http://tools.ietf.org/html/rfc3986#section-1.1
a library can also be resources. Likewise, abstract concepts can be resources, such as the
operators and operands of a mathematical equation, the types of a relationship (e.g.,
???parent??? or ???employee???), or numeric values (e.g., zero, one, and infinity).
The question of resources and their corresponding URIs are not as abstract as they may
sound. In fact, looking at resources may seem rather obvious. For example, for Flickr, which is
self-described as ???almost certainly the best online photo management and sharing application
in the world,??? important entities are, not surprisingly, photos and users. As you will see later in
the chapter, these entities are also resources; you can identify specific photos and users in the
URLs produced by Flickr. For example, this URL:
http://www.flickr.com/photos/raymondyee/508341822/
is for photo 508341822, which belongs to user raymondyee. A Flickr photo is addressable via
a URL; that is, a URL can lead you right to the photo in question. As experienced users of the
Web, we all know the useful things we can do when there are specific URLs. You can bookmark
a link, e-mail it, and use it as a reference in a web page. You don??™t have to tell someone to go to
Flickr and type in the photo number to get to the photo.
As you will see later in this chapter, granular URLs also enable mashups.
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