public Triple getSampleFromAnyEndpoint(Triple triplePattern)
throws SADIException, IOException, NoSampleAvailableException, ExceededMaxAttemptsException {
List<SPARQLEndpoint> endpoints =
new ArrayList<SPARQLEndpoint>(getRegistry().findEndpointsByTriplePattern(triplePattern));
RandomDataImpl generator = new RandomDataImpl();
int attempts = 0;
SPARQLEndpoint endpoint = null;
while (attempts < MAX_ATTEMPTS) {
if (endpoints.size() == 0)
throw new NoSampleAvailableException(
"there are no triples matching "
+ triplePattern
+ " in the data (without blank nodes)");
int endpointIndex = endpoints.size() > 1 ? generator.nextInt(0, endpoints.size() - 1) : 0;
endpoint = endpoints.get(endpointIndex);
if (getRegistry().getServiceStatus(endpoint.getURI()) == ServiceStatus.DEAD) {
endpoints.remove(endpointIndex);
continue;
}
try {
return getSampleFromEndpoint(endpoint, triplePattern);
} catch (NoSampleAvailableException e) {
log.warn(String.format("failed to retrieve sample from %s", endpoint), e);
} catch (IOException e) {
log.warn(String.format("failed to retrieve sample from %s", endpoint), e);
}
endpoints.remove(endpointIndex);
attempts++;
}
throw new ExceededMaxAttemptsException(
"exceeded "
+ MAX_ATTEMPTS
+ " attempts when trying to retrieve triples matching "
+ triplePattern);
}
protected Triple getSampleFromEndpoint(
SPARQLEndpoint endpoint, Triple triplePattern, long sampleIndex)
throws SADIException, IOException, NoSampleAvailableException {
log.trace(
"retrieving triple #"
+ sampleIndex
+ " for "
+ triplePattern
+ " from "
+ endpoint.getURI());
Query query =
getConstructQuery(
triplePattern); // SPARQLStringUtils.getConstructQuery(Collections.singletonList(triplePattern), Collections.singletonList(triplePattern));
query.setOffset(sampleIndex);
query.setLimit(1);
log.trace(String.format("sample query: %s", query.serialize()));
Collection<Triple> triples = endpoint.constructQuery(query.serialize());
if (triples.size() == 0) {
throw new RuntimeException("triple #" + sampleIndex + " doesn't exists in " + endpoint);
}
Triple triple = triples.iterator().next();
// Sanity check. If the index is incomplete or out of date, the sample triple may not satisfy
// the predicate list / regular expressions for the endpoint. The simplest thing to do in
// this case is to fail and take another sample.
if (getRegistry() != null
&& !getRegistry().findEndpointsByTriplePattern(triple).contains(endpoint)) {
throw new NoSampleAvailableException(
"sample triple does not match the regular expressions for "
+ endpoint.getURI()
+ " (from which it was sampled!)");
}
return triple;
}
protected long getUpperSampleLimit(SPARQLEndpoint endpoint, Triple triplePattern)
throws SADIException, IOException {
String uri = endpoint.getURI();
log.trace("determining number of triples matching " + triplePattern + " in " + uri);
ServiceStatus status = ServiceStatus.OK;
if (getRegistry() != null) {
status = getRegistry().getServiceStatus(endpoint.getURI());
}
// check for a cached value first
if (upperSampleLimitCache.contains(endpoint, triplePattern)) {
log.trace("using previously cached value for upper sample limit");
return upperSampleLimitCache.get(endpoint, triplePattern);
}
ElementGroup whereClause = getWhereClauseWithBlankNodeFilter(triplePattern);
/*
Node s = triplePattern.getSubject();
Node o = triplePattern.getObject();
// Build a Jena representation of the WHERE clause
ElementGroup whereClause = new ElementGroup();
whereClause.addTriplePattern(triplePattern);
if(s.isVariable()) {
whereClause.addElementFilter(new ElementFilter(new E_LogicalNot(new E_IsBlank(new ExprVar(s)))));
}
if(o.isVariable()) {
whereClause.addElementFilter(new ElementFilter(new E_LogicalNot(new E_IsBlank(new ExprVar(o)))));
}
*/
if (status != ServiceStatus.SLOW) {
try {
// issue a SELECT COUNT(*) query
Query countStarQuery = new Query();
countStarQuery.setQuerySelectType();
countStarQuery.setQueryPattern(whereClause);
countStarQuery.addResultVar(countStarQuery.allocAggregate(AggCount.get()));
List<Map<String, String>> results = endpoint.selectQuery(countStarQuery.serialize());
Map<String, String> firstRow = results.iterator().next();
String firstColumn = firstRow.keySet().iterator().next();
long limit = Long.parseLong(firstRow.get(firstColumn));
upperSampleLimitCache.put(endpoint, triplePattern, limit);
log.trace(String.format("successful upper limit query: %s", countStarQuery.serialize()));
log.trace(String.format("upper limit: %d", limit));
return limit;
} catch (IOException e) {
log.warn(
"failed to COUNT number of triples matching "
+ triplePattern
+ " in "
+ uri
+ ", trying for a lower bound instead.");
}
}
Query selectStarQuery = new Query();
selectStarQuery.setQuerySelectType();
selectStarQuery.setQueryPattern(whereClause);
selectStarQuery.setQueryResultStar(true);
long limit = endpoint.getResultsCountLowerBound(selectStarQuery.serialize(), 50000);
upperSampleLimitCache.put(endpoint, triplePattern, limit);
log.trace(String.format("successful upper limit query: %s", selectStarQuery.serialize()));
log.trace(String.format("upper limit: %d", limit));
return limit;
}