HTTP Working Group                                 E. J. Whitehead, Jr.
INTERNET-DRAFT                                     U.C. Irvine
<draft-whitehead-http-distreq-00.txt>              September 1996

Expires March, 1997

Rev. 0.4, September 27, 1996

Requirements on HTTP for Distributed Content Editing

Status of this Memo

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The HyperText Transfer Protocol, version 1.1 (HTTP/1.1), provides simple support for applications which allow remote editing of typed data. In practice, the existing capabilities of HTTP/1.1 have proven inadequate to support efficient, scalable remote editing free of overwriting conflicts. This document presents a list of features in the form of requirements which, if implemented, would improve the efficiency of common remote editing operations, provide a locking mechanism to prevent overwrite conflicts, improve relationship management support between non-HTML data types, provide a simple attribute-value metadata facility, and provide for the creation and reading of container data types. These requirements are also supportive of versioning capability.

1. Introduction

This document describes functionality which, if provided in the HyperText Transfer Protocol (HTTP) [4], would support the interoperability of tools which allow remote loading, editing and saving (publishing) of various media types using HTTP. As much as possible, this functionality is described without suggesting a proposed implementation, since there are many ways to perform the functionality within the HTTP framework. It is also possible that a single mechanism within HTTP could simultaneously satisfy several requirements.

Much of the functionality described in this document stems from the assumption that people performing distributed authoring only have access to the objects they are editing via the HTTP protocol. This is in contrast to the majority of current authoring practice, where there is access to the underlying storage media, often with a shell or graphical user interface mediating access to a filesystem. Authors need more than just remote control over their individual documents: they need remote control over the namespace in which those documents reside. Currently, authors control their namespace by interacting directly with the underlying storage system, but when performing distributed authoring this access is not available.

2. Requirements

In the requirement descriptions below, the requirement will be stated, followed by its rationale. If any current distributed authoring tools currently implement the requirement, this is also mentioned. It is assumed that "server" means "a program which receives and responds to HTTP requests," and that "distributed authoring tool" or "intranet enabled tool" means "a program which can retrieve a source resource via HTTP, allow editing of this resource, and then save/publish this resource to a server using HTTP." A "client" is "a program which issues HTTP requests and accepts responses."

  1. Source Retrieval. The source of any given resource should be retrievable via HTTP.

    There are many cases where the source resource stored on a server does not correspond to the entity transmitted in response to an HTTP GET. Current known cases are server side include directives, and Standard Generalized Markup Language (SGML) source entities which are converted on the fly to HyperText Markup Language (HTML) [2] output entities. There are many possible cases, such as automatic conversion of bitmap images into several variant bitmap media types (e.g. GIF, JPEG), and automatic conversion of an application's native media type into HTML. As an example of this last case, a word processor could store its native media type on a server which automatically converts it to HTML. A GET of this resource would retrieve the HTML. Retrieving the source of this resource would result in the transmission of the native word processor entity.

    This requirement should be met by a general mechanism which can handle both the "single-step" source processing described above, where the source is converted into the transmission entity via a single conversion step, as well as "multi-step" source processing, where there are one or more intermediary processing steps and outputs. An example of multi-step source processing is the relationship between an executable binary image, its object files, and its source language files. It should be noted that the relationship between source resource and transmission entity could be expressed using the functionality described below in "Relationships."

  2. Relationships. Via HTTP, it should be possible to create, query, list, and delete typed relationships between resources of any media type.

    A hypertext link is a relationship between resources which is browsable using a hypertext style point-and-click user interface. Relationships, whether they are browsable hypertext links, or simply a means of capturing a interrelation between entities, have many purposes. Relationships can support pushbutton printing of a multi-resource document in a prescribed order, jumping to the access control page for a resource, and quick browsing of related information, such as a table of contents, an index, a glossary, help pages, etc. While relationship support is provided by the HTML "LINK" element, this is limited only to HTML entities, and does not support bitmap image types, and other non-HTML media types.
    AOLpress from America Online [1] currently "allows pages to add toolbar buttons on the fly using the HTML 3.2 <LINK REL....> tag. For example, your page can add toolbar buttons that link to a home page, table of contents, index, glossary, copyright page, next page, previous page, help page, higher level page, or a bookmark in the document."

  3. Write Locks. It should be possible, via HTTP, to restrict modification of a resource to a specific person, or list of persons. It should be possible to set single or multi-person write locks with a single action.
  4. No-Modify Locks. It should be possible, via HTTP, to indicate to the HTTP server that the contents of a resource should not be modified until the read lock is released. It should be possible to assign a no-modify lock to a single person or a list of persons with a single action.
  5. Read Locks. It should be possible, via HTTP, to restrict the ability to read a resource to a specific person, or list of persons. It should be possible to set single or multi-person read locks with a single action.
  6. Partial-Resource Locking. It should be possible to take out a write or a read lock on subsections of a resource.
  7. Lock Query. It should be possible to query for whether a given resource has any active modification restrictions, and if so, who currently has modification permission.
  8. Independence of locks. It should be possible to lock a resource without re-reading the resource, and without committing to editing the resource.
  9. Multi-Resource Locking. It should be possible to take out a lock on multiple resources in the same action, and this locking operation must be atomic across these resources.

