The challenge with a financial services application, is that to make a blue-green deployment work, you have to guarantee that the old app will still be able to transact whilst the new app is being deployed - which requires testing the old app in light of the new code going in. (Potentially double the testing).
So basically blue-green deployments are infeasible for large financial services applications because you double your regression testing cost just to have extra availability during the release. Instead what you need is a compromise position, where you put the application into 'read-only' mode during the deployment, which reduces risk but still maximises availability.
What some designs do is a 'reporting database' with a web-app read-only mode - so the web app can still work providing information, but not transact.
When I google for this - I can't find this pattern, which makes me think I've got the wrong term.
My question is: What is the DevOps term for 'read-only-deployment' mode of a web application?