- From: Jim Whitehead <ejw@ics.uci.edu>
- Date: Wed, 04 Sep 1996 11:27:35 -0700
- To: w3c-dist-auth@w3.org
- cc: taylor@ics.uci.edu
Based on the comments I have received, I have modified "Requirements
on HTTP for Distributed Content Editing." At the bottom of this
message, please find draft v0.2 of this document.
Major changes from v0.1:
o Addition of new sections:
- Status of this Memo
- Acknowledgements
- References
- Author's Address
o Removed sections:
- Abstract
o New Requirements:
- Read Locks
- Lock Query
- Partial Write
o Addition of new rationale text for requirements:
- Source Retrieval
- Locks
- List URL Hierarchy level
I appreciate any comments you may have on this draft. In particular,
it would be very helpful to know if I have omitted any requirements.
Also, if you feel you have made a contribution towards this draft, but
are not listed in my acknowledgements section, please assume that you
have been accidentally omitted, and let me know.
It is my intent to submit this as an official Internet Draft either
later this week or early next week, so it may be considered by the HTTP
Working Group.
- Jim
HTTP Working Group E. J. Whitehead, Jr.
INTERNET-DRAFT U.C. Irvine
<draft-ietf-http-distreq-00.txt> September 1996
Expires March, 1996
Rev. 0.2, September 3, 1996
Requirements on HTTP for Distributed Content Editing
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 information as
Internet drafts.
Internet Drafts are draft documents valid for a maximum of six months and
can be updated, replaced or obsoleted by other documents at any time. It is
inappropriate to use Internet drafts as reference material or to cite them
as other than as "work in progress".
To learn the current status of any Internet draft please check the
"lid-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). Further information about the IETF can be found at URL:
http://www.ietf.org/
Distribution of this document is unlimited. Please send comments to the HTTP
working group at <http-wg@cuckoo.hpl.hp.com>. Discussions of the working
group are archived at <URL:http://www.ics.uci.edu/pub/ietf/http/>. General
discussions about HTTP and the applications which use HTTP should take place
on the <www- talk@w3.org> mailing list.
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 entity via HTTP, allow editing
of this entity, and then save/publish this entity 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 entity should be retrievable
via HTTP.
There are many cases where the source entity stored on a server does
not correspond to the actual 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 entity would retrieve the HTML. Retrieving the
source of this entity would retrieve the word processor native entity.
Ideally, this requirement will be satisfied 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 and
transmission entity could be expressed using the relationship
functionality described below in "Relationships."
2. Relationships. Via HTTP, it should be possible to create, query, and
delete typed relationships (links) between entities of any media type.
Relationships (or links which are not necessarily navigable) between
entities can be used for many purposes. Relationships can support
pushbutton printing of a multi-resource document in a prescribed order,
jumping to the access control page for an entity, 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 an entity to a specific person, or list of persons.
4. Read Locks. It should be possible, via HTTP, to indicate to the HTTP
server that the contents of an entity should not be modified.
5. Lock Query. It should be possible to query for whether a given URL has
any active modification restrictions, and if so, who currently has
modification permission.
6. Independence of locks. It should be possible to lock an entity without
re-reading the entity, and without committing to editing an entity.
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 an entity. 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 entity, 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 entities even though they
are editing just a single entity, to keep the other entities 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 an entity. Since it should not be assumed that because an
entity is locked, that it will necessarily be modified, and since many
people may wish to have simultaneous guarantees that an entity will not
be modified, but still not want to modify the entity themselves, it is
desirable to have a "read" lock capability. A read lock, by being less
restrictive, provides better support than a write lock for providing a
guarantee that an entity will not be modified.
7. Notification of Intention to Edit. It should be possible to notify the
HTTP server that an entity is about to be edited by a given person. It
should be possible to query the HTTP server for the list of people who
have notified the server of their intent to edit an entity.
Experience from configuration management systems has shown that people
need to know when they are about to enter a parallel editing situation.
