Vitess

Vitess lets you shard MySQL horizontally without rewriting your application, solving the scaling limits of a single MySQL instance. It's the system YouTube built to scale their MySQL infrastructure, now a CNCF graduated project. Apache 2.0, Go. Vitess sits between your app and MySQL, routing queries to the right shard transparently. It handles resharding (splitting data when a shard gets too big), connection pooling, and query rewriting. Your app still thinks it's talking to one MySQL database.

Fully free and open source. PlanetScale built a managed Vitess cloud (now pivoted/changed), and some providers offer hosted Vitess, but the core is Apache 2.0 with no restrictions. Self-hosting is where the complexity hits. Vitess has multiple components: vtgate (query router), vttablet (MySQL manager), vtctld (control plane), and an orchestration layer. Production setup requires Kubernetes or careful manual deployment. This is not a weekend project.

Solo or small teams: you almost certainly don't need this. If your MySQL fits on one big server, use that. Medium teams: consider Vitess when you're hitting genuine MySQL limits. Large teams: this is home. Vitess handles the scale that justifies the complexity.

The catch: the operational complexity is significant. This is infrastructure software, not a drop-in upgrade. If you don't have a dedicated database team, the learning curve and maintenance burden will eat you. CockroachDB or PlanetScale (when available) offer distributed SQL with less operational overhead.