Just from a glance at the AlloyDB documentation, it seems to be fundamentally a similar design to PostgreSQL. That is, you have a single primary instance that h

Just from a glance at the AlloyDB documentation, it seems to be fundamentally a similar design to PostgreSQL. That is, you have a single primary instance that holds the entire dataset and handles all write traffic, plus some number of additional replicas within the system for failover and read-only queries. Spanner has a very different implementation: your dataset is split into (possibly a large number of) shards, each of which is replicated across multiple regions. So you get better global availability and scalability, at the cost of more complicated performance characteristics when you have to do operations that involve multiple shards.