So here we have a bit of confusion. Some XML and web services folks are tuned into the idea that the data will require lookups. Some RDF folks are thinking the data will be amassed in large pool. You can start to see a problem. When data is smushed together into repositories (centralized or ones under your own control) you have to watch out for syncronization problems. As in, the data you have in your repository may well be out of sync with the actual data available online.

Where this becomes an issue for me is how to tell that statements made in a Foaf file are authoritative. As in, how can I state that this is MY own Foaf file and that statements I choose to make about resources I control are to be considered authoritative. This, to many people, is a very obvious need. When you start smushing data together it gives rise to possible problems where the statements I make about something under my direct influence are contradicted by others. I'm not expressing this as a control-freak issue but as a genuine concern.

Part of the value behind RDF and the smushing of data can be shown in examples like group pictures. If someone else on the web takes a picture of a group of people, by using RDF it's possible for them to make statements that say they took the picture but that it shows (depicts) other indivisuals. And those people may be known to have a Foaf files at a given external web addresses. When the data gets combined in repository it becomes possible to ask "show me pictures of a given person" and it will be able to traverse the triples to reveal the many different places the notion of 'picture' and 'that person' appear.

But back to the authority issue, how can I make sure that when I ask about a given person (or other entity) that the data I've found is actually fresh or that it's accurate? When everything's dumped into a repository there's going to be a little trouble 'being sure' that you've got correct stuff dumped into it. The web services folks are perhaps scratching their heads here, thinking 'well, duh, just go resolve the URL and parse it' and they're correct. The question becomes, however, what can be done on a reasonably universal scale, to make sure that when they come looking for the document, that it contains statements that can be proved? How, when you come looking for my Foaf file, know that I actually created it and that I consider statements I'm making within it to be correct? Notice I'm saying "statements I'm making within". I'm not trying to force the broader guarantee of truth, just that they're my statements and I support them.

All this boils down to understanding some simple ideas about using markup and making statements that can be parsed in ways that avoid making incorrect statements. Then it's a matter of getting the collective consumers of the data to understand those same simple concepts and handling the statements correctly. This has not been a trivial task.

So here's something to think about, making use of RDF's data model will make searching and combining your data along with that from others. It's not, however, entirely different than the ways web services and other XML applications usually work. But most importantly, think about how you'd want to traverse through a LOT of different data. When you look into how the RDF model works you'll start to understand why it's simplicity makes it so powerful. That simplicty comes with a price of sometimes having to be verbose in the way the data is marked up. Fortunately that same ability to make simple statements makes it possible for a LOT of things to make simple statements. They can be combined in very easy ways. Once combined the big picture starts to emerge. That big picture, what some folks think of as the Semantic Web, is what we're after. Using RDF makes it very easy to start building it.