Archive for March, 2011

Are you confused yet about the word “representation”?

Wordnet entry for “representation”.

S: (n) representation, mental representation, internal representation (a presentation to the mind in the form of an idea or image)
S: (n) representation (a creation that is a visual or tangible rendering of someone or something)
S: (n) representation (the act of representing; standing in for someone or some group and speaking with authority in their behalf)
S: (n) representation, delegacy, agency (the state of serving as an official and authorized delegate or agent)
S: (n) representation (a body of legislators that serve in behalf of some constituency) “a Congressional vacancy occurred in the representation from California”
S: (n) representation (a factual statement made by one party in order to induce another party to enter into a contract) “the sales contract contains several representations by the vendor”
S: (n) theatrical performance, theatrical, representation, histrionics (a performance of a play)
S: (n) representation (a statement of facts and reasons made in appealing or protesting) “certain representations were made concerning police brutality”
S: (n) representation (the right of being represented by delegates who have a voice in some legislative body)
S: (n) representation (an activity that stands as an equivalent of something or results in an equivalent)

Niklaus Wirth.
Program Development by Stepwise Refinement.
Communications of the ACM, Vol. 14, No. 4, April 1971, pp. 221-227.

In order to refine these instructions and predicates further in the direction of instructions and predicates available in common programming languages, it becomes necessary to express them in terms of data representable in those languages. A decision on how to represent the relevant facts in terms of data can therefore no longer be postponed.

Tim Berners-Lee.
Generic Resources.
Web page, 1996-2009.

A resource may be generic in that as a concept it is well specified but not so specifically specified that it can only be represented by a single bit stream.

T. Berners-Lee, R. Fielding, and H. Frystyk.
Hypertext Transfer Protocol — HTTP/1.0.
RFC 1945, IETF, May 1996.

A feature of HTTP is the typing of data representation, allowing systems to be built independently of the data being transferred.

R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, and T. Berners-Lee.
Hypertext Transfer Protocol — HTTP/1.1.
RFC 2616, IETF, June 1999.

A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.

Resources may be available in multiple representations (e.g. multiple languages, data formats, size, and resolutions).

representation [definition from glossary]:
An entity included with a response that is subject to content negotiation.

Tim Berners-Lee.
“The range of the HTTP dereference function.”
Email to www-tag list, March 2002.

HTTP is a protocol which provides, for the client, a mapping (the http URI dereference function) from URI starting with “http:” and not containing a “#” to a representation of a document. The document is the abstract thing and the representation is bits.

Roy T. Fielding and Richard N. Taylor.
Principled Design of the Modern Web Architecture.
ACM Transactions on Internet Technology (TOIT), 2002.
[JAR comments inserted]

… allowing a user to progress through the application by selecting a link or submitting a short data-entry form, with each action resulting in a transition to the next state of the application by transferring a representation of that state to the user.

[A representation is of a state.]

REST components communicate by transferring a representation of the
data …

[A representation is of data.]

Finally, it allows an author to reference the concept rather than some singular representation of that concept, thus removing the need to change all existing links whenever the representation changes.

[A representation is of a concept.]

Depending on the message control data, a given representation may indicate the current state of the requested resource, the desired state for the requested resource, or the value of some other resource, such as a representation of the input data within a client’s query
form, or a representation of some error condition for a response.

[A representation indicates a state.]

… the specification of Web addresses also defines the scope and semantics of what we mean by resource, which has changed since the early Web architecture. REST was used to define the term resource for the URI standard [Berners-Lee et al. 1998], as well as the overall semantics of the generic interface for manipulating resources via their representations.

[A resource can be manipulated via a machine interface.]

A resource does not always map to a singular file, but all resources that are not static are derived from some other resources, and by following the derivation tree an author can eventually find all of the source resources that must be edited in order to modify the representation of a resource. [emphasis JAR’s]

[A resource is derived from editable sources.]

Semantics are a byproduct of the act of assigning resource identifiers and populating those resources with representations. At no time whatsoever do the server or client
software need to know or understand the meaning of a URI — they merely act as a conduit through which the creator of a resource (a human naming authority) can associate representations with the semantics identified by the URI. In other words, there are no resources on the server; just mechanisms that supply answers across an abstract interface defined by resources. [emphasis JAR’s]

[Resources do not reside on servers.]

Ian Jacobs and Norman Walsh, editors.
Architecture of the World Wide Web, Volume One.
W3C Recommendation, December 2004.

[Document approved for advancement to Proposed Recommendation by Roy Fielding and the rest of the TAG.]

[Glossary] A representation is data that encodes information about resource state. Representations do not necessarily describe the resource, or portray a likeness of the resource, or represent the resource in other senses of the word “represent”.

[1] In this travel scenario, the resource is a periodically updated report on the weather in Oaxaca…

[2.2] In the case of this document, the message payload is the representation of this document.

Tim Berners-Lee, Roy Fielding, and Larry Masinter.
Uniform Resource Identifier (URI): Generic Syntax.
RFC 3986, IETF, January 2005.

When URIs are used within information retrieval systems to identify sources of information, the most common form of URI dereference is “retrieval”: making use of a URI in order to retrieve a representation of its associated resource. A “representation” is a sequence of octets, along with representation metadata describing those octets,
that constitutes a record of the state of the resource at the time when the representation is generated.

Tim Berners-Lee.
The meaning of “representation”.
Email to www-tag list, November 2007.

In fact, the relationship includes social as well as technical aspects. It also is defined, often, by high-level protocols. These higher level protocols set common expectations between the publisher and the reader. These are not consistent across the web. That is why you can’t simplistically just give a formula for that relationship.
…There is information in it in RDF which says that I made it and it is my public profile document.
…These expectations are very important to the web working. That’s why you can’t just write down a formula for the constraint on the representations of a given resource.

Roy Fielding.
“Some notes on organizing discussion on WebApps architecture.”
Email to www-tag list, October 2010.

On Oct 14, 2010, at 2:11 PM, Larry Masinter wrote:

Well, I wonder if we might introduce another step between “resource” and “representation” which is “application resource in identified state”, so that the representation isn’t a representation of the resource, but a representation of the resource in that state.

Umm, what? That would be terribly confusing and contrary to why I used the term representation in the first place (it is a representation by the origin server to the recipient of the state of that identified resource at the time of message generation).

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