Overview

How to deal with information which is true at specific times?

• Explicit representation
• Using web documents
• Quoting

1. Explicit representation

• Ontology has a class of event-like thing
• These have start time and end time properties
• Example: iCalendar in RDF
• Example: Explicit employment events in CV:

	  [  cal:dtstart  "1996-01-01";
	    cal:dtend    "1997-09-22";
	    cv:employee  :JXR;
	    cv:employee  [ foaf:name "IBM" ]].		
 

This is reification of the time period.

2. Using documents

• Different web documents apply at different times
• Doc can self-describe its validity using <>
• Provenance-aware applications can process

	<>  valid:start  "2007-01-01T09:00";
	    valid:end    "2007-01-01T10:00";
	    dc:title	 "Temperature at 2007-01-01T09:00".
	room:r17   phys:temp     17.3;
		   phys:humidity  0.8.     
 

Expiration through HTTP

• Configure httpd to give realistic Expires: headers
• Publisher's commitment is until that date
• Caches do "if modified since" fetches
• Systems can track expiry and ripple through recalculation.
• Tabulator code makes metadata available in store

"Feed" architecture is polled

Propagation of changes

• Trigger e.g. from HTTP put or CVS post-commit script
• Send e.g. email or HTTP POST, etc
• Put limits on frequency
• New peer-peer protocols?

3. Reasoning about information and time

• Information is quoted in N3 with { braces }
• It can then be discussed objectively

  { room:r17 phys:temp 17.3; phys:humidity  0.8. }
	    valid:start  "2007-01-01T09:00";
	    valid:end    "2007-01-01T09:30".

  { room:r17 phys:temp 17.4; phys:humidity  0.8. }
	    valid:start  "2007-01-01T09:30";
	    valid:end    "2007-01-01T10:00".
 

Extends RDF. Not (2007) standard.

Reasoning about information and time -rule

• It can then be discussed objectively e.g with rules:

	{ ?EVENT  ex:dataTime  ?TIME.

	  ?TIME string:notLessThan ?READING!valid:start;
	     string:lessThan	   ?READING!valid:start.

	  ?READING log:includes { room:r17 phys:temp ?TEMP }

	}  =>  { ?EVENT ex:ambientTemp ?TEMP } 

 

Extends RDF. Not (2007) standard.

This generalizes

• Is this infromation valid at the relevant time?
• Is this information I can trust?
• Is this information I am allowed to use for this purpose?
• Conclusions from the provenance of info ( courseRules)

Summary: 3 ways

• Explicit representation
• Using web documents
• Quoting

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• This presentation : http://dig.csail.mit.edu/2007/Talks/0108-time-tbl/

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