How to prepare and execute atomic operations with cache

Question

Design campaign management system to distribute 6 M items to users who wants to get free item. The campaign lasts for 10 mins. Each user must get only 1 item.

My approach:

I am more interested to discuss the implementation /algorithm part Basically decrementing a counter and flag a user ID atomically . Since we are looking at peak req of 100k per sec. I think distributed caches would be better suited here may be Redis.

So we are looking at transactions to do a decrement and flagging the userid.

Any suggestions on if this seems the correct approach or your thoughts on better approaches