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RSPQLStarEngine

This document describes the current state of the RSPQLStarEngine as of May, 2019, and clarifies some important terms and design decisions. The engine based on the RSP-QL semantics but implements the RDF* and SPARQL* extensions to represent and query statement-level metadata. The store is based primarily on Apache Jena 3.7.0.

RSP-QL

RSP-QL is an extension of standard SPARQL, developed by the RSP W3C Community Group (www.w3id.org/rsp) to support processing and querying of streaming data. RSP-QL provides language constructs for defining temporal windows that define discrete selections of over potentially infinite RDF streams. We include in here the construct for executing queries at a fixed interval, as described in C-SPARQL.

The query below illustrates some of the core functionality of the language. Here we define a window over a stream with a width of 10 seconds that slides over the stream every second. The query is executed once per second.

PREFIX : <http://example.org#>
REGISTER STREAM :mystream COMPUTED EVERY PT1S AS 

SELECT ?obs ?value
FROM NAMED WINDOW :w ON :stream [RANGE PT10S STEP PT1S]
WHERE {
   WINDOW :w {
      GRAPH ?g {
         ?obs a :Observation ;
              :hasValue ?value .
      }
   }
}

Extensions to RDF and SPARQL

The basic idea of RDF* is that a triple can contain an embedded triple in the subject or object position, e.g.:

<<:obs1 :hasValue 1.5>> :confidence "0.99"^^xsd:float

Similarly, embedded triples can also be referenced as triple patterns in SPARQL*, e.g.:

PREFIX : <http://example.org#>
SELECT ?value ?confidence
WHERE {
   <<?obs :hasValue ?value>> :confidence ?confidence
}

RSP-QL*

RSP-QL* is an extension of RSP-QL that leverages RDF* and SPARQL* to provide statement-level annotations. Let's return to the previous RSP-QL query above, but this time we assume that each value in the stream is associated with some confidence value. The query below shows how this would be accomplished by using RDF reification, and the second query shows the same query expressed using RSP-QL*.

PREFIX : <http://example.org#>
REGISTER STREAM :mystream COMPUTED EVERY PT1S AS 

SELECT ?value ?confidence
FROM NAMED WINDOW :w ON :stream [RANGE PT10S STEP PT1S]
WHERE {
   WINDOW :w {
      GRAPH ?g {
         ?obs a :Observation ;
              :hasValue ?value .
         [] a rdf:Statement ;
            rdf:subhect ?obs ;
            rdf:predicate a ;
            rdf:object ?value ;
            :confidence ?confidence .
       }
   }
}

PREFIX : <http://example.org#>
REGISTER STREAM :mystream COMPUTED EVERY PT1S AS 

SELECT ?value ?confidence
FROM NAMED WINDOW :w ON :stream [RANGE PT10S STEP PT1S]
WHERE {
   WINDOW :w {
      GRAPH ?g {
         ?obs a :Observation .
         <<?obs :hasValue ?value>> :confidence ?confidence . }
   }
}

Node dictionary

All incoming nodes are mapped to IDs (longs) in a NodeDictionary. All mappings in the node dictionary use IDs where the most significant bit (MSB) of the ID is not set. For example, an ID with value 5 would be encoded as:

0-000000000000000000000000000000000000000000000000000000000000101

Reference dictionary

The ReferenceDictionary instead encodes a special type of ID, which maps a a triple to an ID. The reference dictionary uses IDs where the most significant (MSB) of the ID is set. For example, a reference ID with value 5 would be encoded as;

1-000000000000000000000000000000000000000000000000000000000000101

Variable dictionary

The VariableDictionary encodes the variables of the triple patterns in queries as integers. Having this numeric representation of variables makes lowers the comparison cost compared with the default Jena implementation.

Indexes

The main index of the system contains six types of indexes: GSPO, GPOS, GOSP, SPOG, POSG, and OSPG. Currently, a TreeMap is used for storing the encoded quads in the index, but the implementation can easily be extended to use other types of indexes.

Decoding

The bulk of the processing of the system is handled by an extension of the Jena execution environment. For some of the processing, the node IDs have to be decoded into regular Jena nodes. This is done by either: 1) looking up the corresponding values in the dictionaries, 2) wrapping the ID-based iterator representation in a DecodingQuadsIterator, or 3) and iterator with bindings in DecodeBindingsIterator.