|more useful options|
I agree (except for the minor confusion with some numbers in the last paragraph).
Bids probably need to be subtracted from a budget in Redis. You might have a separate "budget" that includes bids while keeping the budget that only includes payments for winning bids.
You might also hedge and say that each bid only has a (say) 60% chance of winning so bidding $50 subtracts only $30 from the "budget less outstanding bids", realizing that this risks going over the budget if you win more than an average of 60% of bids (weighted by bid size).
There are several complications of this approach:
Also, the example Ovid gives seems way too optimistic and thus to greatly understate the risk. If the budget is $100 and $50 of it has been spent and there are no outstanding bids, then 36 systems could each bid $50 and end up spending $1800 and exceeding the budget by 1750%, not a measly 35%. So that makes me suspect that the explanation was incomplete.
But winning bids already have to access the Redis master, so unless winning bids are a small fraction of regular bids, there isn't much benefit to horizontal scaling with Redis slaves. But I could certainly see the timing being less critical for recording winning bids (and thus an outage on the master would prevent bidding but not be as serious of a problem for the recording of winning bids).
I can see a lot of approaches for dealing with better scaling and higher availability while still subtracting each bid from a central balance. But I don't think a simple Redis set-up would be the solution. I'd probably want multiple masters so you might want to look at membase or riak, for example (though that might not really address scaling).
You could also shard your requests so that bids against budget $X only go to (say) 3 of the available servers and then each server can bid upto 1/3 of the available budget (using a server-local central bid amount tracker, perhaps a local Redis instance). Then your 100s of budgets spread load across your dozens of servers but each budget only spreads over a smaller subset. That obviously can get complicated to maintain that mapping in a way where load is sufficiently balanced.
You can also just roll your own somewhat trivial multi-master infrastructure where each master is responsible for broadcasting updates to other masters. You can even use "broadcast" or "multicast" packets for notifying all masters "at once", if you seriously trust the capacity and reliability of your network. This also brings to mind pubsub.