Once notified, they either decide not to edit in parallel with the
other authors, or they use out-of-band communication (face-to-face,
telephone, etc.) to coordinate their editing to minimize the difficulty
of merging their results. Notification is separate from locking, since
a write lock does not necessarily imply an entity will be edited, and
a notification of intention to edit does not carry with it any access
restrictions. This capability is supportive of versioning, since a
check-out is typically involves taking out a write lock, making a
notification of intention to edit, and getting the entity to be edited.
8. Partial Write. After editing an entity, it should be possible, via
HTTP, to only write the changes to an entity, rather than
retransmitting the entire entity.
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 entity 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. Support for
partial entity updates will make small edits more efficient, and allow
distributed authoring tools to scale up for editing of large documents.
9. Attributes. Via HTTP, it should be possible to create, query, and
delete arbitrary attributes on entities 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 entity which is not
available in electronic form, or which will be available later.
10. List URL Hierarchy Level. A listing of all entities, 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
entities 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 entities
at a hierarchy level, or set of levels. It also supports a tree
visualization of the entities 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 entities 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.
11. 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 entities supports
management of a name space by packaging its members into small, related
clusters. This 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.
12. Copy. Via HTTP, it should be possible to make a byte-for-byte duplicate
of an entity without a client loading, then resaving the entity. This
copy should leave an audit trail.
There are many reasons why an entity 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 an entity, it is far preferable to have a
server perform an entity copy than a client. If a copied entity records
which entity it is a copy of, then it would be possible for a cache to
avoid loading the copied entity if it already locally stores the
original.
13. Rename. Via HTTP, it should be possible to change the URL of an entity
without a client loading, then resaving the entity under a different
name.
It is often necessary to change the name of an entity, 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, then resaving the entity,
followed by a delete of the old entity. Ideally an HTTP server should
record the rename operation, and issue a "301 Moved Permanently" status
code for requests on the old URL. Note that moving an entity is
considered the same function as renaming an entity.
3. Acknowledgements
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, Marty_Cagan@continuus.com
Dan Connolly, World Wide Web Consortium, connolly@w3.org
David Durand, Boston University, dgd@cs.bu.edu
Ron Fein, Microsoft, ronfe@microsoft.com
David Fiander, Mortice Kern Systems, davidf@mks.com
Roy Fielding, U.C. Irvine, fielding@ics.uci.edu
Yaron Goland, Microsoft, yarong@microsoft.com
Phill Hallam-Baker, MIT, hallam@ai.mit.edu
Dennis Hamilton, Xerox PARC, hamilton@parc.xerox.com
Andre van der Hoek, University of Colorado, Boulder,
andre@bigtime.cs.colorado.edu
Gail Kaiser, Columbia University, kaiser@cs.columbia.edu
Rohit Khare, World Wide Web Consortium, khare@w3.org
Dave Long, America Online, dave@sb.aol.com
Henrik Frystyk Nielsen, World Wide Web Consortium, frystyk@w3.org
Larry Masinter, Xerox PARC, masinter@parc.xerox.com
Murray Maloney, SoftQuad, murray@sq.com
Jim Miller, World Wide Web Consortium, jmiller@w3.org
Andrew Schulert, Microsoft, andyschu@microsoft.com
Christopher Seiwald, Perforce Software, seiwald@perforce.com
Judith Slein, Xerox, slein@wrc.xeroc.com
Richard Taylor, U.C. Irvine, taylor@ics.uci.edu
Robert Thau, MIT, rst@ai.mit.edu
Fabio Vitali, University of Bologna, Italy, fabio@dm.unibo.it
4. References
[1] America Online, "AOL Web Tools -- AOLpress 1.2 Features." WWW page.
http://www.aolpress.com/press/1.2features.html.
[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.
http://www.microsoft.com/msoffice/frontpage/productinfo/brochure/default.htm.
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
EMail: ejw@ics.uci.edu
Received on Thursday, 5 September 1996 13:11:33 UTC