without bringing down the app?
You can redirect requests from firebase to the private backend
First step is to make a compat layer for your db ops. Convert your db layer to use Mongo instead of Firebase without disturbing the rest of your application. Next, write a migration script. I don't know how Firebase works, but should be fairly simple to map your db to Mongo. Run tests with sample data and live data. Verify all is okay. Run manual and integration tests. Then the final migration happens. It's important that the app shouldn't create new insertions during this process, so you'll have to either disable any insertion requests if your architecture makes this possible, or put the app in maintainence mode (alert users in advance). Keep a full backup of the database, run the migration script, and once it's done, test it with internal accounts. If everything seems fine, should be fine to release maintainence mode and push notification saying you're back up.
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