I'll try to explain this as best I can so bear with me - if anything is unclear, please let me know.
We have a live server which is a 2-node Windows cluster with SQL 2017 on it. It has 2 Availability Groups on it that replicate all of the non-static DBs over to our DR site (a 3rd, SQL 2017 machine, which is not part of the Windows cluster.
My boss has asked me a question and I don't know the answer to it. He is concerned that if, when we do a bubble test of the DR process, and fail the AGs over, that at some point, both our Live server and our DR server could be seen to be live and be receiving updates. Then, if this is the case, how do we deal with that upon fail back?
Now, my thoughts are that:
1/. We'd be doing the bubble test at an agreed time so there should be no-one using the system at the time apart from the resourced testers
2/. The application that is involved here would have all DNS changes done, as per a real DR failure, and so would all be pointing to the DR server
3/. Given the setup, we'd be following the steps from this MS doc for the failover (in the Forced Failover with data loss section) the AG on the live server would be offline
perform-a-planned-manual-failover-of-an-availability-group-sql-server
So, unless someone connected directly to the Live machine, via SSMS for example, and updated some data then I can't see how anything would see Live as being live. But he's more knowlegable than me and needs an answer.