JanusGraph Custom String Vertex Ids for existing Cassandra Keyspaces

I am trying out the Custom Vertex ID feature in JanusGraph as mentioned in the below documentation

https://docs.janusgraph.org/master/advanced-topics/custom-vertex-id/

I am using Cassandra as the backend store for the JanusGraph database and used the below properties for enabling the custom vertex Id feature in Janus Graph, Currently I am using Embedded JanusGraph with Java Code.

storage.cql.keyspace=janusgraph
graph.set-vertex-id=true
graph.allow-custom-vid-types=true

When I try with an existing keyspace where existing records are already present I am getting the below exception

Exception in thread "main" java.lang.UnsupportedOperationException: Vertex does not support user supplied identifiers
    at org.apache.tinkerpop.gremlin.structure.Vertex$Exceptions.userSuppliedIdsNotSupported(Vertex.java:163)
    at org.janusgraph.graphdb.tinkerpop.JanusGraphBlueprintsTransaction.addVertex(JanusGraphBlueprintsTransaction.java:119)
    at org.janusgraph.graphdb.tinkerpop.JanusGraphBlueprintsGraph.addVertex(JanusGraphBlueprintsGraph.java:143)

But once I try the same code on a new Cassandra Key space it is working as expected and able to store the string Custom Vertex Ids using the below Code

 Vertex firstVertex = g.addV("record")
                       .property(T.id,"customVertexId1")
                       .property("recordId", "abc")
                       .next();

So my query is this Custom Vertex Id cannot be applied to existing Keyspaces? We always require to create the new keyspaces for using the Custom String type vertexId in JanusGraph? Please Suggest

Solution

See https://docs.janusgraph.org/advanced-topics/custom-vertex-id/#alter-an-existing-graph

Basically if you want to use this on an existing graph, you should do

mgmt = graph.openManagement();
mgmt.set("graph.set-vertex-id", true);
// optional, if you want to provide string ID
mgmt.set("graph.allow-custom-vid-types", true);
mgmt.commit();