This topic describes how to migrate schemas, full data, and incremental data between ApsaraDB RDS for PostgreSQL instances by using Data Transmission Service (DTS). When you migrate data between ApsaraDB RDS for PostgreSQL instances, you can select all the supported migration types to ensure service continuity.

Prerequisites

  • The source and destination ApsaraDB RDS for PostgreSQL instances are created. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
    Note To ensure compatibility, the version of the destination database must be the same as or later than that of the source database.

    If the version of the destination database is earlier than that of the source database, database compatibility issues may occur.

  • The available storage space of the destination ApsaraDB RDS for PostgreSQL instance is larger than the total size of the data in the source ApsaraDB RDS for PostgreSQL instance.

Limits

CategoryDescription
Limits on the source database
  • The tables to be migrated must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, the destination database may contain duplicate data records.

    The name of the source database cannot contain hyphens (-). For example, dts-testdata is not allowed.

  • If you select tables as the objects to be migrated and you need to edit the tables, such as renaming tables or columns, in the destination database, you can migrate up to 1,000 tables in a single data migration task. If you run a task to migrate more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to migrate the tables, or configure a task to migrate the entire database.
  • If you need to migrate incremental data, make sure that the following requirements are met:
    • The value of the wal_level parameter must be set to logical.

    • For incremental data migration, the write-ahead logging (WAL) logs of the source database must be stored for more than 24 hours. For full data and incremental data migration, the WAL logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the WAL logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After full data migration is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of WAL logs based on the preceding requirements. Otherwise, the service reliability and performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

  • Limits on operations to be performed on the source database:
    • During schema migration and full data migration, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data migration task fails.
    • If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency may occur between the source and destination databases. To ensure data consistency, we recommend that you select Schema Migration, Full Data Migration, and Incremental Data Migration as the migration types.
  • If the source database has one or more long-running transactions and incremental data is migrated in the data migration task, the WAL logs that are generated before the long-running transactions are committed may not be cleared and therefore pile up, resulting in insufficient storage space in the source database.
Other limits
  • If you need to perform a primary/secondary switchover on the source ApsaraDB RDS for PostgreSQL instance, 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.
  • A single data migration task can migrate data from only one database. To migrate data from multiple databases, you must create a data migration task for each database.
  • If you select a schema as the object to be migrated and create a table in the schema or execute the RENAME statement to rename the table during incremental data migration, you must execute the ALTER TABLE schema.table REPLICA IDENTITY FULL; statement before you write data to the table.
    Note Replace the schema and table in the preceding sample statement with your actual schema name and table name.
  • 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 $$
declare
  nsp name;
  rel name;
  val int8;
begin
  for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
  loop
    execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
    raise notice '%',
    format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
  end loop;
end;
$$;
  • DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and the heartbeat information. During data migration, do not delete temporary tables in the source database. Otherwise, exceptions occur. After the DTS instance is released, temporary tables are automatically deleted.

    public.DTS_PG_CLASS, public.DTS_PG_ATTRIBUTE, public.DTS_PG_TYPE, public.DTS_PG_ENUM, public.DTS_POSTGRES_HEARTBEAT, public.DTS_DDL_COMMAND, public.DTS_ARGS_SESSION.

  • If you run a full or incremental data migration task, the tables to be migrated from the source database contain foreign keys, triggers, or event triggers, and the account of the destination database is a privileged account or an account that has the permissions of the superuser role, DTS temporarily sets the session_replication_role parameter to replica at the session level during full or incremental data migration. If the account of the destination database does not have the permissions, you must manually set the session_replication_role parameter to replica in the destination database. After the session_replication_role parameter is set to replica during full migration or incremental migration, if a cascade update or delete operation is performed in the source database, data inconsistency may occur. After the DTS migration task is released, you can change the value of the session_replication_role parameter to origin.
  • To ensure that the latency of incremental data migration is accurate, DTS creates a heartbeat table named dts_postgres_heartbeat in the source database.
  • During incremental data migration, DTS creates a replication slot for the source database. The replication slot is prefixed with dts_sync_. DTS automatically clears historical replication slots every 90 minutes to reduce storage usage.
    Note
    • After the DTS task is released, the replication slot is automatically deleted. If you change the password of the source database or delete the IP address whitelist of DTS, the replication slot cannot be automatically deleted. In that case, you must delete the replication slot from the source database to prevent the replication slot from piling up.
    • If the data migration task is released or fails, DTS automatically clears the replication slot. If a primary/secondary switchover is performed on the source ApsaraDB RDS for PostgreSQL instance, you must log on to the secondary node to clear the replication slot.
    Replication slot information
  • Before you migrate data, evaluate the impact of data migration on the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers.
  • During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data migration is complete, the tablespace of the destination database is larger than that of the source database.
  • Make sure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the ROUND(COLUMN,PRECISION) function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for the FLOAT data type to 38 digits and the precision for the DOUBLE data type to 308 digits.
  • DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination database, stop or release the data migration task. You can also execute the REVOKE statement to revoke the write permissions from the account used by DTS to access the destination database. Otherwise, the data in the source database overwrites the data in the destination database after the failed task is resumed.
Special cases

