I am looking for a Graph database using Scylla or Cassandra as the backend and then expose the web api as GraphQl.
Can you help me verify that I have got the followin stack right:
You've pretty much got it right though just to help clarify:
GraphQL is an abstraction designed to help make development/data access a bit simpler for developers. You would have to create a service that translates GraphQL into Gremlin.
The stack you're envisioning looks like:
GraphQL -> Gremlin/TinkerPop -> JanusGraph -> DataStore (Cassandra, Scylla, etc).
As far as the datastore is concerned, JanusGraph is compatible with both Apache Cassandra and Scylla.