That assumes that the underlying implementation of saveBalance can be updated to push the atomic operation downstream. In general you highlight the problem tho

That assumes that the underlying implementation of saveBalance can be updated to push the atomic operation downstream. In general you highlight the problem though; without the central datastore ensuring atomicity, this isn't really solving anything. I.e., if this app had multiple instances, you STILL have a problem, even with this solution, since nothing guarantees that another -instance- hasn't also loaded the old data.