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Arnon Rotem-Gal-Oz

Dr. Dobb's Bloggers

The web vs. the fallacies of distributed computing

May 28, 2009

I recently read a post by  Tim Bray where he states that building on web technologies let you get away with believing some of the fallacies of distributed computing.

I personally thinks he is a little optimistic in that claim.

On “The network is reliable” – Tim says that that the connectionless of HTTP helps (it does) and that GET, PUT and DELETE are idempotent helps as well. I say that GET, PUT and DELETE only if the people implementing the server side make them so – i.e. consider the fallacy. The fact that the HTTP says they should be idempotent doesn’t automatically make each implementation compliant

On “ Latency is Zero” – Tim says the web makes it worse – but, he claims, users got used to that. Even if they did I think that users are just part of the picture since the programmable web is also making strides. Also as Tim says it is actually worse. Not to mention that “Latency isn’t constant” either

On “Bandwidth is infinite” – Again Tim agrees that it is worse but people learn to note it. Again learning that it is there doesn’t mean the fallacy is gone just that people are less likely to presume it

On “The Network is secure” – Tim says its probably the “least-well-addressed by the web” – no argument here

On “Topology doesn’t change” – Tim says URIs help mitigate it – Again Tim is assuming people make URIs permanent or will always return a temporary redirect/permanent redirect when a URI change – good luck with that.

On “There is one administrator” – Tim says that yes that’s the case but who cares. Well, an example I usually give is that time when I deployed an ASP.NET which worked for a while – until the hosting company decided to change their policy to partial-trust (the app. needed full-trust) – when that happens to you. You care. If you mashup with someone else, you care etc.

On “Transport cost is Zero” – Tim says it is the same as for Bandwidth – i.e. worse.

On “The network is homogeneous” – Tim says that that’s this is the “web’s single greatest triumph”. I actually agree to that as long as all of you stick to using the web’s ubiquitous standards (http, XML/JSON ) if you have parts of your application that can’t use that you still need to pay attention

One thing I am really  puzzled by is Tim’s conclusion :

“If you’re building Web technology, you have to worry about these things. But if you’re building applications on it, mostly you don’t.”

Since even according to him only 4 fallacies are covered by the web… (I think only 1)

In any event, I agree that the web standards and REST in particular, do contain guidelines that take into consideration the fallacies. However it is still up to developers to understand the problems they’ll create if they don’t follow these guidelines. Assuming that that is indeed the case, is well, overly optimistic in my experience.

You can also read a paper I published a few years ago which explains the fallacies  and why they are still relevant today.

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