Overview of Schema and Data Migration.
Depot makes simple schema migrations extremely simple and complex schema migrations pretty easy.
- Automatic schema migration: adding a new column to a persistent record is as simple as adding
the new field to the POJO and incrementing the
SCHEMA_VERSION_NUMBERconstant. - Assisted schema migration: dropping, renaming and retyping columns is very easy, and more sophisticated custom migrations can also be easily incorporated into Depot's schema migration system.
- Data migration: migrations that do not change record schemas but manipulate their data can also be registered and Depot will ensure that they run successfully and only once.
- Distributed migration coordination: Depot is designed so that you can bring up a dozen application servers on a dozen machines and during their initialization they will coordinate (through the database) which server will handle each migration and the other servers will block any database access until those migrations have successfully completed.
The addition of columns is automatic. Dropping, renaming and retyping are very simple:
public DecorRepository (PersistenceContext ctx)
{
super(ctx);
ctx.registerMigration(DecorRecord.class,
new SchemaMigration.Rename(17004, "scale", DecorRecord.ACTOR_SCALE));
ctx.registerMigration(DecorRecord.class, new SchemaMigration.Drop(17004, "offsetX"));
ctx.registerMigration(DecorRecord.class, new SchemaMigration.Drop(17004, "offsetY"));
ctx.registerMigration(DecorRecord.class,
new SchemaMigration.Retype(17004, DecorRecord.FURNI_SCALE));
}More complex migrations are also possible, one has to take care if they wish to preserve database agnosticism:
ctx.registerMigration(FooRecord.class, new SchemaMigration(42) {
@Override
public Integer invoke (Connection conn, DatabaseLiaiason liaison) throws SQLException {
// go crazy with your raw JDBC connection or use the DatabaseLiaison to
// help you do things in a database agnostic way
}
});TBD