This topic describes how to migrate data from a PolarDB for PostgreSQL cluster to an ApsaraDB RDS for PostgreSQL instance by using Data Transmission Service (DTS).

Prerequisites

Limits

Note
  • During schema migration, DTS migrates foreign keys from the source database to the destination database.
  • During full data migration and incremental data migration, DTS temporarily disables the constraint check and cascade operations on foreign keys at the session level. If you perform the cascade and delete operations on the source database during data migration, data inconsistency may occur.
CategoryDescription
Limits on the source database
  • The tables to be migrated from the source PolarDB for PostgreSQL cluster must contain primary keys or UNIQUE NOT NULL indexes.
  • If the source database has long-running transactions and the data migration task contains incremental data migration, the write-ahead logging (WAL) logs that are generated before the long-running transactions are submitted may not be cleared and therefore pile up, resulting in insufficient storage space in the source database.
  • If you want to perform a primary/secondary switchover on the source PolarDB for PostgreSQL cluster, the logical replication slot failover feature must be enabled. This prevents logical subscriptions from being interrupted and ensures that your data migration task can run as expected. For more information, see Logical replication slot failover.
Other limits
  • During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data migration is complete, the size of used tablespace of the destination database is larger than that of the source database.
  • DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination instance, stop or release the failed task. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are used by DTS to access the destination instance. Otherwise, the data in the source database overwrites the data in the destination database after the failed task is resumed.
  • DTS does not check the validity of metadata such as sequences. You must manually check the validity of metadata.
  • After your workloads are switched to the destination database, newly written sequences do not increment from the maximum value of the sequences in the source database. Therefore, you must query the maximum value of the sequences in the source database before you switch your workloads to the destination database. Then, you must specify the queried maximum value as the starting value of the sequences in the destination database. You can execute the following statements to query the maximum value of the sequences in the source database: 
    do language plpgsql $$
declare
  nsp name;
  rel name;
  val int8;
begin
  for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
  loop
    execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
    raise notice '%',
    format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
  end loop;
end;
$$;

Billing

Migration typeInstance configuration feeInternet traffic fee
Schema migration and full data migrationFree of charge. Charged only when data is migrated from Alibaba Cloud over the Internet. For more information, see Billing overview.
Incremental data migrationCharged. For more information, see Billing overview.

Migration types

  • Schema migration

    Data Transmission Service (DTS) migrates the schemas of objects from the source database to the destination database.

  • Full data migration

    DTS migrates the existing data of objects from the source database to the destination database.

  • Incremental data migration

    After full data migration is complete, DTS migrates incremental data from the source database to the destination database. Incremental data migration allows data to be migrated smoothly without interrupting services of self-managed applications during data migration.

SQL operations that can be incrementally migrated

Operation typeSQL statement
DMLINSERT, UPDATE, and DELETE
DDL
  • DDL operations can be migrated only in the data migration tasks that are created after October 1, 2020.
    Important
  • If the database account of the source database is a privileged account, data migration tasks support the following DDL operations:
    • CREATE TABLE and DROP TABLE
    • ALTER TABLE, including RENAME TABLE, ADD COLUMN, ADD COLUMN DEFAULT, ALTER COLUMN TYPE, DROP COLUMN, ADD CONSTRAINT, ADD CONSTRAINT CHECK, and ALTER COLUMN DROP DEFAULT
    • TRUNCATE TABLE (The database engine version of the source PolarDB for PostgreSQL cluster must be PostgreSQL V11 or later.)
    • CREATE INDEX ON TABLE
    Important
    • You cannot migrate additional information of DDL statements, such as CASCADE or RESTRICT.
    • You cannot migrate the DDL statements from a session that executes the SET session_replication_role = replica statement.
    • If the SQL statements submitted by the source database at a time contain both DML and DDL statements, DTS does not migrate the DDL statements.
    • If the SQL statements submitted by the source database at a time contain DDL statements on objects not to be migrated, DTS does not migrate the DDL statements.

Permissions required for database accounts

DatabaseSchema migrationFull data migrationIncremental data migrationReferences
PolarDB for PostgreSQLPermissions of a privileged accountCreate a database account
ApsaraDB RDS for PostgreSQLCREATE and USAGE permissions on the objects to be migratedOwner permissions on schemasOwner permissions on schemasCreate an account on an ApsaraDB RDS for PostgreSQL instance

Procedure

  1. Go to the Data Migration Tasks page.
    1. Log on to the Data Management (DMS) console.
    2. In the top navigation bar, click DTS.
    3. In the left-side navigation pane, choose DTS (DTS) > Data Migration.
    Note
  2. From the drop-down list next to Data Migration Tasks, select the region in which the data migration instance resides.
    Note If you use the new DTS console, you must select the region in which the data migration instance resides in the upper-left corner.
  3. Click Create Task. On the page that appears, configure the source and destination databases.
    SectionParameterDescription
    N/ATask Name

    DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to use a unique task name.

    Source DatabaseDatabase TypeThe type of the source database. Select PolarDB for PostgreSQL.
    Access MethodThe access method of the source database. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the source PolarDB for PostgreSQL cluster resides.
    Replicate Data Across Alibaba Cloud AccountsSpecifies whether to migrate data across Alibaba Cloud accounts. In this example, No is selected.
    Instance IDThe ID of the source PolarDB for PostgreSQL cluster.
    Database NameThe name of the source database in the PolarDB for PostgreSQL cluster.
    Database AccountThe database account of the PolarDB for PostgreSQL cluster.
    Database Password

    The password of the database account.

    Destination DatabaseDatabase TypeThe type of the destination database. Select PostgreSQL.
    Access MethodThe access method of the destination database. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the destination ApsaraDB RDS for PostgreSQL instance resides.
    Instance IDThe ID of the destination ApsaraDB RDS for PostgreSQL instance.
    Database NameThe name of the database to which the objects are migrated in the destination ApsaraDB RDS for PostgreSQL instance.
    Database AccountThe database account of the destination ApsaraDB RDS for PostgreSQL instance.
    Database Password

    The password of the database account.

  4. In the lower part of the page, click Test Connectivity and Proceed.
    If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must make sure that the ECS instance can access the database. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database. For more information about the CIDR blocks of DTS servers, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
    Warning If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of a database or an instance, or to the security group rules of an ECS instance, security risks may arise. Therefore, before you use DTS to migrate data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhance the security of your username and password, limit the ports that are exposed, authenticate API calls, regularly check the whitelist or ECS security group rules and forbid unauthorized CIDR blocks, or connect the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
  5. Select objects for the task and configure advanced settings.
    ParameterDescription
    Migration Types
    • To perform only full data migration, select Schema Migration and Full Data Migration.
    • To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration.
    Note If Incremental Data Migration is not selected, we recommend that you do not write data to the source instance during data migration. This ensures data consistency between the source and destination instances.
    Processing Mode of Conflicting Tables

    • Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started.

      Note You can use the object name mapping feature to rename the tables that are migrated to the destination database. You can use this feature if the source and destination databases contain tables that have identical names and the tables in the destination database cannot be deleted or renamed. For more information, see Map object names.
    • Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.
      Warning If you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks.
      • If the source and destination databases have the same schema, DTS does not migrate data records that have the same primary keys as data records in the destination database.
      • If the source and destination databases have different schemas, only some columns are migrated or the data migration task fails. Proceed with caution.
    Capitalization of Object Names in Destination Instance

    The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with the default capitalization of object names in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.

    Source Objects

    Select one or more objects from the Source Objects section and click the Rightwards arrow icon to move the objects to the Selected Objects section.

    Selected Objects
    • To rename an object that you want to migrate to the destination instance, right-click the object in the Selected Objects section. For more information, see Map the name of a single object.
    • To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time.
    Note If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be migrated.
  6. Click Next: Advanced Settings to configure advanced settings.
    ParameterDescription
    Set Alerts
    Specifies whether to set alerts for the data migration task. If the task fails or the migration latency exceeds the threshold, the alert contacts will receive notifications. Valid values:
    Retry Time for Failed Connection
    The retry time range for failed connections. If the source or destination database fails to be connected after the data migration task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS is reconnected to the source and destination databases within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails.
    Note
    • If you set different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is set later takes precedence.
    • When DTS retries a connection, you are charged for the data migration task. We recommend that you specify the retry time range based on your business requirements. You can also release the data migration task at the earliest opportunity after the source and destination databases are released.
    The wait time before a retry when other issues occur in the source and destination databases.
    The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set the parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data migration task. Otherwise, the data migration task fails.
    Important The value of the The wait time before a retry when other issues occur in the source and destination databases parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
    Configure ETL
    Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values:
  7. In the lower part of the page, click Next: Save Task Settings and Precheck.
    Note
    • Before you can start the data migration task, DTS performs a precheck. You can start the data migration task only after the task passes the precheck.
    • If the task fails to pass the precheck, click View Details next to each failed item. After you troubleshoot the issues based on the causes, run a precheck again.
    • If an alert is triggered for an item during the precheck:
      • If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
      • If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.
  8. Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.
  9. On the Purchase Instance page, specify the Instance Class parameter for the data migration instance. The following table describes the parameter.
    SectionParameterDescription
    ParametersInstance Class

    DTS provides several instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Specifications of data migration instances.

  10. Read and select the check box for Data Transmission Service (Pay-as-you-go) Service Terms.
  11. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.