how to connect external cassandra service with cadence

There's no tool called "cadence" so I have no idea what this "cadence service" you are referring to is. Also, asking a question in the "Feedback, Suggestions and Questions" forum is not going to get you very far because this forum is for issues with the forum system itself, not for specific Cadence tools.

So you need to be more specific about the tools you are talking about and post in a relevant forum. I'm guessing wildly that you might mean the PCB Design tools, but I really have no idea (most posts in this forum I redirect to the relevant forum, but I can't do that here because there's not enough information).