General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

c○: Michael Kohlhase 353
Note that some forms of URIs can be used for actually locating (or accessing) the identified
resources, e.g. for retrieval, if the resource is a document or sending to, if the resource is a mailbox.
Such URIs are called “uniform resource locators”, all others “uniform resource locators”.
Uniform Resource Names and Locators
Definition 522 A uniform resource locator (URL) is a URI that that gives access to a
web resource, by specifying an access method or location. All other URIs are called uniform
resource names (URN).
Idea: A URN defines the identity of a resource, a URL provides a method for finding it.
Example 523 The following URI is a URL (try it in your browser)
http://kwarc.info/kohlhase/index.html
Example 524 urn:isbn:978-3-540-37897-6 only identifies [Koh06] (it is in the library)
Example 525 URNs can be turned into URL via a catalog service, e.g. http://wm-urn.
org/urn:isbn:978-3-540-37897-6
Note: URI/URLs are one of the core features of the web infrastructure, they are considered
to be the plumbing of the WWWeb. (direct the flow of data)
c○: Michael Kohlhase 354
Historically, started out as URLs as short strings used for locating documents on the Internet.
The generalization to identifiers (and the addition of URNs) as a concept only came about when
the concepts evolved and the application layer of the Internet grew and needed more structure.
Note that there are two ways in URIs can fail to be resource locators: first, the scheme does
not support direct access (as the ISBN scheme in our example), or the scheme specifies an access
method, but address does not point to an actual resource that could be accessed. Of course, the
problem of “dangling links” occurs everywhere we have addressing (and change), and so we will
neglect it from our discussion. In practice, the URL/URN distinction is mainly driven by the
scheme part of a URI, which specifies the access/identification scheme.
Running the World Wide Web
The infrastructure of the WWWeb relies on a client-server architecture, where the servers (called
web servers) provide documents and the clients (usually web browsers) present the documents to
the (human) users. Clients and servers communicate via the http protocol. We give an overview
via a concrete example before we go into details.
The World Wide Web as a Client/Server System
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