Zero-Downtime Schema Migrations

Changing a database schema can look simple. On a local project, adding a column or dropping a table is often one SQL command and a restart.

In production, the same change is much riskier. The database is shared, it is under load, and it may be used by old and new application versions at the same time during a deployment.

A bad migration can lock a table, break running code, corrupt data, or run for hours on a large table.

A zero-downtime schema migration changes the database without taking the system down, breaking the application, or putting data at risk.

This chapter explains how to change a live database in small, safe steps. We will cover common changes like adding columns, removing columns, renaming fields, adding indexes, and backfilling large tables. We will also look at how database migrations need to line up with application deployments.