When we were switching a contained part of our codebase from Mongo to MySQL, we ran both in parallel and stored a key in Redis to let us turn off the MySQL back

When we were switching a contained part of our codebase from Mongo to MySQL, we ran both in parallel and stored a key in Redis to let us turn off the MySQL backend (we re-used our feature flag utility for this). This turned out to be quite useful. If we were throwing exceptions in the new MySQL code, or even just suspected the new system of causing problems, we could switch it off. We also switched it off so that we could easily run migrations without taking down the site.