Monday, May 27, 2013

Resources Are Four-Dimensional

The term ROA (Resource-Oriented Architecture) is misleading. It should ideally stand for "Resource/Representation-Oriented Architecture", even though that's quite a mouthful.

I've found in my discussions with people who work on REST-based systems that lots of them are very fuzzy about the notions of "resource" and "representation", even though these are the fundamental concepts underlying REST (and my forthcoming messaging-oriented variant, ROMA).

Let me try and explain this. Let's say I found a space capsule that had fallen out of the sky, and by deciphering the alien glyphs on its surface, I understood that it contained a 4-dimensional object from the planet Royf. Unfortunately, I couldn't take the object out, because ours is a 3-dimensional world and such an operation is impossible. However, I found that it was possible to get the capsule to "project" a 3-dimensional image of the object it contained, so I could "see" it in a way that made sense to my limited mind. I found that I could also ask for the object to be manipulated in 3-dimensional terms. I knew, of course, that the object itself was 4-dimensional and so my instructions had to be somehow translated into terms that made sense in the 4D world. But I found to my satisfaction that the 3D image that resulted from my instructions was exactly what I wanted.

I realised then that my interactions with the space capsule were RESTian. The 4-dimensional object was the resource, an unseeable thing that I had no way of even comprehending and which was therefore mercifully shielded from my vision. What I could ask for (through a GET) was a 3D "representation" of the object, and this was something I could understand. I could also manipulate the object in several ways. I could show the capsule other 3D objects and say, "Change its shape to resemble this", or "Make its colour more like this", and it would happen! Obviously, the objects I was holding up were not the same as the object inside the capsule. They were representations of what I wanted the object to look like, when I saw it in 3D terms.

That's really what REST is. The only aspect of the resource itself that we can directly deal with is its name, or URI. The actual resource is completely unseen, indeed unseeable.  Everything that is actually seen is a representation, whether it's a representation of what the object is like right now, or a representation of what we want the object to be like. Everything that goes "over the wire" in REST is a representation.

Nerds can readily understand what a 3D projection is

[See also that REST is the very opposite of "Distributed Objects", although some industry personalities continue to insist that REST is DO! (JJ, I'm talking to you.) Distributed Objects tries to bring about an illusion that remote objects are local, allowing you to grasp them using virtual reality gloves. REST tries to bring about a discipline that says even local objects like the one inside the space capsule should be treated as remote, and we shouldn't try to grasp them directly, only deal with them indirectly through representations. Distributed Objects works well when it does and fails horribly when it doesn't. REST always works.]

Hopefully, this should set to REST some of the confusion around resources and representations.

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