3. From pages to resources

Encode and decode URLs

URLs only accept ASCII characters. Therefore we need to encode any special characters like accents before we incorporate them into a URL. Try it yourself:

To encode, press the first button...

To decode, press the second button...

You can change and experiment with other input values for the little scripts above.

Here we see the whole point of the IRI for internationalization and accessibility of the identifiers on the Web.

URI IRI

URIs for everything...

As we will see later, URIs are used to name a lot of very different things.

Here are some examples:

• URI for Paris in DBpedia: http://dbpedia.org/resource/Paris
  
• URI for the protein MUC18 in UniProt: http://www.uniprot.org/uniprot/P43121
  
• URI for the great white shark at BBC: http://www.bbc.co.uk/nature/life/Great_white_shark
  
• URI for the "31st of December 2016 at 23:00 in New York": http://www.timeanddate.com/worldclock/meetingdetails.html?year=2017&month=1&day=1&hour=4&min=0&sec=0&p1=179
  
• URI, URI, URI,...

Namespace and URIs

The notion of namespace is one of the most abstract of this MOOC. Now that we have discussed URIs, let us go back to the notion of namespaces a second time.

Quoting Wikipedia: "In computing, a namespace is a set of symbols that are used to organize objects of various kinds, so that these objects may be referred to by name (...) namespaces are typically employed for the purpose of grouping symbols and identifiers around a particular functionality and to avoid name collisions between multiple identifiers that share the same name."

Since URIs can be used to identify anything, they can be used to identify these sets that form namespaces. You can see namespace as a set of terms that where isolated and that we identify by giving a URI to that set. This allows you to do two important things:

• to specify that a term you use belongs to a specific set and as a specific meaning just like when you say "I use the word SCORE with the meaning of the University of NICE" as a way to avoid any misunderstanding;
• to avoid name collisions by prefixing the words (called "qualified name") with prefixes associated with the right namespace for example URI unice: SCORE and sport:SCORE that would make the difference between "results of student exams" and "sport results"; so we can use these two prefixed terms in the same data, documents and applications without any collision or ambiguity.

So you can conceive namespaces as a mechanism to create sets of terms, to identify these sets with URIs and to link these URIs to prefixes that allow to you use terms in a non-ambiguous way.

Famous namespaces include:

• the XML namespace: http://www.w3.org/XML/1998/namespace
• the XHTML namespace: https://www.w3.org/1999/xhtml/
• the SVG namespace: http://www.w3.org/2000/svg
• etc.

REFERENCES

• - 5.2.1.1 Resources and Resource Identifiers, Architectural Styles and the Design of Network-based Software Architectures, PhD Dissertation, Roy Thomas Fielding, University of California, 2000

• - Resource, 1.1. Overview of URIs, Uniform Resource Identifier (URI): Generic Syntax, T. Berners-Lee, R. Fielding, L. Masinter, 2005
• - URI Euler Diagram no lone URIs by David Torres CC BY-SA 3.0 via Wikimedia Commons