There can also be shared, mostly read-only databases, with no transactions crossing database boundaries.
For example, one database per retail store with sharded customers, orders and inventory (most transactions involve one order of one customer fulfilled from that store) and a shared one with item descriptions and prices (modified, say, daily).
For example, one database per retail store with sharded customers, orders and inventory (most transactions involve one order of one customer fulfilled from that store) and a shared one with item descriptions and prices (modified, say, daily).