Presentation for the PhD Symposium workshop @ESWC 2016

1. Continuously Self-Updating Query
Results over Dynamic Linked Data
Ruben Taelman - @rubensworks
iMinds - Ghent University

2. Dynamic Linked Data
E.g. Thermometer measures every minute:
“19,05°C” - 30-05-2016 11:00
“19,06°C” - 30-05-2016 11:01
“19,11°C” - 30-05-2016 11:02
“19,08°C” - 30-05-2016 11:03
…
Typically exposed as an RDF stream = stream of <RDF triple, timestamp>

3. Querying continous data
Clients send queries to server: e.g. What is the current temperature?
Server continuously evaluates the queries
→ Server does all of the work
Cause of low public endpoint availability!
½ have availability of < 95% (Buil-Aranda 2013)
→ Clients just wait for results

4. What if we moved continuous query evaluation to the client?
→ to lower server load

5. Overview
Research questions
Research approach
Evaluation plan
Preliminary results

6. Overview
Research questions
Research approach
Evaluation plan
Preliminary results

7. Research questions
How to publish of dynamic data, to make it queryable together with static data
at a low server cost?
How can we efficiently store dynamic data and allow efficient transfer to clients?
What kind of server interface do we need to enable client-side query evaluation over
both static and dynamic data?

8. Hypotheses
1. Our storage solution can store new data in linear time with respect to the
amount of new data.
2. Our storage solution can retrieve data by time or triple values in linear time with
respect to the amount of retrieved data.
3. The server cost for our solution is lower than the alternatives.
4. Data transfer is the main factor influencing query execution time.

9. Overview
Research questions
Research approach
Evaluation plan
Preliminary results

10. Moving continuous query evaluation to the client

11. Triple Pattern Fragments does this for static data!
Triple pattern fragments (TPF) (Verborgh 2016):
Servers can only respond to triple pattern queries
Clients need to evaluate queries locally
→ Lowers server complexity

12. How I will do this for dynamic data
Storage Transmission Query evaluation

13. Storage
How do we efficiently store / retrieve dynamic data? (Indexing)
It depends on the use cases:
Querying on a certain time (Indexing by time)
What was the temperature in Ghent yesterday?
Querying for a certain time (Indexing by property)
When was it 20°C in Ghent?
Can we / Do we have to combine these indexing techniques?

14. Transmission
Disadvantage:
Moving query evaluation to the client requires more data to be transfered
→ Increases bandwidth usage
→ Slows down query evaluation
→ Limits query frequency
Possible solutions:
Compression within and between versions
Caching
Higher data selectivity

15. Query Evaluation
Scope: Data with a predictable valid time
Some thermometers measure /min → data will not change during that minute.
Otherwise we need to poll or have a persistent server connection
Annotate data with their valid time:
Thermometer_1 : 10°C (10:00 - 10:01)
Thermometer_1 : 20°C (10:01 - 10:02)
Thermometer_1 : 20°C (10:02 - 10:03)
→ Clients can fetch this data as if it was static data

16. Overview
Research questions
Research approach
Evaluation plan
Preliminary results

17. Evaluation of the three parts
Storage
Transmission
Query evaluation
Insertion, lookup, size
Latency, bandwidth, cacheability
Result latency

18. Combined evaluation
Realistic datasets/datastreams and queries
Compare with:
Server-side:
C-SPARQL (Barbieri 2012)
CQELS (Le-Phuoc 2011)
Client-side:
Ztreamy (Fisteus 2014)
Compare by:
latency
completeness
server load
client load
scalability
→ LSBench (Le-Phuoc 2012), SRBench (Zhang 2012), CityBench (Ali 2015), ...

19. Overview
Research questions
Research approach
Evaluation plan
Preliminary results

20. Preliminary scalability test
Query Streamer prototype (Taelman 2016), based on TPF
Test server load for increasing #clients
Compared with C-SPARQL, CQELS

21. Query Streamer moves load from server to client
Server scalability Client load

22. Overview
Research questions
Research approach
Evaluation plan
Preliminary results