    At present, HTTP provides limited support for preventing two or more people from overwriting each other's modifications when they save to a given URL. Furthermore, there is no way for people to discover if someone else is currently making modifications to a resource. This is known as the "lost update problem," or the "overwrite problem." Since there can be significant cost associated with discovering and repairing lost modifications, preventing this problem is crucial for supporting distributed authoring. A "write" lock ensures that only one person (or list of persons) may modify an resource, preventing overwrites. Furthermore, locking support is also a key component of many versioning schemes, a desirable capability for distributed authoring.

    An author may wish to lock an entire web of resources even though they are editing just a single resource, to keep the other resources from changing. In this way, an author can ensure that if a local hypertext web is consistent in their distributed authoring tool, it will then be consistent when they write it to the server. Because of this, it should be possible to take out a lock without also causing transmission of the contents of a resource. Since it should not be assumed that because an resource is locked, that it will necessarily be modified, and since many people may wish to have simultaneous guarantees that a resource will not be modified, but still not want to modify the resource themselves, it is desirable to have a "no-modify" lock capability. A no-modify lock, by being less restrictive, provides better support than a write lock for providing a guarantee that a resource will not be modified. Since it is prudent to frequently save working drafts of a resource, yet undesirable in many cases for working drafts to be readable by persons other than the author, a "read" lock allows a resource to have only a limited set of people (perhaps only the author) who can read the current state of the resource. In summary, a no-modify lock states that the resource is guaranteed not to change for the duration of the lock. A read lock guarantees that only the owner of the lock may read the resource (but there is no guarantee it will not be modified). A write lock states that a resource is guaranteed not to change only if the owner of the lock does not change it, and only the owner of the lock may change it.

    It is often necessary to guarantee that a lock or unlock operation occurs at the same time across multiple resources, a feature which is supported by the multiple-resource locking requirement. This is useful for preventing a collision between two people trying to establish locks on the same set of resources, since with multi-resource locking, one of the two people will get a lock. If this same multiple-resource locking scenario was repeated by using atomic lock operations iterated across the resources, the result would be a splitting of the locks between the two people, based on resource ordering and race conditions.

    Partial resource locking provides support for collaborative editing applications, where multiple users may be editing the same resource simultaneously. Partial resource locking also allows multiple people to simultaneously work on a database type resource.

    When more than one types of lock is requested on a resource, it results in either multiple, complimentary locks, or lock collisions. An implementation of the locking functionality must specify how the locks interact, noting which combinations result in collisions, and which combinations are complimentary.
  10. Partial Write. After editing a resource, it should be possible, via HTTP, to only write the changes to a resource, rather than retransmitting the entire resource.

    During distributed editing which occurs over wide geographic separations and/or over low bandwidth connections, it would be extremely inefficient (and frustrating) to rewrite a large resource after minor changes, such as a one-character spelling correction. Ideally, support will be provided for transmitting "insert" (e.g., add this sentence in the middle of a document) and "delete" (e.g. remove this paragraph from the middle of a document) style updates. These changes could be as small as a byte, or arbitrarily large (perhaps even larger than the original resource), and could result in an arbitrarily large growth or shrinkage of the resource. Support for partial resource updates will make small edits more efficient, and allow distributed authoring tools to scale up for editing of large documents.

  11. Attributes. Via HTTP, it should be possible to create, modify, query, read and delete arbitrary attributes on resources of any media type.

    Attributes can be used to define fields such as author, title, subject, and organization, on resources of any media type. These attributes have many uses, such as supporting searches on attribute contents, and the creation of catalog entries as a placeholder for an resource which is not available in electronic form, or which will be available later.

  12. List URL Hierarchy Level. A listing of all resources, along with their media type, and last modified date, which are located at a specific URL [3] hierarchy level in an http URL scheme should be accessible via HTTP, so long as this operation is meaningful.

    In [3] it states that, "some URL schemes (such as the ftp, http, and file schemes) contain names that can be considered hierarchical." Especially for HTTP servers which directly map all or part of their URL name space into a filesystem, it is very useful to get a listing of all resources located at a particular hierarchy level. This functionality supports "Save As..." dialog boxes, which provide a listing of the resources at a current hierarchy level, and allow navigation through the hierarchy. It also supports the creation of graphical visualizations (typically as a network) of the hypertext structure among the resources at a hierarchy level, or set of levels. It also supports a tree visualization of the resources and their hierarchy levels.

