The talk I gave at the Stream Reasoning workshop in TU Berlin on December 8. I give an overview of RSEP-QL and how it can capture and formalise the behaviour of existing RSP engines, e.g. CSPARQL, EP-SPARQL, CQELS, SPARQLstream

1. Department of Informatics
On unifying query languages
for RDF streams
Daniele Dell’Aglio
dellaglio@ifi.uzh.ch http://dellaglio.org @dandellaglio
Stream Reasoning workshop 2016 Berlin, 08.12.2016

2. Stream Processing and RDF Stream Processing
In literature there are two different main approaches to
process streams
Data Stream Management Systems (DSMSs)
• Aggregations and filters
• Relies on the notion of sliding window
• DSMS-inspired RSPs: C-SPARQL, CQELS, SPARQLstream
Complex Event Processors (CEPs)
• Search of relevant patterns in the stream
• Non-equi-join on timestamps (after, before, etc.)
• CEP-inspired RSP: EP-SPARQL
• C-SPARQL offers some CEP features through a timestamp
function; CQELS has some initial extension on this
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 2/19

3. The problem
Unpredictable behaviour
Exec. 1° answer 2° answer
1 :rRoom [6] :bRoom [11]
2 :rRoom [5] :bRoom [10]
3 :rRoom [6] :bRoom [11]
4 - [7] - [12]
Where are :a and :b, when they are
together?
{:a :isIn ?room . :b :isIn ?room}
Let’s consider a tumbling window
W(ω=β=5)
Let’s execute the experiment 4 times
on CSPARQL
G1 G2 G3
t
{:a :isIn :rRoom}
{:b :isIn :rRoom}
{:a :talksIn :bRoom}
G4S1
{:b :talksIn :bRoom}
3 6 92
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 3/19

4. The problem
Different behaviours (1)
Exec. 1° answer 2° answer
1 :rRoom [6] :bRoom [11]
2 :rRoom [5] :bRoom [10]
3 :rRoom [6] :bRoom [11]
4 - [7] - [12]
Exec. 1° answer 2° answer
1 :rRoom [3] :bRoom [9]
2 No answers
3 :rRoom [3] :bRoom [9]
4 No answers
CSPARQL CQELS
G1 G2 G3
t
{:a :isIn :rRoom}
{:b :isIn :rRoom}
{:a :talksIn :bRoom}
G4S1
{:b :talksIn :bRoom}
3 6 92
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 4/19

5. The problem
Different behaviours (2)
Engine 7 8 9
EP-SPARQL
(unrestricted)
:a, :c [1,5]
:b, :d [3,5]
:b, :d [3,7]
:a, :c [1,9]
EP-SPARQL
(chronological)
:a, :c [1,5]
:b, :d [3,5]
EP-SPARQL
(recent)
:b, :d [3,5]
:b, :d [3,7]
C-SPARQL
(sl. win. 7,1)
:b, :d [3,7]
Who is listening to who?
{?att :isIn ?room}
SEQ
{?spe :talksIn ?room}
G1 G2 G3 G4
t
{:a :isIn :rRoom}
{:b :isIn :bRoom}
{:c :talksIn :rRoom,
:d :talksIn :bRoom}
{:d :talksIn :bRoom}
G6
{:e :isIn :bRoom}
S2 G5
{:c :talksIn :rRoom}
3 5 91 7 8
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 5/19

6. Research Question
In the context of continuous query answering over RDF
streams, how can the behaviour of existing systems be
captured, compared and contrasted?
Why do we need it?
• Comparison and contrast
• Interoperability
• Standard RSP query language
• Study RDF Stream Processing and Stream Reasoning related
problems
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 6/19

7. Background
data
Streams
RSEP-QL
Applications
RSP-QL BGP evaluation
over background
data
BGP evaluation
over streams
Event Pattern
detection operators
Model to express
continous queries
RDF
SPARQL
RSEP-QL
A reference model that formally defines the semantics of
RDF Stream Processing engines
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 7/19

8. RSEP-QL
From SPARQL to RSEP-QL
Evaluator
Data layer
Result
Formatter
Ans(Q)RDF graphs
E
DS
QF
Query
Interface
Q
(E, DS, QF)
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 8/19

9. Q
(E, DS, QF)
RSEP-QL
From SPARQL to RSEP-QL
Evaluator
Data layer
Result
Formatter
Ans(Q)RDF graphs
E
DS
QF
RDF graphs
RDF streams
Query
Interface
SDS
Q
(E, SDS, QF)
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 8/19

