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CS156

SQL: Liquibase

Liquibase is an example of a database migration tool. It helps when you are implementing changes to the code that require a change to the database.

Why use Liquibase?

When implementing backend changes, you may need to alter how data is stored in the database. By default, Spring Boot has a mechanism which attempts to restructure the database to match the structure implied by the Java code. This system works in some cases, but there are many cases where this system is unable to understand how you want the database to be modified. Liquibase allows us to explicitly specify what we want to do to the database, which is generally less error-prone and is safer to use on real production data.

If you use VS Code, you may want to install the XML Language Support plugin. This plugin offers validation and code completion for XML files such as the pom.xml or Liquibase’s changelog.xml, which is very helpful when making changes to those files.

Understanding the changelog.xml

The Liquibase changelog.xml (located in the db folder in CS156 projects) is a list of all the changes that must be made to an empty database in order to create the correct database schema/structure. Any changes you with to make to the database should be encapsulated inside a <changeSet> tag with an id (that must be unique within the entire file) and the author name (which usually should just be your GitHub username). The <changeSet> tag can contain a number of changes, each of which describes an action to be performed on the database. A full list of these change types can be found here.

Updating the database

If you are starting from a fresh database or have changesets in the changelog.xml which have not been applied (because you either wrote a new changeset or pulled changes that add to the changelog), you will have to run ./mvnw liquibase:update in order to apply those changesets.