WEBDAV Working Group J. Slein INTERNET DRAFT Xerox Corporation May 20, 1998 Expires November 20, 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), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe),munnari.oz.au (Pacific Rim), ftp.ietf.org (US EastCoast), 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. 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. Slein Page 1 INTERNET-DRAFT WebDAV Collection Requirements May 1998 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 Strong Reference A reference whose referential integrity is guaranteed by the server Weak Reference A reference whose referential integrity is not guaranteed by the server 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. 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 Slein Page 2 INTERNET-DRAFT WebDAV Collection Requirements May 1998 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 For many applications, it is useful 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. A professor may order a collection of course readings in the sequence that makes sense to coordinate them with her lectures, where there is no property on the member resources that could be used to create this ordering. This set of requirements is primarily concerned with orderings that are not based on property values. Another useful distinction is between server-maintained and client-maintained orderings. In server-maintained orderings, the server enforced the semantics of the ordering by placing each collection member at the appropriate position in the ordering; clients cannot alter the ordering. In client-maintained orderings, the client places each collection member in the ordering based on its understanding of the semantics of the ordering; the server does nothing to validate the client's positioning of the member in the ordering. This set of requirements is concerned only with client-maintained orderings. WebDAV already provides tools that could be used for creating and maintaining ordered collections. For example, using only the base WebDAV specification, an application could create a WebDAV property called "Order" on a collection 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 Slein Page 3 INTERNET-DRAFT WebDAV Collection Requirements May 1998 the same collection without private agreements about how to manage and examine its order. To make this possible, there needs to be a standard way to manipulate and retrieve 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 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 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. Slein Page 4 INTERNET-DRAFT WebDAV Collection Requirements May 1998 Passing operations through to the target resource would expose servers to the risks of circular references and long chains of references that refer to other references. 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. Requirement 3.1.6 makes it unnecessary for the server to pass operations through to the target resource. The client can always obtain the URI of the target resource and operate directly on the target. 3.1.5 Operations on a resource do not affect weak references to them. This requirement is the mirror of 3.1.4. Just as we do not expect operations on a reference to affect its target, so we do not expect operations on a target to affect references to it. Locking a target does not cause the references to it to be locked. Modifying the properties of a target does not cause changes in the properties of references to it. Etc. Just as in 3.1.4 passing operations through to the target can be problematic, so here reflecting operations back to the referencing 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. The requirement cannot apply to all referential members, however. Section 1 defined weak references and strong references. Strong references are those whose referential integrity is guaranteed by the server. Weak references are those whose referential integrity is not guaranteed by the server. For strong references, some operations on targets must cause changes in references to them. For strong references, if the target is moved, the reference must change to reflect the new location of the target, for example. Requirement 3.1.11 makes it possible that some servers will support strong references. Consequently, the requirement cannot apply to all referential members, but only to weak references. 3.1.6 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. 3.1.7 It is possible to add a referential member to a collection. 3.1.8 It is possible to remove a referential member from a collection. It is important to note that this is a different operation from Slein Page 5 INTERNET-DRAFT WebDAV Collection Requirements May 1998 deleting the referential member’s target resource. According to 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. 3.1.9 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. 3.1.10 A listing of the members of a collection shows both the internal members and the referential members. A listing of collection members with Depth = 1 or Depth = infinity shows all members of the collection, whether internal or external. If Depth = infinity, however, the listing will not follow references into any collections to which they may refer. This is to avoid the risk of being caught in a circle of references. 3.1.11 It is possible to request creation of a referential member that the server will guarantee to have referential integrity. For some applications, broken references are unacceptable. 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. For applications that require referential integrity, it must be possible to specify in a request for creation of a referential member that its integrity be guaranteed. If the server cannot honor this request, it must decline to create the referential member. A referential member whose integrity is guaranteed by the server is called a strong reference. 3.1.12 For any member of a collection, it is possible to discover whether it is an internal or a referential member. Since 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. 