Custom SPARQL Functions

I put together a more useful example of how to write a custom SPARQL function with bigdata. It’s up on the wiki here:

The example details a common use case – filtering out solutions based on security credentials for a particular user. For example, if you wanted to return a list of document visible to the user “John”, you could do it with a custom SPARQL function:

PREFIX ex: <>
  ?doc rdf:type ex:Document .
  filter(ex:validate(?doc, ?user)) .
BINDINGS ?user {

The function is called by referencing its unique URI, in this case ex:validate. This URI must be registered with bigdata’s FunctionRegistry along with an appropriate factory and operator. The wiki details how to do that. In the query above, the function is called with two arguments, the document to be validated and the user to validate against. The user in this simple example is a constant included in the BINDINGS clause. Always remember that bigdata custom functions are executed one solution at a time – they do not yet benefit from vectored execution and thus are not suitable for reading data from the indices. (The functions must operate without reading from the index on a per execution call basis.) A custom service (distinct from a custom function) is a more appropriate choice when execution requires touching indices. This is how we implement SPARQL 1.1 Federation.

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