[brightball]

Read replication is easy.

Horizontal scaling for DB typically means distributing the write load across instances either via sharding or master-master. I could be wrong of course, but that’s how I read it.

MySQL is very good at master-master out of the box.

[roncesvalles]

To be pedantic, master-master implies the same row can be committed to at the same time at different master nodes (without an immediate causal connection between the two). Very few databases allow this.