In addition to what jamii said, you can also have multiple write servers (called "transactors) and can join data between them. Furthermore, that transactor isn'

In addition to what jamii said, you can also have multiple write servers (called "transactors) and can join data between them. Furthermore, that transactor isn't serving queries, so it can have higher write throughput than a traditional database. Assuming hot failover, it's a perfectly reasonable approach for pretty much all places a traditional SQL database was reasonable before.