Understanding SPARQLMotion

SPARQLMotion is an entirely declarative language. Scripts are defined by means of instances of the SPARQLMotion ontologies and then interpreted by a SPARQLMotion execution engine. The engine "knows" how to execute each step and executes internal (Java) code to actually perform the necessary steps. If two modules are linked via the sm:next property, then the output of the first module becomes the input of the second module.

Importing Data

SPARQLMotion scripts typically start with modules that import data from the outside into the script. A typical examples are sml:ImportRDFFromURL that imports RDF from some server. A simple way of getting started is to take sml:ImportCurrentRDF as starting point - this will simply pass all the triples of the currently open model (the script itself) into the script, including all imports etc.

Processing Data

Most modules process RDF data. In the default implementation, this means that they operate on Jena RDF Graphs. If a module takes multiple input modules, then it will operate on the union graph that results from merging the single input graphs.

Many modules are controlled by SPARQL queries (Select, Construct and Ask). These queries will be executed over the input RDF graphs.

Using Variables

In addition to passing RDF, modules can bind variables and use variables defined by their predecessors. Some modules such as sml:EnterLiteral create new variable bindings. There are two common ways of using variables in modules:

First, if a bound variable is mentioned in a SPARQL query, then the query will be executed with the current variable's value. Note that variables that are bound to a blank node will be ignored if a SPARQL query calls a remote server (with a FROM clause).

Second, all string properties of a module definition can reference variables bindings using a construct such as {?varName}. For example, if the variable ?placeName contains the value "London", then the URL

http://ws.geonames.org/search?type=rdf&maxRows=10&q={?placeName}

will be converted into

http://ws.geonames.org/search?type=rdf&maxRows=10&q=London

Controlling the Execution Flow

SPARQLMotion scripts are always executed from top to bottom, i.e. starting with the nodes that don't have any predecessors and then walking down into the sm:next nodes. The order of execution is non-predictable when a script branches, but the engine will always make sure that all predecessors have been executed before the module itself. In addition to this execution policy, there are currently two control constructs available: branching (if-then-else) and iterating (for each).

• sml:IterateOverSelect runs a SPARQL Select query and repeats a nested sub-script (the body) for each match of the query. The variable mentioned in the Select query will be bound in each iteration.
• sml:BranchByAsk runs a SPARQL Ask query which delivers a boolean. Depending on the outcome, either the sm:if or the sm:else branch are executed.

In both cases, the main execution thread stops until the nested script is completed. The outcome of the end nodes of the nested scripts will then be passed into the sm:next node of the control module.