Finally, some URL languages have URLs that have high persistence, which means
to last, while others do not, making them difficult to link to.
CHAPTER 2 ?– UNCOVERING THE MASHUP POTENTIAL OF WEB SITES 27
4. Restful Web Services by Leonard Richardson and Sam Ruby (O??™Reilly Media, 2007). The idea of analyzing
URls in terms of addressability, granularity, transparency, and persistence comes from Restful Web
Services.
UNDERSTANDING THE RELATIONSHIP AMONG URI, URL, AND URN
Universal Resource Identifier (URI) is a specific type of identifier. URIs fall into two classes: Universal Resource
Locators (URLs) and Universal Resource Names (URNs). You??™re likely to be much more familiar with the former.
An example of the latter is urn:isbn:159059858X, which refers to this book. Many of the things
I write about URLs in this book apply to URIs in general.
RFC 3986 clarifies the relationship among URI, URL, and URN (http://tools.ietf.org/html/
rfc3986#section-1.1.3):
A URI can be further classified as a locator, a name, or both. The term ???Uniform
Resource Locator??? (URL) refers to the subset of URIs that, in addition to identifying
a resource, provide a means of locating the resource by describing its primary access
mechanism (e.g., its network ???location???). The term ???Uniform Resource Name??? (URN) has
been used historically to refer to both URIs under the ???urn??? scheme [RFC2141], which are
required to remain globally unique and persistent even when the resource ceases to exist
or becomes unavailable, and to any other URI with the properties of a name.
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