WEBDAV Working Group J. Slein INTERNET DRAFT Xerox Corporation March 11, 1998 Expires September 16, 1998 Requirements for Advanced Collection Functionality in WebDAV Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or made obsolete by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this document is unlimited. Please send comments to the Distributed Authoring and Versioning (WebDAV) working group at , which may be joined by sending a message with subject "subscribe" to . Discussions of the WEBDAV working group are archived at URL: . Abstract The base WebDAV protocol [Goland et al., 1998] provides basic support for collections. It defines a MKCOL method for creating collections and specifies how other HTTP and WebDAV methods interact with collections. It supports only internal members of collections, which it defines as members whose URIs are immediately relative to the URI of the collection. This draft sets out requirements for more advanced, optional collection functionality. It extends the base functionality in two general directions: support for referential members, and support for ordered collections. A separate WebDAV specification is expected to define protocol elements providing the functionality described here. Slein [Page 1] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 1 Terminology The terminology used here differs from that in the base WebDAV protocol specification [Goland et al., 1998], particularly in its definition of internal member resource. Collection A resource that contains member resources Member Resource A resource contained by a collection Referential Member Resource A member resource that has no content of its own, but rather is a reference to another resource Internal Member Resource A member resource that either has content of its own or is empty, but is not a reference to another resource Target Resource The resource referenced by a referential member of a collection 2 Introduction and Rationale The simple collections that the base WebDAV specification supports are powerful enough to be widely useful. They provide for the hierarchical organization of resources, with mechanisms for creating and deleting collections, copying and moving them, locking them, adding resources to them and deleting resources from them, and getting listings of their members. Delete, copy, move, list, and lock operations can be applied recursively, so that a client can operate on whole hierarchies with a single request. Many applications, however, need more powerful collections. There are two areas in particular where more powerful functionality is often needed: referential members and ordering. This draft details the additional functionality that is needed in these two areas. The requirements have been discussed to some extent on the WebDAV mailing list, but still contain much that is controversial. Slein [Page 2] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 Consequently, this draft should be regarded primarily as a basis for further discussion. 2.1 Referential Members Referential members make it possible for many collections, on the same or different servers, to share the same resource. Because the collections share the resource by referencing it, only one physical copy of the resource need exist, and any changes made in the resource are visible from all the collections that reference it. So, for example, the mathematics department at one university can create a collection of resources on fractals that contains some local resources, but also references resources at several other universities. A manufacturing company develops and maintains its product maintenance manuals on the Web, with a separate collection for each product manual. Each manual is divided into sections, one section for every product component. Since many of the company's products contain some of the same components, many of the product maintenance manuals have sections in common. Each manual may have some unique sections, which are internal members of its collection. But for product components that are common to multiple products, the manual has a referential member that references a resource in a shared library. 2.2 Ordered Collections It is useful for many applications to be able to impose an ordering on a collection. In the product manual application above, the sections of each manual may be ordered so that they can be printed together as a book. A configuration management application might use a collection to represent a version series, in which case the "derives from" relationship might be represented as an ordering on the collection. A collection ordering may sometimes be based on property values. An example of such an ordering is one that is alphabetical by author's last name, or one from most recent to oldest last- modified-date. An ordering need not be based on property values, however. It may be purely arbitrary. A professor may order a collection of course readings in the sequence that makes sense to coordinate them with her lectures, not based on any property of the member resources. WebDAV already provides the tools for creating and maintaining ordered collections. One obvious approach is for an application to create a WebDAV property called "Order" on a collection Slein [Page 3] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 resource. The value of this property might be a list of the URIs of the collection members. What the base WebDAV specification does not do is standardize a single way to represent orderings for collections. Different applications and services should be able to operate on the same collection without private agreements about how to manage and examine its order. To make this possible, there needs to be a standard mechanism for manipulating and retrieving the order of a collection, and a standard representation of the ordering. In any situation where collaborative management of a collection takes place, and different authoring tools or WebDAV servers might be used by the collaborators, standardization is important. It is also important where a different tool may be used to view the collection from the one that was used to create it. So for example, two users from different organizations, using different authoring tools, are working together to create a collection. One of the tools uses a property on the collection called "Order" to store an ordering of the collection. The other tool uses a property on the member called "SequenceNumber". If each user adds some members to the collection, there will be no reliable ordering. 