Years ago I worked for a startup that provided CMS and ecommerce software for small business. Each of our 3000+ customers had their own MySQL database. We had

Years ago I worked for a startup that provided CMS and ecommerce software for small business. Each of our 3000+ customers had their own MySQL database. We had a long tail of customers with negligible usage and would run several thousand MySQL databases on a single server. As customers scaled we could migrate the database to balance capacity. We could also optionally offer "premium" and "enterprise" services that guaranteed isolation and higher durability. Scaling was never a real issue, but the nature of our clients was steady incremental growth. I don't think we ever had a case of real "overnight success" where a shared host customer suddenly melted the infrastructure for everyone. However, managing and migrating the databases could be a real issue. We had a few ways of handling it, but often would need to handle it in the code, `if schemaVersion == 1 else`. Over time this added up and required discipline to ensure migration, deprecation and cleanuop. As a startup, we mostly didn't have that discipline and we did have a fair bit of drift in versions and old code lying around.