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

Prerequisites

  • A PolarDB for PostgreSQL cluster is created as the source database. For more information, see Create a cluster.
  • An ApsaraDB RDS for PostgreSQL instance is created as the destination database. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
  • The available storage space of the destination database is larger than the total size of the data in the source database.

Limits

Category Description
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.

Other limits
  • 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, DTS uses read and write resources of the source and destination databases. This may increase the loads on the database servers.
  • 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. For example, if you use tools other than DTS to write data to the destination database, data loss may occur in the destination database when you use Data Management (DMS) to perform online DDL operations.

Billing

Synchronization type Task configuration fee
Schema synchronization and full data synchronization Free of charge.
Incremental data synchronization Charged. For more information, see Pricing.

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 type SQL statement
DML INSERT, UPDATE, and DELETE

Procedure

  1. Go to the Data Synchronization Tasks page.
    1. Log on to the DMS console.
    2. In the top navigation bar, click DTS.
    3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
  2. On the right side of Data Synchronization Tasks, select the region where the data synchronization instance resides.
    Note If you use the new DTS console, you must select the region where 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.
    Section Parameter Description
    N/A Task 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 Database Database Type Select PolarDB for PostgreSQL.
    Access Method Select Alibaba Cloud Instance.
    Instance Region The region in which the PolarDB for PostgreSQL cluster resides.
    Replicate Data Across Alibaba Cloud Accounts Specifies whether to replicate data across Alibaba Cloud accounts. In this example, No is selected.
    Instance ID The ID of the PolarDB for PostgreSQL cluster.
    Database Name The name of the source database in the PolarDB for PostgreSQL cluster.
    Database Account The database account of the PolarDB for PostgreSQL cluster.
    Database Password

    Enter the password of the database account.

    Destination Database Database Type Select PostgreSQL.
    Access Method 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 destination database in the ApsaraDB RDS for PostgreSQL instance.
    Database Account The database account of the destination ApsaraDB RDS for PostgreSQL instance. The account must have the permissions of the schema owner. For more information about how to create an account on an ApsaraDB RDS for PostgreSQL instance and grant permissions to this account, see Create an account on an ApsaraDB RDS for PostgreSQL instance.
    Database Password

    Enter the password of the database account.

  4. In the lower part of the page, click Test Connectivity and Proceed.
    Note
    • You do not need to modify the security settings for ApsaraDB instances (such as ApsaraDB RDS for MySQL and ApsaraDB for MongoDB) and ECS-hosted databases. DTS automatically adds the CIDR blocks of DTS servers to the whitelists of ApsaraDB instances or the security group rules of Elastic Compute Service (ECS) instances. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
    • After data synchronization is completed, we recommend that you remove the CIDR blocks of DTS servers from the whitelists or security groups.
  5. Select objects for the task and configure advanced settings.
    • Basic Settings
      Parameter Description
      Task Stages

      Incremental Data Synchronization is selected by default. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is completed, DTS synchronizes the historical data of 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 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 consistency is not guaranteed and your business may be exposed to potential risks.
        • If the source and destination databases have the same schema, and a data record has the same primary key 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

      Specify 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 of 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, and 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.
    • Advanced Settings
      Parameter Description
      Set Alerts
      Specify whether to set alerts for the data synchronization task. If the task fails or the synchronization latency exceeds the threshold, the alert contacts will receive notifications.
      • Select No if you do not want to set alerts.
      • Select Yes to set alerts. In this case, you must also set the alert threshold and alert contacts.
      Retry Time for Failed Connection
      Specify the retry time range for failed connections. If a data synchronization task is disconnected, DTS immediately retries a connection within the specified time range. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the retry time range to more than 30 minutes. 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 multiple DTS instances have the same source or destination database, the lowest value of this parameter takes effect. For example, the retry time range is set to 30 minutes for Instance A and 60 minutes for Instance B, DTS retries failed connections within 30 minutes.
      • 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.
      Configure ETL
      Specify whether you want to enable the ETL feature. For more information, see What is ETL?.
      • If you select Yes, the ETL feature is enabled. You must specify a policy in the text box.
      • If you select No, the ETL feature is disabled.
  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, you can click the Info icon icon next to each failed item to view details.
      • You can troubleshoot the issues based on the causes and run a precheck again.
      • If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
  7. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  8. On the Purchase Instance page, specify the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.
    Section Parameter Description
    Parameters Billing Method
    • Subscription: You pay for your subscription when you create an instance. We recommend that you select the subscription billing method for long-term use because it is more cost-effective than the pay-as-you-go billing method. You can save more costs with longer subscription periods.
    • Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. We recommend that you select the pay-as-you-go billing method for short-term use. If you no longer require a pay-as-you-go instance, you can release the 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 Length If you select the subscription billing method, set the subscription length and the number of instances that you want to create. The subscription length can be one to nine months or one to three years.
    Note This parameter is available only if you select the subscription billing method.
  9. Read and select 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.