There is a lot of optimism around Blue-Green deployments, particularly from luminaries like Martin Fowler.

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?

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    Not everything is a well-defined pattern. There are some things that don't have "terms." – Robert Harvey Nov 27 '16 at 15:28
  • This actually does have a term, it's "Immutable Deployments". I don't see how it was closed as "opinion based". – Paul Sep 13 '17 at 2:09
  • Thanks Paul - if you have a reference for that then we can get this reopened. – hawkeye Sep 13 '17 at 8:57