Most fintech engineering writing is about payments or lending. Deposits get less attention, which is odd, because a savings-and-deposit platform has a harder correctness problem than either. You are moving other people's money between regulated institutions, and you have to be right every time, reconcilable after the fact, and defensible to a regulator.

I designed one. Here are five patterns that held up. None are novel on their own. The value was in choosing them together, early, and refusing to compromise on the correctness ones under delivery pressure.