Database Migration Categories

Most long-running projects need to manage changes to the database schema of the system and data migrations in some way. As the system evolves new datatypes/tables and properties/columns are added, some are removed and others are changed. Relationships between objects also change in unpredictable ways so that you have to deal with these changes in some way. Not all changes are equal in nature, so we handle them differently!

One tool we use to manage our database is liquibase. The units of change are called migrations and are logged in the database itself (table “databasemigrations”) so that you can actually see in which state the schema is. Our experience using such tools for several years is very positive because there is no manual work on the database of some production system needed and new installations automatically create the database schema matching the running software. There are however a few situations when you want to do things manually. So we identified three types of changes and defined how to handle them:

1. structural changes

Structural changes modify the database schema but no data. In some cases you have to care about the default values for not null columns. These changes are handled by database management/versioning tools. They are relevant for all instances and specific for each deployed version of the system. The changes are stored with the source code under version control. Most of the time they are needed when extending the functionality of the system and implementing new features. In SQL the typical commands are CREATE, DROP and ALTER.

2. data rule changes

Changes to the way how the data is stored we call data rule changes. Examples for this are changing the representation of an enum from integer to string or a relation from one-to-many to many-to-many. In such a case the schema and importantly existing data has to be changed. For these migrations you do not need explicit ids of an object in the database but you change all entries in the same way according to the new rules. The changes can be applied to each instance of the system that is update to the new database (and software) version. Like structural changes they are executed using the database migration tool and stored under version control. The typical SQL command after the involveld structural changes needed is UPDATE with an where clause and sometimes CASE WHEN statements.

3. data modifications

Sometimes you have to change individual data sets of one instance of a system. That may be because of a bug in the software or corrupted/wrong entries that cannot by fixed using the system itself, e.g. as super user. Here you fix the entries of one instance of the system manually or with a SQL script. You will usually name specific object ids of the database and perform these exact changes only on this instance. It may be necessary to perfom similar tasks on other instances using different object ids. Because of this one-time and instance-specific nature of the changes we do not use a migration tool but some kind of SQL shell. Such manual changes have to be performed with extra caution and need to be thoroughly documented, e.g. in your issue tracker and wiki. If possible use a non-destructive approach and make backups of the data before executing the changes. Typical SQL statements are UPDATE or DELETE containing ids or business keys.

Conclusion

With categories and guidelines above developers can easily figure out how to deal with changes to the database. They can keep the software, database schema and customer data up-to-date, nice and clean over many years while improving and evolving the system and managing several instances running possibly different versions of the system.

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