Thank you @geraldss for ur response.
It seems to me the connections which held by my java client has been expired at the Couchbase server. I am just assuming this, please ignore if it is not relevant.
The problem comes when I tried to get a doc or select any data using N1QL query. But it normal execution happen when tried to insert or select using view.
I guess someone is facing the same problem:-https://www.couchbase.com/forums/t/couchbase-server-randomly-disconnects-kv-endpoints-keepalive-not-working/9607/13
Thanks,