Database migrations are a common part of any web application. They are used to update the database schema to match the application’s code. In a traditional web application, the database migrations are run synchronously, which means that the application is blocked until the migration is complete. This is not ideal, as it means that the application is unavailable to users during the migration. Long past the days when stopping the service for maintenance was acceptable; we need to be able to run migrations without blocking the application.

It’s easy to perform database migrations in small databases or if you have no load. But what if you have a large database and a lot of users?