During data migration, do not modify the endpoint or zone of the source ApsaraDB RDS for PostgreSQL instance. Otherwise, the data migration task fails.

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 and the minor engine version of the source ApsaraDB RDS for PostgreSQL instance is 20210228 or later, the following DDL statements can be migrated in data migration tasks. For information about how to update the minor engine version of an instance, see Update the minor engine version of an ApsaraDB RDS for PostgreSQL instance.
    • 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 version of the source ApsaraDB RDS for PostgreSQL instance 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 migration
Source ApsaraDB RDS for PostgreSQL instanceUSAGE permission on the pg_catalog schema SELECT permission on the objects to be migrated Permissions of a privileged account. The account must be the owner of the database.
Note If the database engine version of the source ApsaraDB RDS for PostgreSQL instance is 9.4 and you migrate only DML operations, only the REPLICATION permission is required for the database account.
Destination ApsaraDB RDS for PostgreSQL instanceCREATE and USAGE permissions on the objects to be migrated Owner permissions on schemas

For more information about how to create a database account for an ApsaraDB RDS for PostgreSQL instance and grant permissions to this account, see Create an account on an ApsaraDB RDS for PostgreSQL instance and Create a database 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.
  2. From the drop-down list next to Data Migration Tasks, select the region in which your data migration instance resides.
    Note If you use the new DTS console, 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.
    Warning After you select the source and destination instances, we recommend that you read the limits displayed at the top of the page. This helps ensure that the task properly runs or prevent data inconsistency.
    SectionParameterDescription
    N/ATask Name

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

    Source DatabaseSelect an existing DMS database instance
    The database instance that you want to use. You can choose whether to select an existing instance based on your business requirements.
    • If you select an existing instance, DTS automatically populates the parameters for the database.
    • If you do not select an existing instance, you must manually configure parameters for the database.
    Database TypeThe type of the source database. Select PostgreSQL.
    Access MethodThe access method of the source database. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the source ApsaraDB RDS for PostgreSQL instance resides.
    Instance IDThe ID of the source ApsaraDB RDS for PostgreSQL instance.
    Database NameThe name of the database from which the objects are migrated in the source ApsaraDB RDS for PostgreSQL instance.
    Database AccountThe database account of the source ApsaraDB RDS for PostgreSQL instance. For more information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    The password of the database account.

    Destination DatabaseSelect an existing DMS database instance
    The database instance that you want to use. You can choose whether to select an existing instance based on your business requirements.
    • If you select an existing instance, DTS automatically populates the parameters for the database.
    • If you do not select an existing instance, you must manually configure parameters for the database.
    Database 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. For more information about the permissions that are required for the account, see Permissions required for database accounts.
    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. Configure objects to migrate and advanced settings.
    ParameterDescription
    Migration Type
    • 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 you select Schema Migration, DTS migrates the schemas (including foreign keys) of the tables to be migrated from the source database to the destination database.
    • If Incremental Data Migration is not selected, we recommend that you do not write data to the source database during data migration. This ensures data consistency between the source and destination databases.
    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.
    Source Objects

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

    Note You can select columns, tables, or schemas as the objects to be migrated. If you select tables or columns as the objects to be migrated, DTS does not migrate other objects, such as views, triggers, or stored procedures, to the destination database.
    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.
    • 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 depend on the object may fail to be migrated.
    • To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data.
    • To migrate SQL operations performed on a specific database or table, right-click the object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to migrate. For more information about the DML and DDL operations that can be migrated, see the SQL operations that can be incrementally migrated section of this topic.
  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:
    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.

    Retry Time for Failed Connections
    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 reconnects to the source and destination databases within the specified 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.
    • If DTS retries a connection, you are charged for the operation of the DTS instance. We recommend that you specify the retry time based on your business needs and release the DTS instance at your earliest opportunity after the source and destination instances 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 value 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 to agree to 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.