Surely you could just set up a logically-replicated standby server running the new PG version and then hot-failover to the standby, turning it into the new master. That should preserve availability throughout the upgrade.
Changing master for upgrades works great if youre not at their scale though.
I was about to say the same, as much as it pains me to say it. I wonder if Citus or EnterpriseDB has any solutions to a problem like this? Can Citus run temporarily with heterogeneous nodes? E.g. some nodes upgrading with other nodes operating?
Their (currently commercial only) engine, Xpand (previously known as Clustrix), offers distributed SQL as well as online schema changes.
No need for an unncessary database migration if you are already on LAMP and the like to get these fancy features. However handling any kind of change should be handled with care, especially on such a large dataset. Test your rollback procedures!
Disclaimer, I work for MariaDB.
They are a payments processor, not an issuer. That means they are on the merchant's side of the transaction, not the purchaser's. Their role in authorization is simply routing. The issuer is responsible for performing the authorization. The only balance they need to track is how much money they owe the merchant.
Sadly, PostgreSQL doesn't have a built-in solution for online upgrade yet. This is one of the few points why commercial DBMSes are often worth their money.