I'm scheduling a backup operation on Cloud Firestore. I'm not sure if data changes while the backup function is running, if those changes would be reflected in the backup.
For completeness that's not the behavior I'm looking for and if the changes would be reflected, is there anyway I can lockout the db? Perhaps dynamically changing the security rules?
Firestore's export feature is not a true "backup". Notice that nowhere in the documentation is the word "backup" ever used. It's just an export, and that export is effectively just querying every collection and writing the documents to a file in a storage bucket.
Data can change during an export. The export might not contain everything that might have changed while the export happened. You can expect the export to be inconsistent in that case.
Security rules don't affect the export. They just affect web and mobile clients.
It's not really possible to "lock" the entire database, except by preventing your code from writing it entirely by controlling it yourself.