Re: dropStep and BarrierStep

[pieter gmail]

Hi,

Any ideas about this one?

Thanks
Pieter

On 22/11/2017 19:49, pieter gmail wrote:

Hi,

Whilst optimizing the DropStep in Sqlg I came across RepeatStep plus DropStep in a traversal going into a infinite loop.

To test Sqlg I tested on TinkerPop's ComplexTest.playlistPath() changing the gremlin to have a drop()

    @Test
    public void testDropPlaylist() {
        final TinkerGraph g = TinkerGraph.open();

        Io.Builder<GraphSONIo> builder = GraphSONIo.build(GraphSONVersion.V3_0);

        final GraphReader reader = g.io(builder).reader().create();

        try (final InputStream stream = AbstractGremlinTest.class.getResourceAsStream("/grateful-dead-v3d0.json")) {

            reader.readGraph(stream, g);
        } catch (IOException e) {
            Assert.fail(e.getMessage());
        }

        Traversal<Vertex, Vertex> playListTraversal = getPlaylistPaths(g);

        System.out.println(playListTraversal.toString());
        List<Vertex> vertices = playListTraversal.toList();
        Assert.assertEquals(100, vertices.size());
        System.out.println("counted");

        Traversal<Vertex, Vertex> dropTraversal = getPlaylistPaths(g).drop().drop().iterate();

        System.out.println("done");
    }

    public GraphTraversal<Vertex, Vertex> getPlaylistPaths(Graph graph) {

        return graph.traversal().V().has("name", "Bob_Dylan").in("sungBy").as("a").

repeat(__.out().order().by(Order.shuffle).simplePath().from("a")).

                until(__.out("writtenBy").has("name", "Johnny_Cash")).limit(1).as("b"). repeat(__.out().order().by(Order.shuffle).as("c").simplePath().from("b").to("c")).                 until(__.out("sungBy").has("name", "Grateful_Dead")).limit(100);

    }

The second dropTraversal goes into an infinite loop.

A simpler scenario illustrating the infinite loop,

    @Test
    public void testRepeatDrop() {
        final TinkerGraph g = TinkerGraph.open();
        Vertex a1 = g.addVertex(T.label, "A");
        Vertex b1 = g.addVertex(T.label, "B");
        Vertex c1 = g.addVertex(T.label, "C");
        a1.addEdge("ab", b1);
        b1.addEdge("ba", a1);
        b1.addEdge("bc", c1);

        Vertex a2 = g.addVertex(T.label, "A");
        Vertex b2 = g.addVertex(T.label, "B");
        Vertex c2 = g.addVertex(T.label, "C");
        a2.addEdge("ab", b2);
        b2.addEdge("ba", a2);
        b2.addEdge("bc", c2);

        a1.addEdge("ac", c1);
        a1.addEdge("ac", c2);

        List<Vertex> vertices = g.traversal().withoutStrategies(LazyBarrierStrategy.class)

                .V().hasLabel("A")
                .repeat(__.out("ab", "ba"))
                .until(__.out("bc"))
                .out("bc")
                .in("ac")
                .out("ac")
                .toList();
        Assert.assertEquals(4, vertices.size());
g.traversal().withoutStrategies(LazyBarrierStrategy.class)
                .V().hasLabel("A")
                .repeat(__.out("ab", "ba"))
                .until(__.out("bc"))
                .out("bc")
                .in("ac")
                .out("ac")
                .drop()
                .iterate();
        System.out.println("asdasdasd");
    }

Adding a BarrierStep before drop() resolves the issue but probably this should happen automatically in a strategy? Afraid I can not quite tell what the scenarios are that would cause this issue so not sure if a BarrierStep should always be added or only sometimes.

Cheers
Pieter