WebDAV Ordered Collections Protocol

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Copyright © The Internet Society (2003). 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 [RFC2518], 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 [RFC2616]. Since this augmented BNF uses the basic production rules provided in Section 2.2 of HTTP, 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].¶

This document uses XML DTD fragments as a purely notational convention. WebDAV request and response bodies can not be validated due to the specific extensibility rules defined in section 23 of [RFC2518] and due to the fact that all XML elements defined by this specification use the XML namespace name "DAV:". In particular: ¶

1. element names use the "DAV:" namespace,
2. element ordering is irrelevant,
3. extension elements (elements not already defined as valid child elements) may be added anywhere, except when explicitly stated otherwise,
4. extension attributes (attributes not already defined as valid for this element) may be added anywhere, except when explicitly stated otherwise.

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. Section 9 discusses the impact on version-controlled collections (as defined in [RFC3253]. Section 10 describes capability discovery. Section 11 through Section 13 discuss security, internationalization, and IANA considerations. The remaining sections provide supporting information.¶

The terminology used here follows that in [RFC2518] and [RFC3253]. Definitions of the terms resource, Uniform Resource Identifier (URI), and Uniform Resource Locator (URL) are provided in [RFC2396].¶

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

This document uses the terms "precondition", "postcondition" and "protected property" as defined in [RFC3253]. Servers MUST report pre-/postcondition failures as described in section 1.6 of this document.¶

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 6)) or later (using the ORDERPATCH (Section 7) 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.¶

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 add 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.¶

The ORDERPATCH method is used to change the ordering semantics of a collection or to change the order of the collection's members in the ordering or both.¶

The server MUST apply the changes in the order they appear in the order XML element. The server MUST either apply all the changes or apply none of them. If any error occurs during processing, all executed changes MUST be undone and a proper error result returned.¶

If an ORDERPATCH request changes the ordering semantics, but does not completely specify the order of the collection members, the server MUST assign a position in the ordering to each collection member for which a position was not specified. These server-assigned positions MUST all follow the last one specified by the client. The result is that all members for which the client specified a position are at the beginning of the ordering, followed by any members for which the server assigned positions.¶

If an ORDERPATCH request does not change the ordering semantics, any member positions not specified in the request MUST remain unchanged.¶

A request to reposition a collection member at the same place in the ordering is not an error.¶

Additional Marshalling: ¶

• The request body MUST be DAV:orderpatch element.

• <!ELEMENT orderpatch (ordering-type?, order-member*) >
  
  <!ELEMENT order-member (segment, position) >
  <!ELEMENT position (first | last | before | after)>
  <!ELEMENT segment (#PCDATA)>
  <!ELEMENT first EMPTY >
  <!ELEMENT last EMPTY >
  <!ELEMENT before segment >
  <!ELEMENT after segment >
  

• PCDATA value: segment, as defined in section 3.3 of [RFC2396].
• The DAV:ordering-type property is modified according to the DAV:ordering-type element.
• The ordering of internal member URIs in the collection identified by the Request-URI is changed based on instructions in the order-member XML elements in the order they appear in the request. The order-member XML elements identify the internal member URIs whose positions are to be changed, and describe their new positions in the ordering. Each new position can be specified as first in the ordering, last in the ordering, immediately before some other internal member URI, or immediately after some other internal member URI.
• If a response body for a successful request is included, it MUST be a DAV:orderpatch-response XML element. Note that this document does not define any elements for the ORDERPATCH response body, but the DAV:orderpatch-response element is defined to ensure interoperability between future extensions that do define elements for the ORDERPATCH response body.

  <!ELEMENT orderpatch-response ANY>
  

• Since multiple changes can be requested in a single ORDERPATCH request, if any problems are encountered, the server MUST return a 207 (Multi-Status) response (defined in [RFC2518]), containing DAV:response elements for either the request-URI (when the DAV:ordering-type could not be modified) or URIs of collection members to be repositioned (when an individual positioning request expressed as DAV:order-member could not be fulfilled).

Preconditions: ¶

• (DAV:collection-must-be-ordered): see Section 6.1.
• (DAV:segment-must-identify-member): see Section 6.1.

Postconditions: ¶

• (DAV:ordering-type-set): if the request body contained a DAV:ordering-type element, the request MUST have set the DAV:ordering-type property of the collection to the value specified in the request.
• (DAV:ordering-modified): if the request body contained DAV:order-member elements, the request MUST have set the ordering of internal member URIs in the collection identified by the request-URI based on the instructions in the DAV:order-member elements.

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.¶

The Versioning Extensions to WebDAV [RFC3253] introduce the concept of versioned collections, recording both the dead properties and the set of internal version-controlled bindings. This section defines how this feature interacts with ordered collections.¶

This specification considers both the ordering type (DAV:ordering-type property) and the ordering of collection members to be part of the state of a collection. Therefore both MUST be recorded upon CHECKIN or VERSION-CONTROL, and both MUST be restored upon CHECKOUT, UNCHECKOUT or UPDATE (where for compatibility with RFC3253, only the ordering of version-controlled members needs to be maintained).¶

Sections 9.1 and 15 of [RFC2518] 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 [RFC2518]. It defines a new compliance class, called ordered-collections, 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 ordered-collections compliance class in the DAV header.¶

When responding to an OPTIONS request, only a collection or a null resource can include ordered-collections in the value of the DAV header. By including ordered-collections, 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.¶

Furthermore, RFC 3253 [RFC3253] introduces the live properties DAV:supported-method-set (section 3.1.3) and DAV:supported-live-property-set (section 3.1.4). Servers MUST support these properties as defined in RFC 3253.¶

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 [RFC2518] in encoding all human-readable content using [XML] and in the treatment of names. Consequently, this specification complies with the IETF Character Set Policy [RFC2277].¶

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].¶

As in [RFC2518], 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, [RFC2518] 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 [RFC2518].¶

This document uses the namespaces defined by [RFC2518] for properties and XML elements. All other IANA considerations mentioned in [RFC2518] 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, Geoff Clemm, Ken Coar, Ellis Cohen, Bruce Cragun, Jim Davis, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Lisa Dusseault, Chuck Fay, Roy Fielding, Yaron Goland, Fred Hitt, Alex Hopmann, Marcus Jager, Chris Kaler, Manoj Kasichainula, Rohit Khare, Daniel LaLiberte, 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 (2003). All Rights Reserved.¶

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