when it is added to the collection.¶

Only one ordering can be attached to any collection. Multiple orderings of the same resources can be achieved by creating multiple collections referencing those resources, and attaching a different ordering to each collection.¶

An ordering is considered to be part of the state of a collection resource. Consequently, the ordering is the same no matter which URI is used to access the collection and is protected by locks or access control constraints on the collection.¶

A PROPFIND request is used to retrieve a listing of the members of an ordered collection, just as it is used to retrieve a listing of the members of an unordered collection.¶

However, when responding to a PROPFIND on an ordered collection, the server MUST order the response elements according to the ordering defined on the collection. If a collection is unordered, the client cannot depend on the repeatability of the ordering of results from a PROPFIND request.¶

In a response to a PROPFIND with Depth: infinity, members of different collections may be interleaved. That is, the server is not required to do a breadth-first traversal. The only requirement is that the members of any ordered collection appear in the order defined for the collection. Thus for the hierarchy illustrated in the following figure, where collection A is an ordered collection with the ordering B C D,¶

it would be acceptable for the server to return response elements in the order A B E C F G H D. In this response, B, C, and D appear in the correct order, separated by members of other collections. Clients can use a series of Depth: 1 PROPFIND requests to avoid the complexity of processing Depth: infinity responses based on depth-first traversals.¶

Sections 9.1 and 15 of WebDAV [6] describe the use of compliance classes with the DAV header in responses to OPTIONS, to indicate which parts of the Web Distributed Authoring protocols the resource supports. This specification defines an OPTIONAL extension to WebDAV [6]. It defines a new compliance class, called orderedcoll, for use with the DAV header in responses to OPTIONS requests. If a collection resource does support ordering, its response to an OPTIONS request may indicate that it does, by listing the new ORDERPATCH method as one it supports, and by listing the new orderedcoll compliance class in the DAV header.¶

When responding to an OPTIONS request, only a collection or a null resource can include orderedcoll in the value of the DAV header. By including orderedcoll, the resource indicates that its internal member URIs can be ordered. It implies nothing about whether any collections identified by its internal member URIs can be ordered.¶

This section is provided to make WebDAV applications aware of the security implications of this protocol.¶

All of the security considerations of HTTP/1.1 and the WebDAV Distributed Authoring Protocol specification also apply to this protocol specification. In addition, ordered collections introduce a new security concern. This issue is detailed here.¶

This specification follows the practices of WebDAV [6] in encoding all human-readable content using XML [4] and in the treatment of names. Consequently, this specification complies with the IETF Character Set Policy RFC2277 [1].¶

WebDAV applications MUST support the character set tagging, character set encoding, and the language tagging functionality of the XML specification. This constraint ensures that the human-readable content of this specification complies with RFC2277 [1].¶

As in WebDAV [6], names in this specification fall into three categories: names of protocol elements such as methods and headers, names of XML elements, and names of properties. Naming of protocol elements follows the precedent of HTTP, using English names encoded in USASCII for methods and headers. The names of XML elements used in this specification are English names encoded in UTF-8.¶

For error reporting, WebDAV [6] follows the convention of HTTP/1.1 status codes, including with each status code a short, English description of the code (e.g., 423 Locked). Internationalized applications will ignore this message, and display an appropriate message in the user's language and character set.¶

This specification introduces no new strings that are displayed to users as part of normal, error-free operation of the protocol.¶

For rationales for these decisions and advice for application implementors, see WebDAV [6].¶

This document uses the namespaces defined by WebDAV [6] for properties and XML elements. All other IANA considerations mentioned in WebDAV [6] also apply to this document.¶

To be supplied by the RFC Editor.¶

To be supplied by the RFC Editor.¶

This draft has benefited from thoughtful discussion by Jim Amsden, Steve Carter, Tyson Chihaya, Ken Coar, Ellis Cohen, Bruce Cragun, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Roy Fielding, Yaron Goland, Fred Hitt, Alex Hopmann, Marcus Jager, Chris Kaler, Manoj Kasichainula, Rohit Khare, Daniel LaLiberte, Lisa Lippert, Steve Martin, Larry Masinter, Jeff McAffer, Surendra Koduru Reddy, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness, John Stracke, John Tigue, John Turner, Kevin Wiggen, and others.¶

Copyright © The Internet Society (1999). All Rights Reserved.¶

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