Tinkerpop Gremlin Depth First Search Order

Question

I have a very simple example graph which I am trying to get a depth first query on. Let's say the graph edges look like this

A->B
A->C
B->D
B->E
C->F
C->G

A depth first search from A should return

A-B-D-E-C-F-G

But if I could get the below order it would be even better

D-E-B-A-F-G-C-A

How can I create a Gremlin query that will output this order? If I do something like this

g.V('A').repeat(outE().inV()).emit()

I get an order of A,B,C,D,E,F,G which is breadth first. I can't work out how to get the order I want above.

score 5 · Accepted Answer · answered Oct 31 '17 at 18:12

For others to reproduce, here's the sample graph:

g = TinkerGraph.open().traversal()
g.addV().property(id, 'A').
  addV().property(id, 'B').
  addV().property(id, 'C').
  addV().property(id, 'D').
  addV().property(id, 'E').
  addV().property(id, 'F').
  addV().property(id, 'G').
  addE('link').from(V('A')).to(V('B')).
  addE('link').from(V('A')).to(V('C')).
  addE('link').from(V('B')).to(V('D')).
  addE('link').from(V('B')).to(V('E')).
  addE('link').from(V('C')).to(V('F')).
  addE('link').from(V('C')).to(V('G')).iterate()

A depth first search from A should return

A-B-D-E-C-F-G

gremlin> g.V('A').repeat(out('link')).until(__.not(outE('link'))).path().
           unfold().dedup().id().fold()
==>[A,B,D,E,C,F,G]

But if I could get the below order it would be even better

D-E-B-F-G-C-A

This one is kinda rolling up the paths from behind. It's tricky, but doable:

gremlin> g.V('A').
           repeat(outE('link').aggregate('edges').inV()).
             until(__.not(outE('link'))).
           flatMap(
             union(identity(),
                   repeat(inE('link').where(within('edges')).as('current').
                          map(select('edges').unfold().
                                where(neq('current').and(without('done'))).
                              outV().where(without('ad')).fold()).as('bl').
                          select(last, 'current').store('done').
                            filter(outV().where(without('bl').and(without('ad')))).
                          outV().store('ad')).
                     emit())).
           id().fold()
==>[D,E,B,F,G,C,A]

However, it's probably a lot easier to just get the paths and do the ordering on the application side.

score 1 · Answer 2 · answered Oct 31 '17 at 08:06

1

At the moment: you can't.

We are co-sufferers. This is a known issue that I initially raised on the mailing list and then attempted a fix which unfortunately only works if there's no emit step in the traversal. You are invited to help fix it, I didn't yet have time to dig deeper.

answered Oct 31 '17 at 08:06

Michael Pollmeier

1,370
11
20

1

It's really surprising that Gremlin can't do depth first search of arbitrary depth graphs. It's a very common algorithm that you would want to perform on a graph. Thanks for your answer though as it has told me that this isn't ready for production systems. – TimR75 Oct 31 '17 at 09:43
3

I'm sorry that Gremlin doesn't fit your use case, but please don't go so far as to think it isn't ready for production. It is in use in many high profile systems that use graph databases. – stephen mallette Oct 31 '17 at 10:36

Tinkerpop Gremlin Depth First Search Order

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