How to migrate a database schema at scale

Database migrations are essential for evolving schemas in applications, enabling changes without significant downtime or user disruption. They are small code snippets that update the database schema and can be managed using various tools, often integrated with ORMs or database frameworks like Ruby on Rails, Sequelize, knex.js, or Prisma/Lift. The key to successful migrations lies in ensuring that the application code remains compatible with both the previous and new database schema, thus allowing seamless transitions. This approach includes running tests after migrations in Continuous Integration (CI) processes and decoupling migrations from deployment to handle long-running migrations and data backfilling. Examples of multi-step migrations include adding new columns, deleting old ones, or moving data between columns, ensuring that the application code adapts progressively to these changes. This method avoids service disruptions and improves user experience by eliminating the need for maintenance windows, although care must be taken with long-running migrations to prevent locking tables and service degradation.