Cannot Access Vertices with g.V(*vertexID*).next() in Java App

[Hayden Freedman]

This is an extension of a problem that I was having in the Console. I am now trying to write a Java App incorporating Gremlin graph traversals. I have added the Blazegraph-Gremlin library to my app and I am working with data from a Blazegraph database. 

Here, I've written out a small demo to show my problem. Sorry for the excess of print statements, but I wanted to demonstrate in the output exactly what was happening at each step.

System.out.println("Show me first vertex you find");

System.out.println(g.V().next());

Vertex v1 = g.V().next();

System.out.println("Prove that the variable is set properly (it is)");

System.out.println(v1);

Object v1id = g.V().id().next();

System.out.println("Get the ID of the object to pass to g.V()");

System.out.println(v1id);

GraphTraversal<Vertex, Vertex> v2 = g.V(v1id);

System.out.println("Try to find the vertex given the ID");

Vertex v3 = g.V(v1id).next();

System.out.println(v3);

Output of the above code:

Show me first vertex you find

v[8080/source/sampledata1/008a46ce-d7f9-4c60-b7ca-1ccaa6ce7bbd/identifier]

Prove that the variable is set properly (it is)

v[8080/source/sampledata1/008a46ce-d7f9-4c60-b7ca-1ccaa6ce7bbd/identifier]

Get the ID of the object to pass to g.V()

8080/source/sampledata1/008a46ce-d7f9-4c60-b7ca-1ccaa6ce7bbd/identifier

ERROR 2017-03-23 16:20:44,463 1530 com.bigdata.Banner [main] Uncaught exception in thread

org.apache.tinkerpop.gremlin.process.traversal.util.FastNoSuchElementException

From the responses I received to previous posts, I was given the impression that this may be a bug in Blazegraph. With that said, I would appreciate any help/support I can get on the Gremlin side of things, since I am fairly committed to using Blazegraph at this point. If anyone has any syntax suggestions/sees something I am doing improperly/wants more details, please respond. I will be happy to provide more examples if necessary.

Thanks!

[Stephen Mallette]

I don't see anything really wrong with what your Gremlin code looks like. I mean - it looks like just some basic code to try things out and not really any complex logic that could be optimized further. I will note a few things:

With this:

> Object v1id = g.V().id().next();

it seems that BlazeGraph was nice enough to return the same Vertex id as your prior command. TinkerPop does not enforce a deterministic order for element iteration. So don't rely on that unless you're sure about what BlazeGraph is doing. 

I also thing this part is weird give that you aren't running this in the Gremlin Console (which iterates results for you):

> System.out.println(v1id);

> GraphTraversal<Vertex, Vertex> v2 = g.V(v1id);

> System.out.println("Try to find the vertex given the ID");

I don't get why your output doesn't include that last println there. You get a FastNoSuchElementException which should only occur on iteration of the GraphTraversal which you didn't do. You merely initialized it. I don't know how that can happen. Are you sure you got the output right?  Given the problems you've had with id looks ups from BlazeGraph, I think the FastNoSuchElement is expected (unless they've patched something for you), but you should get it on this call:

> Vertex v3 = g.V(v1id).next();

when you next() the Traversal. I sorta wonder why you have this line at all:

> GraphTraversal<Vertex, Vertex> v2 = g.V(v1id);

It's not really doing anything at all in your sample code.

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[Hayden Freedman]

Hi Stephen, thanks again for responding.

So it was my mistake to include the line GraphTraversal<Vertex, Vertex> v2 = g.V(v1id); You are correct that this line does not do anything. I had included it for a previous test and overlooked taking it out for my post on this page. So the code for the test should read: 

System.out.println("Show me first vertex you find");

System.out.println(g.V().next());

Vertex v1 = g.V().next();

System.out.println("Prove that the variable is set properly (it is)");

System.out.println(v1);

Object v1id = g.V().id().next();

System.out.println("Get the ID of the object to pass to g.V()");

System.out.println(v1id);

System.out.println("Try to find the vertex given the ID");

Vertex v3 = g.V(v1id).next();

System.out.println(v3);

Since I wasn't doing anything with that variable as you noted, the output will be the same.

It's really frustrating and perplexing for me that I am not able to complete these basic tasks. Being able to retrieve a specific vertex when given its vertex ID is a really critical functionality for the app I am writing. I feel like there must be a good way to do this and that I am just missing something obvious. 

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[Stephen Mallette]

> Being able to retrieve a specific vertex when given its vertex ID is a really critical functionality for the app I am writing

That's critical to every graph application. I don't think the problem can be solved here on this list though. This doesn't appear to be a problem with Gremlin. It is an issue with the BlazeGraph implementation somehow. I think you will have to get some support directly from them.

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[Hayden Freedman]

OK, understood. I'll try to seek out some help from the Blazegraph side of things.

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[Stephen Mallette]

Basically, every working TinkerPop Graph implementation should be able to do this:

gremlin> graph = TinkerGraph.open()

==>tinkergraph[vertices:0 edges:0]

gremlin> g = graph.traversal()

==>graphtraversalsource[tinkergraph[vertices:0 edges:0], standard]

gremlin> v = g.addV().next()

==>v[0]

gremlin> g.V(v)

==>v[0]

gremlin> g.V(v.id())

==>v[0]

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[Hayden Freedman]

This is really good to know, thank you.

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