This topic describes how to synchronize data between PolarDB for PostgreSQL clusters by using Data Transmission Service (DTS).

Prerequisites

  • The source and destination PolarDB for PostgreSQL clusters are created. For more information, see Create a PolarDB for PostgreSQL cluster.
  • The wal_level parameter of the source and destination PolarDB for PostgreSQL clusters is set to logical. For more information, see Configure cluster parameters.
  • The available storage space of the destination database is larger than the total size of the data in the source database.

Limits

Note
  • During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.
  • During full synchronization and incremental synchronization, DTS temporarily disables checking of foreign key constraints and foreign key cascade operations at the session level. If you perform the cascade update and delete operations on the source database during data synchronization, data inconsistency may occur.
CategoryDescription
Limits on the source database
  • The tables to be synchronized from the source PolarDB for PostgreSQL cluster must contain primary keys or UNIQUE NOT NULL indexes.
  • If the source database has a long-running transaction and an incremental data synchronization task is in progress, the write-ahead logging (WAL) logs that record the long-running transaction cannot be cleared and large storage space is occupied.
  • 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 synchronization task can run as expected. For more information, see Logical replication slot failover.
Other limits
  • During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours.
  • During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After initial full data synchronization is complete, the tablespace of the destination database is larger than that of the source database.
  • During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases.
  • If a table is synchronized in both the forward and reverse synchronization and both the full data and incremental data of the table are synchronized in the forward synchronization, DTS synchronizes only the incremental data of the table in the reverse synchronization.
  • 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

Synchronization typeTask configuration fee
Schema synchronization and full data synchronizationFree of charge.
Incremental data synchronizationCharged. For more information, see Billing overview.

Supported synchronization topologies

  • One-way one-to-one synchronization
  • One-way one-to-many synchronization
  • One-way many-to-one synchronization
For more information about synchronization topologies that are supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

Operation typeSQL statement
DMLINSERT, UPDATE, and DELETE
DDL
  • DDL operations can be synchronized only in the data synchronization 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 ApsaraDB RDS for PostgreSQL instance is 20210228 or later, DTS can synchronize the following DDL statements. 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 major engine version of the source ApsaraDB RDS for PostgreSQL instance must be PostgreSQL 11 or later.)
    • CREATE INDEX ON TABLE
    Important
    • You cannot synchronize additional information of DDL statements, such as CASCADE or RESTRICT.
    • You cannot synchronize the DDL statements from a session for which the SET session_replication_role = replica command is executed.
    • If the SQL statements submitted by the source database at a time contain both DML and DDL statements, DTS does not synchronize the DDL statements.
    • If the SQL statements submitted by the source database at a time contain DDL statements on objects not to be synchronized, DTS does not synchronize the DDL statements.

Procedure

  1. Go to the Data Synchronization 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 Synchronization.
    Note
  2. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
    Note If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.
  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 DatabaseSelect Instance
    Select whether to use an existing instance.
    • If you select an existing instance, DTS automatically applies the parameter settings of the instance. You do not need to configure the corresponding parameters again.
    • If you do not use an existing instance, you must configure parameters for the source database.
    Database 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 data is synchronized 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 source PolarDB for PostgreSQL cluster.
    Database Password

    The password of the database account.

    Destination DatabaseSelect Instance
    Select whether to use an existing instance.
    • If you select an existing instance, DTS automatically applies the parameter settings of the instance. You do not need to configure the corresponding parameters again.
    • If you do not use an existing instance, you must configure parameters for the source database.
    Database TypeThe type of the destination database. Select PolarDB for PostgreSQL.
    Access MethodThe access method of the destination cluster. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the destination PolarDB for PostgreSQL cluster resides.
    Instance IDThe ID of the source PolarDB for PostgreSQL cluster.
    Database NameThe name of the destination database in the PolarDB for PostgreSQL cluster.
    Database AccountA privileged account of the destination PolarDB for PostgreSQL cluster. For more information about how to create and grant permissions to a privileged account, see Create a database account.
    Database Password

    The password of the database account.

  4. In the lower part of the page, click Test Connectivity and Proceed.
    Warning
    • If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. 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 need to manually add the CIDR blocks of DTS servers to the whitelist of the self-managed database on the ECS instance to allow DTS to 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.
    • If the CIDR blocks of DTS servers are automatically or manually added to the whitelist or ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize 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 username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
    • After the DTS task is complete or released, we recommend that you manually remove the CIDR blocks of DTS servers from the whitelist or ECS security group rules. You must remove the IP address whitelist group whose name contains dts from the whitelist of the Alibaba Cloud database instance or the security group rules of the ECS instance. For more information about the CIDR blocks that you must remove from the whitelist of the self-managed databases that are deployed in data centers or databases that are hosted on third-party cloud services, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
  5. Select objects for the task and configure advanced settings.
    • Basic Settings
      ParameterDescription
      Task Stages

      By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.

      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 synchronization task cannot be started.

        Note You can use the object name mapping feature to rename the tables that are synchronized to the destination database. You can use this feature if the source and destination databases contain 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 potential risks.
        • If the source and destination databases have the same schemas, and a data record has the same primary key value as an existing data record in the destination database:
          • During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained.
          • During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten.
        • If the source and destination databases have different schemas, data may fail to be initialized. In this case, only some columns are synchronized or the data synchronization task fails.
      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.

      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 databases as the objects to be synchronized. If you select tables or columns as the objects to be synchronized, DTS does not synchronize other objects such as views, triggers, or stored procedures to the destination database.
      Selected Objects
      • To rename an object that you want to synchronize 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
      • To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information about SQL operations that can be synchronized, see SQL operations that can be synchronized.
      • 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 about how to specify the conditions, see Use SQL conditions to filter data.
    • Advanced Settings
      ParameterDescription
      Set Alerts
      Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, 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 synchronization 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 synchronization task. Otherwise, the data synchronization task fails.
      Note
      • If you set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set 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 the earliest opportunity after the source and destination instances are released.
      Configure ETL
      Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values:
  6. In the lower part of the page, click Next: Save Task Settings and Precheck.
    Note
    • Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization 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 analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
    • If an alert is generated for an item during the precheck, perform the following operations based on the scenario:
      • In scenarios where you cannot ignore the alert item, click View Details next to the failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
      • In scenarios where you can ignore the alert item, click Confirm Alert Details next to the failed item. 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.
  7. Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.
  8. On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.
    Section Parameter Description
    New Instance Class Billing Method
    • Subscription: You pay for the instance when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use.
    • Pay-as-you-go: A pay-as-you-go instance is charged on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go instance, you can release the pay-as-you-go instance to reduce costs.
    Instance Class DTS provides several instance classes that have different performance in synchronization speed. You can select an instance class based on your business scenario. For more information, see Specifications of data synchronization instances.
    Subscription Duration If you select the subscription billing method, set the subscription duration and the number of instances that you want to create. The subscription duration can be one to nine months or one to three years.
    Note This parameter is displayed only if you select the subscription billing method.
  9. Read and select the check box for Data Transmission Service (Pay-as-you-go) Service Terms.
  10. Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.