That's why banks have an available balance and a posted balance. One is the best available estimate of how much money you can spend. The other is the reconciled

That's why banks have an available balance and a posted balance. One is the best available estimate of how much money you can spend. The other is the reconciled balance after transactions have been posted. It would have been possible for the exchanges to have used a NoSQL db with this strategy, but they didn't. They should have been keeping transaction logs and reconciling balances, but they didn't. While, I'm no fan of MongoDB, it isn't really their fault. These exchanges were designed by people who didn't know any better.