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