Concurrency in working with cache?

Question

ASP.NET

Hello, I want to achieve great performance in api.
Now the model is simple: Request -> in the database -> response
And there are no problems with competition because transactions help and the additional condition helps.
I want to do everything in the cache, and periodically upload the cache to the database.
There was a question on concurrency.
Here is a case
2 flows simultaneously want to change the balance of the user
Both took 1 state (100 UAH), but one of them was the first to fill in a new state (120) The
second, respectively, does not know about it and dumps its value there from 100
I thought I would record the change events (ChangeBalance +100) (ChangeBalance +40)
and aggregate everything into a heap before uploading it to the database and save it.
But here is the same problem, the collection of events in the cache, I pull the collection from the cache, add the event there and save it, and there is a possibility that I will overwrite the changes of another thread.
How can this problem be solved? At least cancel overwriting the cache so as not to lose data

Answer 1

I'm not a concurrency specialist, but I think that the rebalancing itself should not be done competitively.
I think it's worth putting tasks to change the balance in a queue and that the queue has a lot of parallel handlers. Thus, only one handler will be responsible for each balance change.