doc: explain RDF4J endpoint

doc/endpoints.md

@@ -19,18 +19,45 @@ programming language or environment.
 SPARQL is inspired by SQL, a simple query that returns all triplets
 in the store looks like
 
-	SELECT *
-	WHERE { ?s ?p ?o }
+		SELECT *
+		WHERE { ?s ?p ?o }
 
 where `s`, `p` and `o` are query variables. The result of a query are
 valid substitutions for the query variables. In this case, the
 database would return a table of all triplets in the store sorted by
 subject `s`, predicate `p` and object `o`.
 
-Of course, queries might return a lot of data. Importing just the
-Isabelle exports [3] into GraphDB results in >200M triplets. This
+Of course, queries might return a lot of data. Importing just the Isabelle exports [3] into GraphDB results in >200M triplets. This
 is solved with pagination techniques [4].
 
+RDF4J
+-----
+
+RDF4J [5] is a Java API for interacting with triplet stores, implemented
+based on a superset of SPARQL. GraphDB supports RDF4J, in fact it is the
+recommended way of interacting with GraphDB repositories [6].
+
+Instead of formulating textual queries, RDF4J allows developers to query
+a repository by calling Java API methods. Above query that returns all triplets
+in the store looks like
+
+		connection.getStatements(null, null, null);
+
+in RDF4J. Method `getStatements(s, p, o)` returns all triplets that
+have matching subject `s`, predicate `p` and object `o`. If any of
+these arguments is `null`, it can be any value, i.e. it is a query
+variable that is to be filled by the call to `getStatements`.
+
+Comparing SPARQL to RDF4J
+-------------------------
+
+While plain SPARQL offers independence from the JVM, RDF4J is a
+convenient interface that makes it easy to write applications with
+problems like pagination already taken care of.
+
+Of course the interesting question for the task of building an ULO/RDF
+endpoint is what works better for this particular application.
+
 References
 ----------
 
@@ -41,3 +68,7 @@ References
 [3] https://gl.mathhub.info/Isabelle
 
 [4] https://stackoverflow.com/questions/27488403/paginating-sparql-results
+
+[5] https://rdf4j.org/
+
+[6] http://graphdb.ontotext.com/documentation/free/using-graphdb-with-the-rdf4j-api.html