10. Q
(E, DS, QF)
RSEP-QL
From SPARQL to RSEP-QL
Evaluator
Data layer
Result
Formatter
Ans(Q)RDF graphs
E
DS
QF
Continuous
EvaluatorET
RDF graphs
RDF streams
Query
Interface
SDS
Q
(E, SDS, QF)
Q
(E, SDS, ET, QF)
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 8/19

11. Q
(E, DS, QF)
RSEP-QL
From SPARQL to RSEP-QL
Evaluator
Data layer
Result
Formatter
Ans(Q)RDF graphs
E
DS
QF
Continuous
EvaluatorET
RDF graphs
RDF streams
Query
Interface
SDS
Q
(E, SDS, QF)
Q
(E, SDS, ET, QF)
Q
(SE, SDS, ET, QF)
SE
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 8/19

12. RSEP-QL: Data
RDF Streams and windows
An RDF stream is an (infinite) ordered sequence of time-
annotated data items (RDF graphs)
𝕎(3,1,0)
𝕃(1)
𝑊(2,5] (Fixed) Window
Landmark window
Sliding window
G1 G2 G3 G4
{:a :isIn :rRoom}
{:b :isIn :bRoom}
{:c :talksIn :rRoom,
:d :talksIn :bRoom}
{:d :talksIn :bRoom}
G6
{:e :isIn :bRoom}
S2 G5
{:c :talksIn :rRoom}
3 5 91 7 8
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 9/19

13. RSEP-QL: Operators
• SPARQL operators
• Stream Processing operators (RSP-QL)
• WINDOW to specify that the active element is a window
(similar to GRAPH)
• RStream, IStream, DStream to create the output stream
• Event Processing operators (RSEP-QL)
• MATCH to describe an event pattern
• EVENT w P Basic Event Pattern
• FIRST E
• LAST E
• E1 SEQ E2
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 10/19

14. RSEP-QL: Instantaneous Evaluation Semantics
The SPARQL evaluation function is defined as
⟦𝑃⟧ 𝐷𝑆(𝐺)
Two evaluation functions in RSEP-QL:
• For SPARQL and RSP-QL operators: ⟪𝑃⟫ 𝑆𝐷𝑆(𝐴)
𝑡
• For event processing operators: ⦅𝐸⦆ 𝑜,𝑐
𝑡
The two functions co-exist
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 11/19

15. The evaluations usually do not occur at every time instant,
but only at some of them (evaluation time instants)
• We indicate with ET the set of evaluation time instants
It is not always possible to define ET a priori
• ET is expressed through a Report Policy (from the SECRET
framework)
Report Policy examples:
• WC Window Close: an evaluation occurs every time the active
window closes
• C-SPARQL, SPARQLstream
• CC Content Change: an evaluation occurs every time the stream
content changes.
• CQELS, EP-SPARQL
RSEP-QL: Continuous Evaluation
Evaluation time instants
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 12/19

16. RSEP-QL: Continuous Evaluation
Consumption policies
The evaluations in a sequence are not always independent
• Some data may be involved in the current evaluation only if it
was not before
We need a mechanism to move this information between
evaluations
• Consumption policies are functions to describe the input for
the next evaluation
Two policies for our needs
• 𝒫 𝑢: the input is the one selected by the window
• 𝒫 𝑛
: the input is the one selected by the window and that
was not used to produce results in the previous evaluations
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 13/19

17. Execution 1° answer 2° answer
1 :rRoom [6] :bRoom [11]
2 :rRoom [5] :bRoom [10]
3 :rRoom [6] :bRoom [11]
4 - [7] - [12]
Window 1° answer 2° answer
t0=0 :rRoom [5] :bRoom [10]
RSEP-QL in action
Unpredictable behaviours
G1 G2 G3
t
{:a :isIn :rRoom}
{:b :isIn :rRoom}
{:a :talksIn :bRoom}
G4S1
{:b :talksIn :bRoom}
3 6 92
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 14/19

