Upgrading from 2.1.5 to 4.0

Couchbase Server
9

I have similar problem.
We have huge Java 6 web application, that was developed 6-7 years ago.
It is running using Couchbase 2.2 (CE) 10 nodes cluster. Couchbase-client version is 1.4.9.
I just recently joined the team and to get my feet wet, has been given a project to refactor and upgrade the whole thing.
We’ll be upgrading to 14 node cluster running 4.1.1 (CE), 2.2 Java SDK, Java 8, etc.
I need to develop a strategy to ensure:

  1. Zero data loss;

  2. Minimum application downtime.

I was thinking about using XDCR, but now read about possible incompatibilities.
Will cbtransfer be a safer approach?
Apparently, it is not. I have try to do it and got an error message

error: cannot back up 2.x or older clusters with 3.x tools

Now I am at a loss, XDCR has incompatibilities, cbbackup/restore/transfer won’t work. :cry: