Database Migration Service for AlloyDB FAQ

MySQL   |  PostgreSQL   |  PostgreSQL to AlloyDB

• What is Database Migration Service?
• Which sources are supported?
• Is there cross-version support?
• Which data, schema, and metadata components are migrated?
• Which changes are replicated during continuous migration?
• What isn't migrated?
• Which networking methods are used?
• What are the known limitations?

What is Database Migration Service?

Database Migration Service is a service that makes it easier for you to migrate your data to Google Cloud. Database Migration Service helps you lift and shift your PostgreSQL workloads into AlloyDB.

Which sources are supported?

• Amazon RDS 9.6.10+, 10.5+, 11.1+, 12, 13, 14, 15
• Amazon Aurora 10.11+, 11.6+, 12.4+, 13.3+, 14, 15
• Self-managed PostgreSQL (on premises or on any cloud VM that you fully control) 9.4, 9.5, 9.6, 10, 11, 12, 13, 14, 15
• Cloud SQL 9.6, 10, 11, 12, 13, 14, 15

Which destinations are supported?

• AlloyDB for PostgreSQL 14, 15, 16

Is there cross-version support?

Database Migration Service supports PostgreSQL to AlloyDB migrations from any of the supported source database versions.

Which data, schema, and metadata components are migrated?

Database Migration Service migrates schema, data, and metadata from the source to the destination. All of the following data, schema, and metadata components are migrated as part of the database migration:

Data Migration

• All schemas and all tables from the selected database.

Schema Migration

• Naming
• Primary key
• Data type
• Ordinal position
• Default value
• Nullability
• Auto-increment attributes
• Secondary indexes

Metadata Migration

• Stored Procedures
• Functions
• Triggers
• Views
• Foreign key constraints

Which changes are replicated during continuous migration?

Only DML changes are automatically updated during the migration. Managing DDL so that the source and destination database(s) remain compatible is the responsibility of the user, and can be achieved in two ways:

1. Stop writes to the source and run the DDL commands in both the source and the destination. Before running DDL commands on the destination, grant the alloydbexternalsync role to the Cloud SQL user applying the DDL changes. To enable querying or changing the data, grant the alloydbexternalsync role to the relevant Cloud SQL users.

2. Use the pglogical.replicate_ddl_command to run DDL 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.

   To add a column to a database table, run the following command:

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

   To change the name of a database table, run the following command:

   select pglogical.replicate_ddl_command('ALTER TABLE [schema].[table] RENAME TO [table_name]','{default}');

   To create a database table, run the following commands:

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

What isn't migrated?

To add users to the AlloyDB destination instance, add them from the PostgreSQL client. Learn more about creating and managing PostgreSQL users.

Large objects can't be replicated because PostgreSQL's logical decoding facility doesn't support decoding changes to large objects. For tables that have column type oid referencing large objects, the rows are still 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 doesn't 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 AlloyDB destination might vary from the source SEQUENCE states.

Which networking methods are used?

To create a migration in Database Migration Service, connectivity must be established between the source and the Cloud SQL destination instance. There are a variety of methods supported. Choose the one that works best for the specific workload.

Networking method	Description	Pros	Cons
Reverse SSH tunnel through cloud-hosted VM	Establishes connectivity from the destination to the source through a secure reverse SSH tunnel. Requires a bastion host VM in the Google Cloud project and a machine (for example, a laptop on the network) that has connectivity to the source. Database Migration Service collects the required information at migration creation time, and auto-generates the script for setting it up.	Easy to configure. Doesn't require any custom firewall configuration. Recommended for short-lived migration scenarios (POC or small database migrations).	You own and manage the Bastion VM. May incur additional costs.
TCP Proxy via a cloud-hosted VM	Establishes connectivity from the destination to the source through a TCP proxy via Cloud hosted VM. Database Migration Service collects the required information at migration creation time, and auto-generates the script for setting it up. Relevant on AlloyDB migrations where the source is on the old network architecture.	Easy to configure. Doesn't require any custom firewall configuration.	The Bastion VM is owned and managed by you, and may incur additional costs.
VPC peering	This method works by configuring the VPCs to communicate with one another.	Native Google Cloud solution. Easy to configure. High-bandwidth Recommended for long-running or high-volume migrations.	Only applicable if both the source and destination databases are hosted in Google Cloud.
VPN	Sets up an IPSec VPN tunnel connecting the internal network and Google Cloud VPC through a secure connection over the public Internet. Use Google Cloud VPN or any VPN solution that is set up for the internal network.	Robust and scalable connectivity solution. Medium-high bandwidth. Security built-in. Offered as Google Cloud solutions or from other 3rd parties.	Additional cost. Non-trivial configuration (unless already in-place).
Cloud Interconnect	Uses a highly available, low latency connection between the on-premises network and Google Cloud.	Highest bandwidth, ideal for long-running high-volume migrations.	Additional cost. Connection isn't secure by default. Non-trivial configuration (unless already in-place).

What are the known limitations?

See Known limitations.

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