Evolving Your Database Schema Without Downtime

The Importance of Migrations

In small applications, making manual changes to the database schema might be acceptable. However, as your application scales, this approach becomes unsustainable. That’s where migrations come in – small pieces of code that update your database schema. By using migrations, you can reproduce and test changes in different environments, ensuring consistency and reliability.

Running Migrations Safely

When running migrations, it’s essential to consider the potential impact on your application. In a small setup, you might be able to run migrations before launching the application. However, in larger applications, this approach can lead to errors and downtime. A better approach is to follow the golden rule:

Ensure your current code works with both the previous and new database schema.

Decoupling Migrations from Deployment

By separating migrations from your deployment process, you can run long-running migrations without affecting your application’s performance. This approach also allows you to backfill data, making it an essential strategy for evolving your database schema.

Real-World Examples

Let’s explore some examples of multi-step migrations:

• Deleting a table or column: Stop writing to the table or column, then drop it physically from the database.
• Moving data between columns or tables: Create a new column or table, backfill data, and then switch to the new column or table.

Caveats and Best Practices

When running long-running migrations, it’s crucial to avoid locking tables and triggering excessive write operations. Instead, query records that need updating and then run the SQL statement only on those records. This approach allows you to stop and resume the migration as needed.

SELECT * FROM users WHERE updated_at < '2022-01-01';

This approach ensures that only the necessary records are updated, minimizing the impact on your application.

Additional Tips

Remember to:

• Test your migrations in different environments before running them in production.
• Monitor your application’s performance during and after migrations.
• Communicate with your team and stakeholders about upcoming migrations and potential downtime.

By following these best practices and separating migrations from deployment, you can ensure a smooth transition without disrupting your users.