Migration fidelity

Overview

When you're migrating your schema, data, and metadata from a source database to a destination database, you want to ensure that all of this information is migrated accurately. Database Migration Service provides a high-fidelity way to migrate database objects (including the schema, data, and metadata) from one database to another.

All of the following data, schema, and metadata components are migrated as part of the database migration:

Data

• All tables from all databases and schemas, excluding the following schemas:

  • The information schema information_schema
  • Any schemas beginning with pg (for example, pg_catalog)

  For more information about these schemas, see Known limitations.

Schema

• Naming

• Primary key

• Data type

• Ordinal position

• Default value

• Nullability

• Auto-increment attributes

• Secondary indexes

Metadata

• Stored procedures

• Functions

• Triggers

• Views

• Foreign key constraints

Continuous migration

Only data manipulation language (DML) changes are updated automatically during continuous migrations. Managing data definition language (DDL) changes so that the source and destination databases remain compatible is the responsibility of the user, and can be achieved in two ways:

1. Stopping writes to the source and running the DDL commands in both source and destination. Before running DDL commands on the destination, grant alloydbexternalsync to the AlloyDB user applying the DDL changes. To enable querying or changing the data, grant the alloydbexternalsync role to the relevant AlloyDB users.
2. Using the pglogical.replicate_ddl_command to allow DDL to be run on the source and destination at a consistent point. The user running this command must have the same username on both the source and the destination, and should be the superuser or the owner of the artifact being migrated (for example, the table, sequence, view, or database).

   Here are a few examples of using the pglogical.replicate_ddl_command.

   Replace:

   • [SCHEMA] with the name of the table schema you want to use
   • [TABLE_NAME] with the table name
   • [NEW_NAME_FOR_TABLE] with the new name for the table when when performing the rename operation

   Add a column to a database table with a primary key

   select pglogical.replicate_ddl_command(
     'ALTER TABLE [SCHEMA].[TABLE_NAME] add column surname varchar(20)',
     '{default}'
   );

   Add a column to a database table without a primary key

   select pglogical.replicate_ddl_command(
     'ALTER TABLE [SCHEMA].[TABLE_NAME] add column surname varchar(20)',
     '{default_insert_only}'
   );

   Change the name of a database table with a primary key

   select pglogical.replicate_ddl_command(
     'ALTER TABLE [SCHEMA].[TABLE_NAME] RENAME TO [NEW_NAME_FOR_TABLE]',
     '{default}'
   );

   Change the name of a database table without a primary key

   select pglogical.replicate_ddl_command(
     'ALTER TABLE [SCHEMA].[TABLE_NAME] RENAME TO [NEW_NAME_FOR_TABLE]',
     '{default_insert_only}'
   );

   Create a database table with a primary key

   Run the following commands:

   1. select pglogical.replicate_ddl_command(
        command := 'CREATE TABLE [SCHEMA].[TABLE_NAME]
          (id INTEGER PRIMARY KEY, name VARCHAR);',
        replication_sets := ARRAY['default']
      );

   2. select pglogical.replication_set_add_table('default', '[SCHEMA].[TABLE_NAME]');

   Create a database table without a primary key

   Run the following commands:

   1. select pglogical.replicate_ddl_command(
        command := 'CREATE TABLE [SCHEMA].[TABLE_NAME]
         (id INTEGER PRIMARY KEY, name VARCHAR);',
        replication_sets := ARRAY['default_insert_only']
      );

   2. select pglogical.replication_set_add_table(
        'default_insert_only', '[SCHEMA].[TABLE_NAME]'
      );

What isn't migrated

• Large objects can't be replicated, as PostgreSQL's logical decoding facility does not support decoding changes to large objects. For tables that have column type oid referencing large objects, the rows are synced, and new rows are replicated. However, trying to access the large object on the destination database (read using lo_get, export using lo_export, or check the catalog pg_largeobject for the given oid), fails with a message saying that the large object does not exist.

• For tables that don't have primary keys, Database Migration Service supports migration of the initial snapshot and INSERT statements during the change data capture (CDC) phase. You should migrate UPDATE and DELETE statements manually.

• Database Migration Service doesn't migrate data from materialized views, just the view schema. To populate the views, run the following command: REFRESH MATERIALIZED VIEW view_name.

• The SEQUENCE states (for example, last_value) on the new destination might vary from the source SEQUENCE states.

• Customized tablespaces aren't supported in the destination Cloud SQL instance. All the data inside customized tablespaces is migrated to the default pg_default tablespace in Cloud SQL.