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
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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.
This document describes functionality which, if provided in the
HyperText Transfer Protocol (HTTP) , 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.
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."
- 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)  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."
- 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  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."
- 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.
- 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
- 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.
- Independence of locks. It should be possible to
lock an entity without re-reading the entity, and without committing to
editing an entity.
- 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.
- 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
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.
- 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.
- 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
- Attributes. Via HTTP, it should be possible to create,
modify, query, read and delete arbitrary attributes on entities of any
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
- 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  hierarchy level in an http URL scheme should be
accessible via HTTP, so long as this operation is meaningful.
In  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
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
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  also currently supports a graphical
network visualization and additionally supports a tree visualization of
a portion of a site's structure.
- 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
- 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.
- 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
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, email@example.com
David Durand, Boston University, firstname.lastname@example.org
Ron Fein, Microsoft, email@example.com
David Fiander, Mortice Kern Systems, firstname.lastname@example.org
Roy Fielding, U.C. Irvine, email@example.com
Yaron Goland, Microsoft, firstname.lastname@example.org
Phill Hallam-Baker, MIT, email@example.com
Dennis Hamilton, Xerox PARC, firstname.lastname@example.org
Andre van der Hoek, University of Colorado, Boulder, email@example.com
Gail Kaiser, Columbia University, firstname.lastname@example.org
Rohit Khare, World Wide Web Consortium, email@example.com
Dave Long, America Online, firstname.lastname@example.org
Henrik Frystyk Nielsen, World Wide Web Consortium, email@example.com
Ora Lassila, Nokia Research Center, firstname.lastname@example.org
Larry Masinter, Xerox PARC, email@example.com
Murray Maloney, SoftQuad, firstname.lastname@example.org
Jim Miller, World Wide Web Consortium, email@example.com
Andrew Schulert, Microsoft, firstname.lastname@example.org
Christopher Seiwald, Perforce Software, email@example.com
Judith Slein, Xerox, firstname.lastname@example.org
Richard Taylor, U.C. Irvine, email@example.com
Robert Thau, MIT, firstname.lastname@example.org
Fabio Vitali, University of Bologna, Italy, email@example.com
 America Online, "AOL Web Tools -- AOLpress 1.2 Features." WWW page.
 T. Berners-Lee, D. Connolly. "HyperText Markup Language Specification - 2.0."
RFC 1866, MIT/LCS, November 1995.
 T. Berners-Lee, L. Masinter, M. McCahill. "Uniform Resource Locators (URL)."
RFC 1738, CERN, Xerox PARC, University of Minnesota, December 1994.
 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.
 Microsoft. "Microsoft FrontPage for Windows Data Sheet." WWW page.
E. James Whitehead, Jr.
Department of Information and Computer Science
University of California
Irvine, CA 92697-3425