In software engineering, a schema migration (also database migration, database change management) refers to the management of version-controlled, incremental and sometimes reversible changes to relational database schemas. A schema migration is performed on a database whenever it is necessary to update or revert that database's schema to some newer or older version.

Migrations are performed programmatically by using a schema migration tool. When invoked with a specified desired schema version, the tool automates the successive application or reversal of an appropriate sequence of schema changes until it is brought to the desired state.

Most schema migration tools aim to minimize the impact of schema changes on any existing data in the database. Despite this, preservation of data in general is not guaranteed because schema changes such as the deletion of a database column can destroy data (i.e. all values stored under that column for all rows in that table are deleted). Instead, the tools help to preserve the meaning of the data or to reorganize existing data to meet new requirements. Since meaning of the data often cannot be encoded, the configuration of the tools usually needs manual intervention.

Schema migration allows for fixing mistakes and adapting the data as requirements change. They are an essential part of software evolution, especially in agile environments (see below).

Applying a schema migration to a production database is always a risk. Development and test databases tend to be smaller and cleaner. The data in them is better understood, and if everything else fails, the amount of data is small enough for a human to process. Production databases are usually huge, old and full of surprises. The surprises can come from many sources:

For these reasons, the migration process needs a high level of discipline, thorough testing and a sound backup strategy.

In the steady state, one version of an application only understands one version of a schema. So the most basic strategy is to shut down the application, execute the schema migration, and then start the newer version of the application. While simple, this strategy causes a downtime. Depending on the criticality of the system and its usage patterns, downtimes of various duration may be tolerated, but in some cases none may be tolerated at all. In those cases, one of the following zero-downtime strategies may be used.

These are the general steps of dual writing (also called double writing):