Is it a small wonder then that we are beginning to see articles like this one ("Why is REST so hard to understand?") pop up in the publications devoted to the web architecture? It is absolutely clear that we won't be in the position to advance ROA practices unless we reach a critical mass of people who do possess a solid grasp of what is the difference between a resource and its representations.
Right now, it seems like only an infinitesimally small percentage of software developers are aware of the significance of that difference. For example, if we say that there are ten million software developers worldwide, 9,999,000 of these developers are blissfully ignorant that such a thing as resources and their representation even exists. Which leaves us with only about 1,000 developers who truly understand what it all means. Not a very encouraging statistics. As a matter of fact, the situation today is right down dismal.
We need to do something quickly in order to remedy this catastrophic situation. My small contribution here is to attempt to clarify the difference between resources and their representation. I will use mostly concepts from everyday life for illustration purposes. I will then attempt to draw the parallel between these concepts and the concepts one encounters in the arena of software development.
What is a Resource?
Let's start from the top: a resource is anything that humans find useful. For example, a house might be a resource. Or, money might be a resource. We find houses very useful, because they offer certain level of comfort in our daily living. We also find money very useful, because it's a resource that enables us to get other resources, such as houses.Ten years ago, when I was buying my first house, I wanted to see what kind of a house can I afford. To that end I met with a mortgage broker, who interviewed me in the attempt to assess my buying power. Among other things, he needed to know how much money can I allocate towards the down payment on the house.
Once I told him how much money I have for the down payment, he was able to plug that information into his spreadsheet and then tell me what kind of mortgage do I qualify for. So I was then all set to go out and shop for the house, right?
Well, not so fast! Just because I've blurted out a dollar figure that I had in mind for the down payment, didn't automatically mean that I fully qualify for that mortgage. Because, you see, my word in these matters was viewed as being largely subjective, that is, prone to miscalculations and all kinds of other aberrations. What the bank needed is a more objective assessment of my buying power. Talk is cheap, and anyone can claim that they have one hundred thousand dollars tucked away in their bank account, ready to be plopped in toward the down payment, but the reality might be somewhat different.
Because of that, the banks have devised a more impartial practice, whereby the applicant must provide what is deemed to be a more objective proof that the money really is in the applicant's bank account.
In this example, the money that has been allocated for the down payment on the house is regarded as a resource.
What is a Representation?
Now, the down payment money we are talking about may indeed be sitting in my bank account. But how am I to truly convince the mortgage broker that it is really there? Well, I use the means of representation, that is to say, I use something else that acts on behalf of that money. For example, I use my words to convey the fact that I do have $100,000 in my bank account. My words then represent my money.But, as we've seen, that representation (my words uttered in a casual conversation) is apparently not good enough for the mortgage broker. He needs something more 'solid' before he is fully convinced that the money is actually there. Meaning, he needs a different form of representation.
Typically, what the mortgage broker would deem a fairly impartial and objective representation of my money is a piece of paper issued by my bank that claims that it is true that I do have $100,000 in my account. In other words, once the mortgage broker receives that piece of paper, he can 'take it to the bank', so to speak.
That piece of paper is what we call a representation of a resource. The real money is nowhere to be found in that representation, yet in a somewhat superstitious manner, all the parties involved almost religiously obey and agree that a piece of paper is as good as the real thing.
Back on the Web
The exact same protocol abides on the web. There, as in everyday, non-virtual life, we have resources which are tucked away somewhere, and all we get to see, feel and touch are the representations. Resources on the web are merely concepts that are being represented using other concepts. On the web, as in real life, no one ever gets to see, hear, feel and touch a resource. All we get is mere representations.We are free to attempt to manipulate those representations. And even in the cases when our manipulation yields perceptible differences, we have no grounds to claim the we have actually manipulated the resource itself. All we can conclude is that the resource representation has changed under the influence of our actions, nothing more, nothing less.
For example, if there is a resource such as a volleyball tournament published on the web, I can get its representation by sending a request for it. I may then decide to destroy that tournament by manipulating the tournament's representation. And my action may result in an affirmative representation (i.e. the resource may respond by sending me its representation which states that it has destroyed itself). I may then conclude that the volleyball tournament has, indeed, been destroyed. But I really have no way of ascertaining that. All I can claim is that I have acted upon the initial representation of that volleyball tournament, I have sent it the request to destroy itself, and I have received the confirmation that it is now destroyed. However, the resource itself may still be alive and well, existing somewhere beyond the reach of the resource representation. The only thing that is manifestly certain is that from that moment on, the resource will refuse to render its representation for the incoming requests.
This is similar to how, in the real world, no one would ever suggest that I take the mortgage broker into my bank's safety vault and point him to the pile of cash that's sitting there, and tell him: "Here, here's my $100,000, please count them and let's get on with it!" No one goes straight to the resource itself in real life, and the same holds true in the digital life as it gets distributed on the web (not to mention that even the physical cash is merely a representation of some other concept, etc.; one can never possibly get to the bottom of it, that is, the real resource is nouminal, unreachable by us mere mortals).
Understanding this distinction is vitally important if we are to move forward successfully in building the functional web of information. It is my experience that none of the developers I've ever worked with understand this situation, and that all of them erroneously believe that they are, at all times, working with the 'real thing'. Nothing could be farther from the truth, so I implore all developers to take a moment and try to digest the difference between resources and their representations.
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