How to scale MySQL horizontally?

If you truly need read/write to scale, multi-masrer clustering with MariaDB Galera is probably the best way to do it. They (MariaDB) also sell a load balancer/query proxy Max scale that can do a lot of surprisingly complex stuff (like publish new data to Kafka or centrally ship binlogs from one place to multiple read replicas).

However generally my advice is that if you're finding yourself trying to build a big relational database and writing to it a significant amount of times, it might be time to consider a different or at least modified architecture. Especially if your use case starts to scale to more than what Galera can handle. At some point, all these solutions become eventually consistent the more you scale. And if you are willing to accept eventual consistency, there are some clever ways to do storage using things like queueing, batching, caching etc that can scale horizontally much further than any relational database can.