Tons of complexity behind this, but the simple problem is source of truth. When you store data in multiple places, if they become unsynchronized, which is the t

Tons of complexity behind this, but the simple problem is source of truth. When you store data in multiple places, if they become unsynchronized, which is the true answer??? Within a monolithic system, you don't have differing clocks, network partitions that can be very weird, etc. And, a related problem, which is race conditions. What if you are reading one copy of the data while the other replica is being updated. Most complexity, imo, arises from this fundamental problem and the various techniques to deal with it.