Find shortest path using GremlinPipeline

Question

My data is stored in a Titan graph database. I am trying to find the shortest path between two vertices (v1 and v2). Currently I have the following code:

    final Vertex v1 = titanGraph.getVertices("nodeId", "110969224").iterator().next();
    final Vertex v2 = titanGraph.getVertices("nodeId", "141396276").iterator().next();
    System.out.println(v2);

    final GremlinPipeline<String, List> pipe = new GremlinPipeline<String, List>(v1)
            .as("similar")
            .both("similar")
            .loop("similar", new PipeFunction<LoopBundle<Vertex>, Boolean>() {
                @Override
                public Boolean compute(LoopBundle<Vertex> bundle) {
                    return bundle.getLoops() < 4 && bundle.getObject() != v2;
                }
            })  
            .path();

which returns alls the paths. I have the following questions:

• Is this tha fastest way to find the shortest path?
• How can I get the shortest of all these paths?

EDIT: I am trying to do the same work but with GremlinGroovyScriptEngine. I have the following code:

    List results = new ArrayList();
    Bindings bindings = engine.createBindings();
    bindings.put("v1", v1); 
    bindings.put("v2", v2); 
    bindings.put("results", results);

    engine.eval("v1.both.filter{it.nodeId!='nodeId'}.loop('similar'){!it.object.equals(v2) && it.loop < 5}.paths.fill(results)", bindings);

but I get the following error:

Exception in thread "main" javax.script.ScriptException: javax.script.ScriptException: java.lang.NullPointerException
    at com.tinkerpop.gremlin.groovy.jsr223.GremlinGroovyScriptEngine.eval(GremlinGroovyScriptEngine.java:94)
    at javax.script.AbstractScriptEngine.eval(AbstractScriptEngine.java:233)
    at TitanQuery.findShortestPath(TitanQuery.java:89)
    at TitanQuery.main(TitanQuery.java:40)
Caused by: javax.script.ScriptException: java.lang.NullPointerException
    at com.tinkerpop.gremlin.groovy.jsr223.GremlinGroovyScriptEngine.eval(GremlinGroovyScriptEngine.java:221)
    at com.tinkerpop.gremlin.groovy.jsr223.GremlinGroovyScriptEngine.eval(GremlinGroovyScriptEngine.java:90)
    ... 3 more
Caused by: java.lang.NullPointerException
    at com.tinkerpop.pipes.branch.LoopPipe.getLoops(LoopPipe.java:75)
    at com.tinkerpop.pipes.branch.LoopPipe.processNextStart(LoopPipe.java:49)
    at com.tinkerpop.pipes.AbstractPipe.next(AbstractPipe.java:89)
    at com.tinkerpop.pipes.transform.PropertyPipe.processNextStart(PropertyPipe.java:29)
    at com.tinkerpop.pipes.AbstractPipe.next(AbstractPipe.java:89)
    at com.tinkerpop.pipes.util.Pipeline.next(Pipeline.java:115)
    at com.tinkerpop.pipes.util.PipeHelper.fillCollection(PipeHelper.java:52)
    at com.tinkerpop.gremlin.java.GremlinPipeline.fill(GremlinPipeline.java:1575)
    at com.tinkerpop.gremlin.java.GremlinFluentPipeline$fill.call(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:108)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:116)
    at Script1.run(Script1.groovy:1)
    at com.tinkerpop.gremlin.groovy.jsr223.GremlinGroovyScriptEngine.eval(GremlinGroovyScriptEngine.java:219)
    ... 4 more

Any advice for any of these issues would be great.

Answer 1

Your code looks to be somewhat similar to the the shortest path recipe in GremlinDocs:

http://gremlindocs.com/#recipes/shortest-path

You might want to read that section in full as you are evaluating both directions of a vertex which has consequences and has been shown to be better handled with the store/except pattern.

Once you have all the paths need to just select the shortest one from the returned list. In pure Java that's a bit more work than with Groovy, but it basically boils down to a sort on the path length and then choosing the shortest one. In groovy that would be something like:

gremlin> g.v(1).out.loop(1){it.object.id != "3" && it.loops < 6}.path.sort{a,b->a.size()<=>b.size()}   
==>[v[1], v[3]]
==>[v[1], v[4], v[3]]

Looking at that made me wonder if you could always just pop-off the first item in the pipeline as it would be the earliest path detected and hence the shortest:

gremlin> g.v(1).out.loop(1){it.object.id != "3" && it.loops < 6}.path[0]                            
==>[v[1], v[3]]

You might want to experiment with that a bit, but it sounds like a promising theory that would allow you to short-circuit the pipeline if you just needed the first shortest path detected.