[RSP CG] Interval streams?

Hi All!

Thank you very much for the good discussion yesterday! Since Jean-Paul invited discussion on the email reflector, I'll give it a try. To me this is also related to the joint action point of timestamps and their relation to use cases.

In the wiki<http://www.w3.org/community/rsp/wiki/RDF_Stream_Models> under RDF Stream Models / Temporal Graphs we are listing "variant 2 - interval based" graphs. These can be described as temporally valid facts, where validity is indicated by the specified time interval. It is clear to me that such temporally valid facts exist and that there must be a method to archive and query them.

However, what I cannot easily motivate, is when would we want to stream temporally valid facts?

The problem is that a fact can be added to a stream only when the ending time is known. In many cases this will be the time, when the fact is no longer valid.

Taking the example on Daniele's slide<http://www.dellaglio.org/uploads/rsp-phone-call-0925.pdf> 8 (second last), if we create a stream of meeting durations, we can only stream the information on each meeting *after* the meeting has finished. We can do historical queries on how long meetings lasted, or which meetings had conflicts, but we cannot query which meetings are in progress *now* or generate an alert about a resource conflict when it happens. Which, to me, is pretty much the essence of why we process real-time streams instead of archived datasets.

It is certainly valid that in many cases the duration of a temporally valid fact can be anticipated at the time when it is initiated. But if the duration can be anticipated, then there is quite often also a rule for the duration. In many such cases we could probably save such rules either in a dataset or as event processing rules on our event processing platform and add them on-the-fly without explicitly streaming the duration with each fact.

To me the more flexible and "real-time" solution to the said example would be to generate a stream of events signalling the starting and ending points of meetings:
:alice :meets :bob @t1
:alice :meets :carl @t3
:alice :finishesmeetingwith :bob @t5
:bob :meets :diana @t6
:alice :finishesmeetingwith :carl @t7
...

The status (which meetings are in progress now?) would be maintained on the event processing platform and the historical data (a graph of meeting starting and ending times) could be saved to a dataset for later querying.

Of course this might all be just my lack of imagination, but it would be helpful for me, if we could get a use case motivating the streaming of temporally valid facts. Anyone?

BR,

Mikko