3 Requirements 3.1 Referential Members of Collections Requirements 3.1.1 - 3.1.5 follow naturally from the definition of Referential Member Resource. Although the behavior of referential members could be forced to be different from what is described in these requirements, the fact that referential members are references to other resources makes the behavior described here the natural one, and the easy one to implement. 3.1.1 The same resource may be referenced by referential members of multiple collections. This is the primary benefit that referential members bring. Resources can be shared by multiple collections, which may reside on the same server as the shared resource or on other servers. 3.1.2 The same resource may be referenced by more than one referential member of the same collection. It is often useful to allow the same resource to be referenced in a collection multiple times. Typically, these are cases where the Slein [Page 4] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 collection is ordered. Consider a case where a collection represents a book, with one member resource for each page in the book. A particular graphic needs to appear in several places in the book, and so needs to appear in the collection several times. 3.1.3 It is possible for the same resource to be an internal member of a collection and also to be referenced by one or more referential members of that same collection. In the example just described, the collection might contain the graphic as an internal member, which is also referenced by referential members of the same collection so that it can appear multiple times in the book. 3.1.4 Operations on a referential member do not affect the resource it references. There needs to be some way to operate on the referential member itself. If requests to the referential member were automatically redirected to its target resource, this would not be possible. In addition, passing operations through to the target resource can be problematic if the referential member and the target resource are on different servers. Issues about what credentials to use would need to be addressed. For some operations, an option to pass the operation through to the target resource may be desirable as an efficiency measure. For example, a lock operation on a collection might have a parameter that determines whether the targets of referential members should be locked. As long as requirement 3.1.5 is satisfied, however, the client can obtain the URI of the target resource and operate directly on the target. 3.1.5 For any referential member of a collection, it is possible to obtain the URI of the resource it references. This will allow clients to resolve references themselves in order to operate on the target resources. 3.1.6 It is possible to add a referential member to a collection. 3.1.7 It is possible to remove a referential member from a collection. It is important to note that this is a different operation from deleting the referential member's target resource. According to Slein [Page 5] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 requirement 3.1.4, operations on a referential member do not affect the target resource, so removing a referential member from a collection does not cause its target resource to be deleted. 3.1.8 It is possible for a referential member of a collection to carry its own properties, distinct from those of the resource it refers to. There are properties like "who added this resource to this collection" and "when was this resource added to this collection" that clearly belong to the referential member, and not to its target resource, which may be referenced by referential members of many collections. 3.1.9 A referential member of a collection also inherits the properties of the resource it refers to. We might expect internal member resources and the target resources of referential members to have properties like "Author" and "Title", but referential members themselves to have properties like "AddedBy" and "AddedDate". An end user who asked to see "Author" and "Title" for all members of a collection would expect to see these properties for both internal and referential members. This behavior will be easier for clients to provide if referential members inherit the properties of their target resources. Otherwise, the client must go to each target resource separately and retrieve its properties for display to the user. This requirement can be regarded as a useful exception to 3.1.4, with all the problems of violating 3.1.4. 3.1.10 A listing of the members of a collection shows both the internal members and the referential members. 