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What is RDF?

Spanish Translation available here.

This article was first published as "RDF and Metadata" on XML.com in June 1998. It has been updated by ILRT's Dan Brickley, chair of the W3C's RDF Interest Group, to reflect the growing use of RDF and updates to the specification since 1998.

The Right Way to Find Things

RDF stands for Resource Description Framework. RDF is built for the Web, but let's leave the Web behind for now and think about how we find things in the real world.

Scenario 1: The Library

You're in a library to find books on raising donkeys as pets. In most libraries these days you'd use the computer lookup system, basically an electronic version of the old card file. This system allows you to list books by author, title, subject, and so on. The list includes the date, author, title, and lots of other useful information, including (most important of all) where each book is.

Scenario 2: The Video Store

You're in a video store and you want a movie by John Huston. A large modern video store offers a lookup facility that's similar to the library's. Of course, the search properties are different (director, actors, and so on) but the results are more or less the same.

Scenario 3: The Phone Book

You're working late at a customer's office in South Denver, and it seems that a pizza is essential if work is to continue. Fortunately, every office comes equipped with a set of Yellow Pages that, when properly used, can lead to quick pizza delivery.

The Common Thread

What do all these situations have in common, and what differences lie behind the scenes? First of all, each of these systems is based on metadata, that is, information about information. In each case, you need a piece of information (the book's location, the video's name, the pizza joint's phone number) you don't have. In each case, you use metadata (information about information) to get it.

We're all used to this stuff; metadata ordinarily comes in named chunks (subject, director, business category) that associate lookup information ("donkeys", "John Huston", "Pizza, South Side") with the information you're really after.

Here's a subtle but important point -- in theory, metadata is not really necessary: you could go through the library one book at a time looking for donkey books, or through the video store shelves until you found your movie, or call all the numbers in your area code until you find pizza delivery. But that would be very wasteful, in fact, it would be stupid. Metadata is the way to go.

It's All Different Behind the Scenes

In each of our scenarios, we used metadata, and we used it in remarkably similar ways. Does this mean that the library, the video store, and the phone company all use the same metadata setup? Of course not. Every library has a choice among at least two systems for organizing their books, and among many vendors who will sell them software to do the looking-up. The same is obviously true for video stores and phone companies.

In fact most such products define their own system of metadata and their own facilities for storing and managing it. They typically do not offer facilities for sharing or interchanging it. This doesn't cause too much of a problem, assuming they do a decent job with the user interface. We are comfortable enough with the general process we call "looking things up" (really, searching via metadata) that we are able to adapt and use all these different systems.

Not Just For Searching

The most common daily use of metadata is to aid our discovery of things. But there are lots of other uses going on behind the scenes. The library and video store are storing other metadata that you don't see: how often the books and videos are being used, how much it cost to buy them, where to go for a replacement, etc. Running a library or a video store would be unthinkable without metadata. Similarly, the phone company, of course, uses its metadata, most obviously to print the Yellow Pages, but for many other internal management and administration tasks.

What About the Web?

The Web is a lot like a really really big library. There are millions of things out there, and if you know the URL (in effect a kind of call number) you can get them. Since the Web has books, movies, and pizza joints, the number of ways you might want to look things up includes all the things a library uses, plus all the things the video store uses, plus all the things the Yellow Pages use, and lots more.

The problem at the moment is that there is hardly any metadata on the Web. So how do we find things? Mostly by using dumb, brute force techniques. The dumb, brute force is supplied by the wandering web robots of search engine sites like Altavista, Infoseek, and Excite. These sites do the equivalent of going through the library, reading every book, and allowing us to look things up based on the words in the text. It's not surprising that people complain about search results, or that the robots are always way behind the growth and change of the Web.

In fact there is one metadata-based general purpose lookup facility: Yahoo! Yahoo doesn't use a robot. When you search through Yahoo, you're searching through human-generated subject categories and site labels. Compared to the amount of metadata that a library maintains for its books, Yahoo! is pitiful; but its popularity is clear evidence of the power of (even limited) metadata.