    There are many instances where there is not a strong correlation between a URL hierarchy level and the notion of a container. One example is a server in which the URL hierarchy level maps to a computational process which performs some resolution on the name. In this case, the contents of the URL hierarchy level can vary depending on the input to the computation, and the number of resources accessible via the computation can be very large. It does not make sense to implement a directory feature for such a namespace. However, the utility of listing the contents of those URL hierarchy levels which do correspond to containers, such as the large number of HTTP servers which map their namespace to a filesystem, argue for the inclusion of this capability, despite not being meaningful in all cases. If listing the contents of a URL hierarchy level does not makes sense for a particular URL, then a "405 Method Not Allowed" status code could be issued.

    AOLpress from America Online currently supports "Save As..." dialog boxes, and graphical network visualization of a portion of a site's hypertext structure, which they term a "mini-web."
    FrontPage from Microsoft [5] also currently supports a graphical network visualization and additionally supports a tree visualization of a portion of a site's structure.

  13. Make URL Hierarchy Level. Via HTTP, it should be possible to create a new URL hierarchy level in an http URL scheme.

    The ability to create containers to hold related resources supports management of a name space by packaging its members into small, related clusters. The utility of this capability is demonstrated by the broad implementation of directories in recent operating systems. The ability to create a URL hierarchy level also supports the creation of "Save As..." dialog boxes with "New Level/Folder/Directory" capability, common in many applications.
    AOLpress from America Online, currently supports this capability through their "Save As..." dialog box, and their custom MKDIR method.

  14. Copy. Via HTTP, it should be possible to make a byte-for-byte duplicate of a resource without a client loading, then resaving the resource. This copy should leave an audit trail.

    There are many reasons why a resource might need to be duplicated, such as change of ownership, a precursor to major modifications, or to make a backup. In combination with delete functionality, copy can be used to implement rename and move capabilities, by performing a copy to a new name, and a delete of the old name. Due to network costs associated with loading and saving a resource, it is far preferable to have a server perform a resource copy than a client. If a copied resource records which resource it is a copy of, then it would be possible for a cache to avoid loading the copied resource if it already locally stores the original.
    Note that in the case of CGI scripts, or other executable content, the intent of the copy operation is to make a byte-for-byte copy of the source of the resource, rather than require the server to guarantee that the resource will execute in the same manner in the new location as it did in the old location. It is the responsibility of the user to ensure the resource behaves properly in its new location.
  15. Move/Rename. Via HTTP, it should be possible to change the URL of a resource without a client loading, then resaving the resource under a different name.

    It is often necessary to change the name of a resource, for example due to adoption of a new naming convention, or if a typing error was made entering the name originally. Due to network costs, it is undesirable to perform this operation by loading and resaving the resource under the new name, followed by a delete of the old resource. Similarly, a single rename operation is more efficient than a copy followed by a delete operation. Ideally an HTTP server should record the move operation, and issue a "301 Moved Permanently" status code for requests on the old URL. A move operation, if implemented with attribute support, should also preserve most attributes across a move. Note that moving a resource is considered the same function as renaming an resource.

3. Acknowledgments

My understanding of these issues has emerged as the result of much thoughtful discussion, email, and assistance by many people, who deserve recognition for their effort.

Martin Cagan, Continuus Software,
Dan Connolly, World Wide Web Consortium,
David Durand, Boston University,
Ron Fein, Microsoft,
David Fiander, Mortice Kern Systems,
Roy Fielding, U.C. Irvine,
Yaron Goland, Microsoft,
Phill Hallam-Baker, MIT,
Dennis Hamilton, Xerox PARC,
Andre van der Hoek, University of Colorado, Boulder,
Gail Kaiser, Columbia University,
Rohit Khare, World Wide Web Consortium,
Dave Long, America Online,
Henrik Frystyk Nielsen, World Wide Web Consortium,
Ora Lassila, Nokia Research Center,
Larry Masinter, Xerox PARC,
Murray Maloney, SoftQuad,
Jim Miller, World Wide Web Consortium,
Andrew Schulert, Microsoft,
Christopher Seiwald, Perforce Software,
Judith Slein, Xerox,
Richard Taylor, U.C. Irvine,
Robert Thau, MIT,
Fabio Vitali, University of Bologna, Italy,

4. References

[1] America Online, "AOL Web Tools -- AOLpress 1.2 Features." WWW page.

[2] T. Berners-Lee, D. Connolly. "HyperText Markup Language Specification - 2.0." RFC 1866, MIT/LCS, November 1995.

[3] T. Berners-Lee, L. Masinter, M. McCahill. "Uniform Resource Locators (URL)." RFC 1738, CERN, Xerox PARC, University of Minnesota, December 1994.

[4] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and T. Berners-Lee. "Hypertext Transfer Protocol -- HTTP/1.1." RFC XXXX, U.C. Irvine, DEC, MIT/LCS, August 1996.

[5] Microsoft. "Microsoft FrontPage for Windows Data Sheet." WWW page.

Author's Address

E. James Whitehead, Jr.
Department of Information and Computer Science
University of California
Irvine, CA 92697-3425

Phone: 714-824-4121
Fax: 714-824-4056