3.1.13 It is possible to discover whether a referential member is a strong reference or a weak reference. Knowing whether a referential member is strong or weak allows a client to intelligently choose its own strategy for working with referential members. For example, if a client does not know Slein Page 6 INTERNET-DRAFT WebDAV Collection Requirements May 1998 whether a particular reference is strong or weak, it may choose to recreate that referential member to be sure of referential integrity; but if it knows that the reference is strong, it will not bother to do this. 3.1.14 It is possible to discover whether a resource is the target of a strong reference. This requirement insures that both ends of a referential integrity relationship have the same information available. 3.1.15 A referential member of a collection is itself a resource. This makes explicit what is suggested by several of the preceding requirements. Requirement 3.1.9 in particular, that referential members carry their own properties, forces referential members to be resources. WebDAV properties can belong only to resources. 3.2 Ordered Collections 3.2.1 Ordering is sufficiently standardized that different applications and servers can operate on the same ordering without private agreements. Applications and servers can apply an ordering to a collection’s members or discover the ordering of 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 3.2.3), 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 3.2.6. 3.2.2 A collection is not required to be ordered. A WebDAV server may support collections without supporting ordered collections. Even if the server supports ordered collections, there is no requirement that every collection on that server be ordered. Since these requirements concern only client-maintained orderings, clients will decide whether any given collection is ordered. The remaining requirements apply only to collections that are ordered. 3.2.3 The semantics of an ordering are discoverable. The semantics of an ordering is the principle or rule according to which the collection members are ordered. This principle must be discoverable if someone (or some application) other than the one that created a collection is to be able to add a member to it and Slein Page 7 INTERNET-DRAFT WebDAV Collection Requirements May 1998 determine where it makes sense to position the new member in the collection's ordering. In some cases it may be possible for the semantics to be expressed in a machine-usable way, so that an application could automatically position a new member in the ordering. In other cases the semantics may require a human user to apply them. In either case they should be discoverable. 3.2.4 Each collection member must appear in the ordering exactly once. It would be possible to support orderings that contain only a subset of the collection members, or orderings that can contain a single collection member more than once. It is not necessary, however, since the same result can be achieved by creating a new collection with exactly the desired members, and including each member of the new collection in its ordering exactly once. This requirement implies that the server will check, whenever a member is added to an ordering, to make sure that it is not already in the ordering. It also implies that either the protocol itself or the server will insure that whenever a new member is added to a collection, it is also added to the collection ordering. 3.2.5 An ordering must not include any resources that are not members of the collection. The server must insure that when a member is removed from a collection, it is also removed from the collection's ordering. 3.2.6 When a client requests a listing of the members of a collection, this listing is returned in the order specified by the collection. This requirement frees clients from the burden of applying the ordering to the member listing. 3.2.7 It is possible to order the members of a collection in a client-specified way, not necessarily based on property values. Orderings that are based on property values can be obtained by a search protocol that supports sorted result sets. This set of requirements is not concerned with such orderings. It is intended primarily to support orderings that cannot be obtained by sorting on property values. A property is not always available that can serve as the basis for a desired ordering. For example, a professor may wish to order a collection of course readings in the sequence that makes sense to coordinate the readings with her lectures. But the properties of resources at the Web site are standardized and do not include one that is appropriate to use for this purpose. Even if the professor in the example could create a Slein Page 8 INTERNET-DRAFT WebDAV Collection Requirements May 1998 "sequencenumber" property to use in sorting the collection, this strategy would be undesirable unless she knew she would not be adding any readings or changing the order of her lectures once the values of sequencenumber were set. Inserting a new reading into the sequence would require updating the sequencenumber property of each reading that comes after the new one in the sequence. Ordered collections are intended to support this sort of case, where sorting based on a property value is impossible or inefficient. 3.2.8 Internal and referential members may be 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. 4 Acknowledgements This draft has benefited from thoughtful discussion by Steve Carter, Ellis Cohen, Jim Davis, Spencer Dawkins, Yaron Goland, Alex Hopmann, Rohit Khare, Daniel LaLiberte, Steve Martin, Surendra Koduru Reddy, Nick Shelness, John Turner, Jim Whitehead, and others. 5 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-08. Microsoft, U.C. Irvine, Netscape, Novell. April, 1998. 6 Authors' Addresses J. Slein Xerox Corporation 800 Phillips Road Webster, NY 14580 Email: slein@wrc.xerox.com Expires November 20, 1998 Slein Page 9