[cryptica]

Having to support two different versions of the Elixir service during the rollout period is risky...

What if something goes wrong during the rollout? For example, if you change the database schema or upgraded your database engine or changed your back end authentication approach, it can break the old code. Then how will you know whether it's a problem with the old code or new code if many nodes are running both?

[brightball]

Those are things you have to account for in any zero downtime deploy situation though. It’s mostly minor changes in how you roll out schema changes.