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Data Transmission Service:Migrate data between ApsaraDB RDS for PostgreSQL instances

Last Updated:Oct 27, 2023

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

Category

Description

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 (-). Example: dts-testdata.

  • If you select tables as the objects to be migrated and you need to edit tables, such as renaming tables or columns in the destination database, up to 1,000 tables can be migrated 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, you must make sure that the following requirements are met:

    • The value of the wal_level parameter must be set to logical.

    • For an incremental data migration, the WAL logs of the source database must be stored for more than 24 hours. For a 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 or performance 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 between the source and destination databases occurs. 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 a table in the schema 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 the 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 initial 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, and 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_. By using this replication slot, DTS can obtain the incremental logs of the source database within the last 15 minutes.

    Note
    • After the DTS instance is released, the replication slot is automatically deleted. If you modify 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 manually 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 for PostgreSQL instance, you must log on to the secondary instance to clear the replication slot.

  • 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 of 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are used by DTS to access the destination database. Otherwise, the data in the source database will overwrite 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

    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 the services of self-managed applications during data migration.

SQL operations that can be incrementally migrated

Operation type

SQL statement

DML

INSERT, 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

Database

Schema migration

Full data migration

Incremental data migration

Source ApsaraDB RDS for PostgreSQL instance

USAGE 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 instance

CREATE 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 and Create a database.

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. In the Create Task wizard, configure the source and destination databases.

    Warning

    After you configure the source and destination databases, we recommend that you read the limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.

    Section

    Parameter

    Description

    N/A

    Task Name

    The name of the task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.

    Source Database

    Select 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 Type

    The type of the source database. Select PostgreSQL.

    Access Method

    The access method of the source database. Select Alibaba Cloud Instance.

    Instance Region

    The region in which the source ApsaraDB RDS for PostgreSQL instance resides.

    Instance ID

    The ID of the source ApsaraDB RDS for PostgreSQL instance.

    Database Name

    The name of the database from which the objects are migrated in the source ApsaraDB RDS for PostgreSQL instance.

    Database Account

    The 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 Database

    Select 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 Type

    The type of the destination database. Select PostgreSQL.

    Access Method

    The access method of the destination database. Select Alibaba Cloud Instance.

    Instance Region

    The region in which the destination ApsaraDB RDS for PostgreSQL instance resides.

    Instance ID

    The ID of the destination ApsaraDB RDS for PostgreSQL instance.

    Database Name

    The name of the database to which the objects are migrated in the destination ApsaraDB RDS for PostgreSQL instance.

    Database Account

    The 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 IP address 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 self-managed database is hosted on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. 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 IP address whitelist of the database to allow DTS to access the database. For more information, see the "CIDR blocks of DTS servers" section of the Add the CIDR blocks of DTS servers to the security settings of on-premises databases topic.

    Warning

    If the public CIDR blocks of DTS servers are automatically or manually added to the IP address whitelist of a database 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: enhancing the security of your account and password, limiting the ports that are exposed, authenticating API calls, regularly checking the IP address whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, and connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

  5. Configure the objects to be migrated and advanced settings.

    Parameter

    Description

    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 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 tables that have 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 table 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 the following potential risks:

      • If the source and destination databases have the same schemas, DTS does not migrate data records that have the same primary key values as data records in the destination database.

      • If the source and destination databases have different schemas, only specific 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 向右小箭头 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 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.

    Parameter

    Description

    Set Alerts

    Specifies whether to configure alerting for the data migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts will receive notifications. Valid values:

    • No: does not configure alerting.

    • Yes: configures alerting. If you select Yes, you must also specify the alert threshold and alert contacts. For more information, see Configure monitoring and alerting.

    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 make sure that the capitalization of object names is consistent with that 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 retry 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 specify different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is specified later takes precedence.

    • When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also 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 DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry 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 retry 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.

    You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameters to be specified when you call the relevant API operation to configure the DTS task.

    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 error message, you can run a precheck again.

    • If an alert is triggered for an item during the precheck:

      • If the alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.

      • If the 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, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

    Section

    Parameter

    Description

    New Instance Class

    Resource Group

    The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?

    Instance Class

    DTS provides 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 agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.

  11. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.