What is 'Representational State Transfer'?

rubyonrails-talk
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I have read about REST, but never really understood meaning of 'Representational State Transfer'. I tried to read Fielding's dissertation, but did not understand this term and why has he used it. What does it mean? Which state? What transfer?

Thanks, Amita.

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Amita Bhatkhande wrote:

I have read about REST, but never really understood meaning of 'Representational State Transfer'. I tried to read Fielding's dissertation, but did not understand this term and why has he used it. What does it mean? Which state? What transfer?

I'm not sure anyone knows why Fielding used this term. I suppose he had to call it something, so that's what he chose.

However, I'll try to explain it as I understand it:

Representational: The web consists of resources in the form of web pages that are represented using a Uniform Resource Identifier (URI). REST extends this definition to refer to other types of resources of an application (i.e. database tables). Which, can also be represented using a URI. http://example.com/posts/1 represents the first Post resource in the posts collection.

State: This is the current state of the resources.

Transfer (or more precisely State Transfer): The act of modifying the current state of the resources. Adding a new one, modifying an existing one, deleting an existing one, etc.

REST defines a convention of using HTTP to both represent and change the state of resources. That's a close as I can come to defining "Representational State Transfer."

Hope that helps.

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I for one found this post very useful.

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Hi --

Robert Walker wrote:

Amita Bhatkhande wrote:

I have read about REST, but never really understood meaning of 'Representational State Transfer'. I tried to read Fielding's dissertation, but did not understand this term and why has he used it. What does it mean? Which state? What transfer?

I'm not sure anyone knows why Fielding used this term. I suppose he had to call it something, so that's what he chose.

However, I'll try to explain it as I understand it:

Representational: The web consists of resources in the form of web pages that are represented using a Uniform Resource Identifier (URI). REST extends this definition to refer to other types of resources of an application (i.e. database tables). Which, can also be represented using a URI. http://example.com/posts/1 represents the first Post resource in the posts collection.

A resource in the REST sense isn't anything as concrete as a database table, though. It's more conceptual. As Fielding says: "The resource is not the storage object. The resource is not a mechanism that the server uses to handle the storage object."

State: This is the current state of the resources.

Transfer (or more precisely State Transfer): The act of modifying the current state of the resources. Adding a new one, modifying an existing one, deleting an existing one, etc.

REST defines a convention of using HTTP to both represent and change the state of resources. That's a close as I can come to defining "Representational State Transfer."

My nutshell take on it is something like this:

A given system gives and receives representations of resources. A resource is a very high-level construct; a representation is more specific, and has a content-type and so forth. Resources don't. (Even more low-level are matters of storage, which may vary and which are not, themselves, either resources or representations.)

A resource is always in a given state. Resource states are altered via the exchange of representations. So if I want to correct the spelling of my name on a site, I submit a representation of my profile with the new spelling (probably in the form of a Web form). If I want to make sure it's OK, I request a representation (probably in the form of an HTML document) to inspect.

I've written a bit more about this on these blog posts:

http://dablog.rubypal.com/2008/3/23/splitting-hairs-over-resource http://dablog.rubypal.com/2008/4/24/splitting-hairs-over-resource-part-2

specifically in a Rails context. (Wow, was that more than a year ago?)

David

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David,

Your posts are certainly an interesting aside on the IMproper usage of the term "resource", but it doesn't (for me) answer the original question: "What is Representational State Transfer?"

Like Amita, I struggle with this. My hope is to someday have a succinct way of expressing the answer, without being forced to convey all of the detailed nuamce of Fielding's dissertation (which I don't really grasp in any great detail).

I kinda like where Robet was going, but I'll suggest a slight rephrasing:

"Representational State Transfer defines a convention for using HTTP to both identify and manipulate related information that is conceptually mapped into an aggrregate construct referred to as a Resource."

I repeat - I don't really understand all of the detailed nuance. I'm struggling to create a simple articulation of the basic idea.

Is my articulation sorta, kinda, in a way, maybe correct? How can my articulation be improved?

Jim Maher

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JDM wrote:

"Representational State Transfer defines a convention for using HTTP to both identify and manipulate related information that is conceptually mapped into an aggrregate construct referred to as a Resource."

Jim Maher

Quite succinct. I like it.

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JDM wrote:

I kinda like where Robet was going, but I'll suggest a slight rephrasing:

"Representational State Transfer defines a convention for using HTTP to both identify and manipulate related information that is conceptually mapped into an aggrregate construct referred to as a Resource."

Thanks for the vote of confidence. I certainly don't claim to be an expert on the subject. There are a lot of people smarter on this stuff than I, which is why I tried to remain somewhat abstract. I do like your rewording of my original thought. It certainly seems like a good start. However, as David pointed out in his excellent blog postings, this definition may still be omitting one important aspect of REST, which is the distinction between a "resource" and a "representation" of a resource. I feel that I have a good mental grasp on this distinction, even if I'm not so great at applying the concepts to the written word.

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I guess I was trying to duck the issue, but I see that does nothiing to elucidate the use of the word "Representational" in the REST name.

I'll suggest this change (though its getting quite wordy):

"Representational State Transfer defines a convention for using HTTP to both identify and manipulate related information that is conceptually mapped into an aggrregate construct referred to as a Resource, via the transfer between application components of Representations that contain the current or intended state of that Resource."

Whadda ya think?

Jim Maher