- From: Jim Whitehead <ejw@kleber.ICS.UCI.EDU>
- Date: Tue, 10 Sep 1996 13:14:31 -0700
- To: w3c-dist-auth@w3.org
Here is the third, and hopefully final initial revision of the
document "Requirements on HTTP for Distributed Content Editing."
This document is also available at URL:
http://www.ics.uci.edu/~ejw/authoring/dist-auth-req.html
Changes from the previous draft:
o Addition of mandatory Abstract section
o Addition of requirements and rationale text:
- multi-entity locking
- partial-entity locking
o Updated Attributes requirement to also discuss modification
and reading of attributes.
o Updated rationale text for Move/Rename to discuss attributes,
and also network efficiency of a Move over a Copy/Delete.
o Misc. spelling and wording corrections.
o Changed name of document to "draft-whitehead-http-distreq-00.txt"
per request by Larry Masinter.
I believe that there has been sufficient time for comments on this
document, so unless I receive major, vocal feedback, I will ship
this draft to the IETF and make it a W3C working draft at the
end of the day Wednesday. There will still be many opportunities
to revise and refine this document once this happens.
- Jim
HTTP Working Group E. J. Whitehead, Jr.
INTERNET-DRAFT U.C. Irvine
<draft-whitehead-http-distreq-00.txt> September 1996
Expires March, 1997
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
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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
WWW Distributed Authoring and Versioning mailing list,
<w3c-dist-auth@w3.org>, which may be joined by sending a message with
subject "subscribe" to <w3c-dist-auth-request@w3.org>. Discussions are
archived at URL:
http://www.w3.org/pub/WWW/Archives/Public/w3c-dist-auth/. The HTTP
working group at <http-wg@cuckoo.hpl.hp.com> also discusses the HTTP
protocol. Discussions of the HTTP 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.
Abstract
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 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.
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 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 between entities of any media type.
A hypertext link is a relationship between entities 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 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. It should be possible to set single or multi-person write
locks with a single action.
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
until the read lock is released. It should be possible to assign a
read lock to a single person or a list of persons with a single
action.
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.
7. Multi-Entity Locking. It should be possible to take out a write or
read lock on multiple entities in the same action, and this
locking operation must be atomic across these entities.
8. Partial-Entity Locking. It should be possible to take out a write
or a read lock on subsections of 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. Put differently, a read lock states that the entity is
guaranteed not to change for the duration of the lock. A write
lock states that an entity 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 entities, a feature which
is supported by the multiple-entity locking requirement. This is
useful for preventing a collision between two people trying to
establish locks on the same set of entities, since with
multi-entity locking, one of the two people will get a lock. If
this same multiple-entity locking scenario was repeated by using
atomic lock operations iterated across the entities, the result
would be a splitting of the locks between the two people, based on
entity ordering and race conditions.
Partial entity locking provides support for collaborative editing
applications, where multiple users may be editing the same entity
simultaneously. Partial entity locking also allows multiple people
to simultaneously work on a database type entity.
9. 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.
10. 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.
11. Attributes. Via HTTP, it should be possible to create, modify,
query, read 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.
12. 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.
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 entities 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 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.
15. Move/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. 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 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
Ora Lassila, Nokia Research Center, ora.lassila@research.nokia.com
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 Tuesday, 10 September 1996 16:19:00 UTC