[TINKERPOP-1519] TinkerGraphComputer doesn't handle multiple MessageScopes in single iteration

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 3.1.1-incubating
• Fix Version/s: 3.1.8
• Component/s: tinkergraph
• Environment: Mac OSX
• External issue URL: https://groups.google.com/forum/#!topic/gremlin-users/t5eiOX3bZRM
• Assignee: Marko A. Rodriguez
• Reporter: Felix Chapman
• Created: 18/Oct/16 16:25
• Updated: 27/Jun/17 18:20
• Resolved: 27/Jun/17 18:20

Description

When executing a VertexProgram that sends messages on multiple MessageScopes in a single iteration, then the messages behave as if they were sent on all scopes within that iteration.

e.g. if you send message A on out edges, and message B on in edges, then A and B will instead be sent over both in and out edges.

The problem can be resolved by using only a single MessageScope per iteration, but this involves increasing the number of iterations.

An example of this behaviour is below:

public class TinkerTest {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        TinkerGraph graph = TinkerGraph.open();
        Vertex a = graph.addVertex("a");
        Vertex b = graph.addVertex("b");
        Vertex c = graph.addVertex("c");
        a.addEdge("edge", b);
        b.addEdge("edge", c);

        // Simple graph:
        // a -> b -> c

        // Execute a traversal program that sends an incoming message of "2" and an outgoing message of "1" from "b"
        // then each vertex sums any received messages
        ComputerResult result = graph.compute().program(new MyVertexProgram()).submit().get();

        // We expect the results to be {a=2, b=0, c=1}. Instead it is {a=3, b=0, c=3}
        System.out.println(result.graph().traversal().V().group().by(Element::label).by("count").next());
    }
}

class MyVertexProgram implements VertexProgram<Long> {

    private final MessageScope.Local<Long> countMessageScopeIn = MessageScope.Local.of(__::inE);
    private final MessageScope.Local<Long> countMessageScopeOut = MessageScope.Local.of(__::outE);

    private static final String MEMORY_KEY = "count";

    private static final Set<String> COMPUTE_KEYS = Collections.singleton(MEMORY_KEY);

    @Override
    public void setup(final Memory memory) {}

    @Override
    public GraphComputer.Persist getPreferredPersist() {
        return GraphComputer.Persist.VERTEX_PROPERTIES;
    }

    @Override
    public Set<String> getElementComputeKeys() {
        return COMPUTE_KEYS;
    }

    @Override
    public Set<MessageScope> getMessageScopes(final Memory memory) {
        return Sets.newHashSet(countMessageScopeIn, countMessageScopeOut);
    }

    @Override
    public void execute(Vertex vertex, Messenger<Long> messenger, Memory memory) {
        switch (memory.getIteration()) {
            case 0:
                if (vertex.label().equals("b")) {
                    messenger.sendMessage(this.countMessageScopeIn, 2L);
                    messenger.sendMessage(this.countMessageScopeOut, 1L);
                }
                break;
            case 1:
                long edgeCount = IteratorUtils.reduce(messenger.receiveMessages(), 0L, (a, b) -> a + b);
                vertex.property(MEMORY_KEY, edgeCount);
                break;
        }
    }

    @Override
    public boolean terminate(final Memory memory) {
        return memory.getIteration() == 1;
    }

    @Override
    public GraphComputer.ResultGraph getPreferredResultGraph() {
        return GraphComputer.ResultGraph.NEW;
    }

    @Override
    public MyVertexProgram clone() {
        try {
            return (MyVertexProgram) super.clone();
        } catch (final CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }

}

Issue Links

• links to: GitHub Pull Request #632