Interface to implement when creating a custom change that does not actually generate SQL. If you are updating a database through SQL, implementing CustomSqlChange is preferred because the SQL can either be executed directly or saved to a text file for later use depending on the migration mode used. To allow the change to be rolled back, also implement the CustomTaskRollback interface. If your change requires sql-based logic and non-sql-based logic, it is best to create a changeset that contains a mix of CustomSqlChange and CustomTaskChange calls.