18. Execution 1° answer 2° answer
1 :rRoom [6] :bRoom [11]
2 :rRoom [5] :bRoom [10]
3 :rRoom [6] :bRoom [11]
4 - [7] - [12]
Window 1° answer 2° answer
t0=0 :rRoom [5] :bRoom [10]
t0=1 :rRoom [6] :bRoom [11]
RSEP-QL in action
Unpredictable behaviours
G1 G2 G3
t
{:a :isIn :rRoom}
{:b :isIn :rRoom}
{:a :talksIn :bRoom}
G4S1
{:b :talksIn :bRoom}
3 6 92
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 14/19

19. Execution 1° answer 2° answer
1 :rRoom [6] :bRoom [11]
2 :rRoom [5] :bRoom [10]
3 :rRoom [6] :bRoom [11]
4 - [7] - [12]
Window 1° answer 2° answer
t0=0 :rRoom [5] :bRoom [10]
t0=1 :rRoom [6] :bRoom [11]
t0=2 - [7] - [12]
RSEP-QL in action
Unpredictable behaviours
G1 G2 G3
t
{:a :isIn :rRoom}
{:b :isIn :rRoom}
{:a :talksIn :bRoom}
G4S1
{:b :talksIn :bRoom}
3 6 92
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 14/19

20. RSEP-QL in action
Different behaviours (1)
Exec. 1° answer 2° answer
1 :rRoom [6] :bRoom [11]
2 :rRoom [5] :bRoom [10]
3 :rRoom [6] :bRoom [11]
4 - [7] - [12]
Exec. 1° answer 2° answer
1 :rRoom [3] :bRoom [9]
2 No answers
3 :rRoom [3] :bRoom [9]
4 No answers
CSPARQL CQELS
G1 G2 G3
t
{:a :isIn :rRoom}
{:b :isIn :rRoom}
{:a :talksIn :bRoom}
G4S1
{:b :talksIn :bRoom}
3 6 92
CQELS
CSPARQL
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 15/19

21. Engine 7 9
EP-SPARQL
(unrestricted)
:a, :c [1,5]
:b, :d [3,5]
:b, :d [3,7]
:a, :c [1,9]
EP-SPARQL
(chronological)
:a, :c [1,5]
:b, :d [3,5]
EP-SPARQL
(recent)
:b, :d [3,5]
:b, :d [3,7]
C-SPARQL
(sl. win. 7,1)
:b, :d [3,7]
G1 G2 G3 G4
t
{:a :isIn :rRoom}
{:b :isIn :bRoom}
{:c :talksIn :rRoom,
:d :talksIn :bRoom}
{:d :talksIn :bRoom}
G6
{:e :isIn :bRoom}
S2 G5
{:c :talksIn :rRoom}
3 5 91 7 8
RSEP-QL in action
Different behaviours (2)
SEQ operators with
different selection
policies
• E1 SEQ E2
• EP-SPARQL (unr.)
• E1 SEQc E2
• EP-SPARQL (chr.)
• E1 SEQr E2
• EP-SPARQL (rec.)
• E1 SEQn E2
•C-SPARQL
Different
consumption policies
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 16/19

22. Captured behaviours
System Report
policy
Streaming
operator
Window
operator
Sequencin
g operator
Consumpti
on policy
CQELS Content
change
Istream Sliding SEQ 𝒫 𝑢
SPARQLstream Window
close
Rstream,
Dstream,
Istream
Sliding - 𝒫 𝑢
C-SPARQL Window
close
Rstream Sliding SEQn
(timestamp
function)
𝒫 𝑢
EP-SPARQL
(unrestricted)
Content
change
Rstream Landmark SEQ 𝒫 𝑢
EP-SPARQL
(chronological)
Content
change
Rstream Landmark SEQc 𝒫 𝑛
EP-SPARQL
(recent)
Content
change
Rstream Landmark SEQr 𝒫 𝑛
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 17/19

23. Conclusions
The dynamics introduced in the continuous query
evlauation process have not been totally understood
• Not fully captured by existing models
RSEP-QL captures those dynamics, and the behaviours of
existing RDF Stream Processing engines
• DSMS-based (RSP-QL)
• CEP-based (RSEP-QL)
What’s next
• Continuous query answering and reasoning
• Further investigation about the relation of RSEP-QL with other
models (SPARQL, LARS)
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 18/19

24. Thank you! Questions?
On unifying query languages for RDF
streams
Daniele Dell’Aglio
dellaglio@ifi.uzh.ch
http://dellaglio.org
@dandellaglio
08.12.2016 Daniele Dell'Aglio - Stream Reasoning worksho 2016 19/19