Overview
- How the Semantic Web Works
- Choosing URIs
- Supporting URIs
How the web Works
- A URI identifies a document
- HTTP allows a client to get a representation of the document
How the Semantic Web works
- A URI identifies any thing
- That URI is related to the URI of the document
- The document is data about the thing
Tree Vs Web
| Tree | Web |
|---|
| OO systems | Semantic Web |
| Information hiding | Information sharing |
| Social Hierarchies | Social web |
| Top-down elaboration | Top-down, Bottom-up & middle-out |
| Fixed slots | Anyone can say anything about anything |
| Can't be a web | Can be a tree |
| Info stored with subject | Can also be stored anywhere else |
| One ID per object | Things can have many URIs |
Put aside Object Oriented and XML paradigms.
URI of Thing to URI of document
Two alternatives
- The URI has a hash:
- Eg: http://example.com/products#sku105
- The part on the left of the # is the document URI
- The URI has no hash:
- Eg: http://example.com/products/sku105
- This URI must HTTP redirect 303 See Also to the document URI
Do NOT confuse the URI of a thing and the URI of a web page about it.
Each URI identifies one thing only
Hash vs Slash
- Hash allows document URI to be generated instantly
- Hash works on local file system too
- Hash does not work when document would be huge
- Useful idiom: http://example.org/product/sku105#it
Criteria for Choosing URIs
- Rational classification of the subjects of the data.
- Persistence
- Persistence
- Persistence
Persistence tips
- Institutional commitment.
- Domain name controlled by those ensuring persistence.
- Cleanliness of URI names.
- No mechanisms /cgi-bin/foo.asp?id=x
- No passing attributes /~harriet/latest/
- No internals of file type /foo.rdf
- Technique: Date space
- Technique: Apache rewrite rules
- See "Cool URIs don't change"
- URN's, DOIs, etc don't help... you need exactly the same...
- Institutional commitment. (Try asking W3C)
See also: W3C TAG finding Metadata in URIs,
W3C Persistence Policy
Multiple URIs for the same thing
- Not the end of the world
- Advisable when many agents have data on same thing.
- E.G. http://www.w3.org/People/Berners-Lee/card#i and http://www4.wiwiss.fu-berlin.de/dblp/resource/person/100007
- Connect using owl:sameAs ('=' in N3)
- Store and serve other people's URIs: link!
Be part of the Semantic Web
Persitence - examples: discuss
What to serve: Linked data
- Use URIs as names for things
- Use HTTP URIs so that people can look up
those names.
- When someone looks up a URI, provide useful
information.
- Include links to other URIs. so that they can discover
more things.
- Ideally: all links out and in
- Reasonable size of data returned
- Not knowledge bases in zip/tar files!
- Link: The use in document d of URI which is in document d'
Examples of linked data
- Most ontologies
- dbview-generated from SQL virtual RDF data
- New: D2R Server provides linked data as well as SPARQL
- FOAF (home brew, LiveJournal, Opera Community etc)
- Semantic wikipedia
- Place-names ... etc
- The biggest challenge is links to other systems
Lots of data? SPARQL service
- SPARQL is the RDF query language
- In final stages of standardization
- Many implementations
- Graph match, optional match,
- More concise than equivalent SQL or XQ
- Higher conceptual level
- More robust against implementation changes
See Jim Melton's W3C Tech Plenary talk (
slides and XTech paper.)
Self-describing Web
- "Follow your nose" from URI to what it means.
- Ladder of authority
- Domin owner controls IP address of server
- HTTP URIs have owners who say what they mean
- Serve links in both directions
Remember web architecture
- Server files with correct Content-type
- eg application/rdf+xml, text/rdf+n3
- Not text/plain or application/binary
- Serve links in both directions
Summary
- URIs for things, URIs for documents
- Pick URIs to be persistent
- Serve data about the thing in response to HTTP GET
- Use SPARQL for large-scale access.