Design a voting system. 100M users will be logging in within a window of 24h (not necessarily uniformly). Every user will be able to choose from a fixed list of options. If the user has already voted the system should not let them to vote a second time. Additional constraint: only the first 100K votes are accepted. If the quota is exceeded any attempt to vote should be rejected.
Since we can only allow 100k votes, and it would be a bad (but not terrible UX) to invalidate anyone who makes a vote after the 100k one. We can try to make this work by having a leader follower setup, with the leader writing ahead to the followers, and if a leader falls, having a new leader election.