How Can I Return Meaningful Errors In Gremlin?

Question

Let's say I have a huge gremlin query with 100 or more steps. One part of this query has a failure and I want it to return a meaningful error message. With a short and sweet query this would not be too difficult, as we can do something like this:

g.V().coalesce(hasId("123"), constant("ERROR - ID does not exist"))

Of course we're asking if a Vertex with an ID of 123 exists. If it does not exist we return a string.

So now let's take this example and make it more complex

g.V().coalesce(hasId("123"), constant("ERROR - ID does not exist")).as("a").V().coalesce(hasId("123"), constant("ERROR - ID does not exist")).as("b").select("a").valueMap(false)

If a vertex with ID: "123" exists we return all properties stored on the vertex.

Lets say a vertex with ID: "123" does not exist in the database. How can I get a meaningful error returned without getting a type error for trying to do a .valueMap() on a string?

stephen mallette stephen mallette · Accepted Answer · 2018-12-12T11:59:36

First of all, if you have a single line of Gremlin with 100 or more steps (not counting anonymous child traversals steps of course), I'd suggest you re-examine your approach in general. When I encounter Gremlin of that size, it usually means that someone is generating a large traversal for purpose of mutating the graph in some way. That's considered an anti-pattern and something to avoid as the larger the Gremlin grows the greater the chance of hitting the Xss JVM limits for a StackOverflowException and traversal compilation times tend to add up and get expensive. All of that can be avoided in many cases by using inject() or withSideEffect() in some way to pass the data in on the traversal itself and then use Gremlin to be the loop that iterates that data into mutation steps. The result is a slightly more complex Gremlin statement, but one that will perform better and avoid the StackOverflowException.

Second, note that this traversal will likely not behave as you want on any graph provider - see this example on TinkerGraph:

gremlin> g.V().coalesce(hasId(1),constant('x'))
==>v[1]
==>x
==>x
==>x
==>x
==>x
gremlin> g.V().hasId(1)
==>v[1]

The hasId() inside the coalesce() won't be optimized by the graph as an fast id lookup but will instead be treated as a full table scan with a filter.

In answer to your question though, I'd say that the easiest option open to you is to just move the valueMap() inside the coalesce():

g.V().coalesce(hasId("123").valueMap(false), 
               constant("ERROR - ID does not exist")).as("a").
  V().coalesce(hasId("123").valueMap(false), 
               constant("ERROR - ID does not exist")).as("b").
  select("a")

I see why that might be bad if you lots of steps other than valueMap() because then you have replicate the same steps over and over again making the code even larger. I guess that goes back to my first point.

I suppose you could use a lambda though not all graph providers support that - note that I've modified your code to ensure a lookup by id for purpose of demonstration:

gremlin> g.V(1).fold().coalesce(unfold(),map{throw new IllegalStateException("bad")})
==>v[1]
gremlin> g.V(10).fold().coalesce(unfold(),map{throw new IllegalStateException("bad")})
bad

At this time, I'm not sure there's much else you can do. Maybe you could make a "error" Vertex that you could return in constant() that way valueMap() would work but it's hard to say if that would be helpful given what I know about the overall intent of your traversal. I suppose you could maybe come up with a fancy evaluation of an if-then using choose() but that might be hard to read and look awkward. The only other option I can think of is to store the error as a side-effect:

gremlin> g.V(10).fold().coalesce(unfold(),store('error').by(constant('x'))).cap('error')
==>[x]

I don't think Gremlin gives you any really elegant way to do what you want right now.

How Can I Return Meaningful Errors In Gremlin?

1 Answers