Questions about implementing and deploying custom traversal extensions

[horak90]

Hello,

I’m currently working on a project that has been using Neo4j, and we’re exploring migrating to JanusGraph (which uses tinkerpop). The main challenge we’re facing relates to how we currently use Neo4j: we’ve developed several stored procedures, including a custom traversal procedure (a recursive function that decides which path to traverse based on edge properties).

From what I’ve gathered so far, it seems that a similar feature could be implemented in Tinkerpop by extending the DSL and deploying it as a JAR into the extensions directory. However, I’m not completely sure if this is the best approach.

Our key requirements are performance-related — this traversal processes a large number of nodes and edges, and it should return complete paths. Therefore, it’s important for us to execute it server-side rather than client-side.

I created a proof of concept for the extension, but I’ve encountered issues during deployment. I placed the JAR file in the /opt/janusgraph/ext directory and added it to plugins.txt. However, when running the Gremlin console, I get the following error:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke method activate() on null object
    at org.codehaus.groovy.runtime.NullObject.invokeMethod(NullObject.java:113)

Could you please help clarify the following points?

Is extending the DSL the recommended approach for implementing a custom traversal?

If so, is the deployment method I described (placing the JAR in /opt/janusgraph/ext and declaring it in plugins.txt) correct?

How can I debug the above error? Could it be related to an incomplete plugin JAR? If possible, could you share an example of a working extension that I could use as a reference?

Thank you in advance for your help and any insights you can provide.

Best regards,
Michael

[porunov]

I didn't have experience extending Gremlin with custom steps (i.e. g.V().myCustomStep()), so can't say for sure, but that would probably be more relevant to be asked in Gremlin group itself (Gremlin users group or Gremlin Discord server).

If you don't need to extend Gremlin language with your custom steps, but simply apply you business logic during traversal, I would suggest to look into some of these solutions:

• SideEffect step - accepts lambda (Groovy) code to be executed.
• Custom graph traversal strategy. You could implement your own traversal strategy and register it either per traversal or globally for all traversals (like JanusGraph does here)
• If your strategy is related to authentication / authorization (for example, you have many users, where users are limited to be able to traverse only to specific sub-graphs) then you could implement your own Authentication and Authorization as described here. During Authorization phase you could inject custom steps into the received Gremlin query and eliminate / change some paths from the traversal.

Again, I didn't have experience implementing custom steps (that doesn't exist in Gremlin), but only changing behavior of existing steps or modifying the Gremlin query by applying different rules, so not sure if that is helpful for your use-case.