3.1.11 Servers are encouraged to maintain referential integrity for referential members as far as possible, but are not required to do so. Broken references are, of course, undesirable. Breakage may be unavoidable when a target resource resides on a different server from the referential member that references it. Servers can, however, maintain the integrity of referential collection members when they receive MOVE or DELETE requests for resources under their own control. They are encouraged, but not required, to do so. 3.1.12 For any member of a collection, it is possible to discover whether it is an internal or a referential member. Slein [Page 6] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 Particularly if operations on referential members are not passed through to their targets, it is important for clients to be able to discover which members are referential. Then the client can resolve the references for the referential members to perform operations on their targets. 3.2 Ordered Collections Requirements 3.2.1 - 3.2.3 in this section push beyond what the WebDAV base specification provides, requiring standardization on a single mechanism for ordering collections. The remaining requirements describe what that standard mechanism must be like. 3.2.1 The ordering mechanism is sufficiently standardized that different applications and servers can operate on the same ordering without private agreements. Applications and servers can find out what orderings are present and can apply an ordering to a collection's members without private agreements. They can also modify an ordering, at least with the help of a human user for semantics (See 3.2.2), without private agreements. This is the minimum that is needed to support collaborative management of an ordered collection, where different authoring tools might be used by the collaborators. It is also what allows a different tool to be used to view the collection from the one that was used to create it. Finally, it is needed in order for servers to list collection members in order, as required by 3.2.3. 3.2.2 The semantics of an ordering are discoverable. If someone other than the creator of a collection adds a member to the collection, that person should be able to determine where it makes sense to position the new member in any orderings on the collection. In some cases it may be possible for the semantics to be expressed in a machine-usable way. These are likely to be only the cases where the ordering is based on property values. 3.2.3 When a client requests a listing of the members of a collection, it can request that a particular ordering be applied to the response. Although servers may ignore the request that the member listing be ordered, defining a request syntax that includes an ordering makes it possible for servers to free clients from the burden of applying the ordering to the member listing. Slein [Page 7] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 3.2.4 It is possible to order the members of a collection in an arbitrary way, not necessarily based on property values. A professor may order a collection of course readings in the sequence that makes sense to coordinate the readings with her lectures, not based on any property of the member resources. 3.2.5 Internal and referential members may by intermixed in the same ordering. The professor in the previous example may store some readings as internal resources of the collection, but reference others from servers at another university. Nevertheless, all the readings need to be included in the ordering for her students' use. 3.2.6 It is possible to impose multiple orderings on the same collection. Two instructors may teach different sections of the same course, and share a collection of readings. They may prefer to assign the readings in a different order. Then two orderings are needed for the same collection. 3.2.7 An ordering is not required to contain all members of the collection. Two instructors may teach different sections of the same course, and share a collection of readings. One instructor assigns his students only a subset of the readings, so his ordering contains only a subset of the collection members. 3.2.8 A collection member may belong to the same ordering more than once. An instructor may like to have her students read a particular article near the beginning of the course, and re-read it at the end. She would like the article to appear twice in her ordering of the collection. 3.2.9 It is possible to modify an existing ordering efficiently. The implementation of orderings and operations on them should minimize the number of round trips and the amount of data transferred when modifying an existing ordering. This includes cases where a single collection member is inserted into an ordering or removed from an ordering, as well as cases where many collection members are moved to different positions in the ordering. Slein [Page 8] INTERNET-DRAFT WebDAV Collection Requirements March 11, 1998 4 Issues What are we really trying to capture in distinguishing between internal and referential members? Are there important differences in functionality that are not captured by these requirements? What should be our position on passing requests through to the target of a referential member? Never? Only for certain operations? Provide an option for all / some operations? Is there any requirement for unnamed referential members of collections? How far should a server be expected to maintain orderings? If a member is deleted from a collection, should the server delete it from all orderings? Should a server perform all maintenance on orderings, with clients operating on them only indirectly by submitting requests to the server? 5 Acknowledgements This draft has benefited from thoughtful discussion by Ellis Cohen, Jim Davis, Daniel LaLiberte, Steve Martin, Surendra Koduru Reddy, John Turner, Jim Whitehead, and others. 6 References [Goland et al., 1998] Y. Y. Goland, E. J. Whitehead, Jr., A. Faizi, S. R. Carter, D. Jensen, "Extensions for Distributed Authoring on the World Wide Web - WebDAV." Draft-ietf-webdav- protocol-07. Microsoft, U.C. Irvine, Netscape, Novell. February, 1998. 7 Authors' Addresses J. Slein Xerox Corporation 800 Phillips Road Webster, NY 14580 Email: slein@wrc.xerox.com Expires September 16, 1998 Slein [Page 9]