Data Transmission Service (DTS) supports two-way data synchronization between two PostgreSQL databases, such as ApsaraDB RDS for PostgreSQL instances and self-managed PostgreSQL databases. This feature is applicable to scenarios such as active geo-redundancy (unit-based) and geo-disaster recovery. This topic describes how to configure two-way data synchronization between ApsaraDB RDS for PostgreSQL instances. You can also follow the procedure to configure data synchronization tasks for self-managed PostgreSQL databases.

Prerequisites

Limits

Supported synchronization topologies

DTS supports two-way data synchronization only between two PostgreSQL databases. DTS does not support two-way data synchronization between multiple PostgreSQL databases.

Conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you specify for the data synchronization task.

DTS checks and fixes conflicts to maximize the stability of two-way synchronization instances. DTS can detect the following types of conflicts:
  • Uniqueness conflicts caused by INSERT operations

    INSERT operations that do not comply with the uniqueness constraint cannot be synchronized. For example, if a record with the same primary key value is inserted into the two synchronization nodes at almost the same time, one of the inserted records fails to be synchronized. The synchronization fails because a record with the same primary key value already exists in the other node.

  • Inconsistent records caused by UPDATE operations
    • If the records to be updated do not exist in the destination instance, DTS converts the UPDATE operation into an INSERT operation. However, uniqueness conflicts may occur.
    • The primary keys or unique keys of the records to be inserted may conflict with those of existing records in the destination instance.
  • Non-existent records to be deleted

    The records to be deleted do not exist in the destination instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

Notice
  • During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS cannot guarantee that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
  • DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.

SQL operations that can be synchronized

Operation type SQL statements
DML INSERT, UPDATE, and DELETE
DDL CREATE, ALTER, DROP, RENAME, and TRUNCATE
Notice
  • Only two-way data synchronization tasks in the China (Shanghai) and China (Hangzhou) regions can synchronize DDL operations.
  • DDL operations can be synchronized only in the forward direction, that is, from the source database to the destination database. DDL operations cannot be synchronized from the destination database to the source database because DDL operations are ignored in the reverse direction.

Procedure

  1. Purchase an instance for two-way data synchronization. For more information, see Purchase a DTS instance.
    Notice On the buy page, set both Source Instance and Destination Instance to PostgreSQL and set Synchronization Topology to Two-way Synchronization.
  2. Go to the Data Synchronization page of the new DTS console.
    Note You can also log on to the Data Management (DMS) console. In the top navigation bar, choose DTS > Data Synchronization.
  3. In the upper-left corner of the page, select the region where the destination instance resides.
  4. Find the data synchronization instance and click Configure Task in the Actions column of the first data synchronization task.
  5. 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 you create and run the data synchronization task.
    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 Select Template
    If the connection settings of the instance has been saved as a template, you can select the connection name. DTS automatically applies the saved connection settings.
    Note
    • You can modify the connection settings of a template.
    • To edit the template name, you can click Edit Template Name at the bottom of the Source Database section.
    • The modified connection template will take effect the next time you select the connection template. The database instance that you have configured by using the template is not affected.
    Database Type Select PostgreSQL.
    Access Method Select Alibaba Cloud Instance.
    Instance Region The source region that you selected on the buy page. You cannot change the value of this parameter.
    Instance ID Select the ID of the source ApsaraDB RDS for PostgreSQL instance.
    Database Name Enter the name of the source database in the ApsaraDB RDS for PostgreSQL instance.
    Database Account Enter the database account of the source ApsaraDB RDS for PostgreSQL instance. A privileged account has the required permissions.
    Note If the version of the source ApsaraDB RDS for PostgreSQL instance is 9.4 and you synchronize only DML operations, the database account must have the REPLICATION permission.
    Database Password

    Enter the password of the database account.

    Save as Template
    Click Save as Template. In the dialog box that appears, set the name of the database connection, and click OK. The connection settings of the database instance is saved as a template.
    Note We recommend that you specify an informative name to identify the database connection. You do not need to use a unique name.
    Destination Database Select Template
    If the connection settings of the instance has been saved as a template, you can select the connection name. DTS automatically applies the saved connection settings.
    Note
    • You can modify the connection settings of a template.
    • To edit the template name, you can click Edit Template Name at the bottom of the Source Database section.
    • The modified connection template will take effect the next time you select the connection template. The database instance that you have configured by using the template is not affected.
    Database Type Select PostgreSQL.
    Access Method Select Alibaba Cloud Instance.
    Instance Region The destination region that you selected on the buy page. You cannot change the value of this parameter.
    Instance ID Select the ID of the destination ApsaraDB RDS for PostgreSQL instance.
    Database Name Enter the name of the destination database in the ApsaraDB RDS for PostgreSQL instance.
    Database Account Enter the database account of the destination ApsaraDB RDS for PostgreSQL instance. A privileged account has the required permissions.
    Database Password

    Enter the password of the database account.

    Save as Template
    Click Save as Template. In the dialog box that appears, set the name of the database connection, and click OK. The connection settings of the database instance is saved as a template.
    Note We recommend that you specify an informative name to identify the database connection. You do not need to use a unique name.
  6. In the lower part of the page, click Test Connectivity and Proceed.
  7. Select objects for the task and configure advanced settings.
    • Basic SettingsBasic 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, DTS synchronizes the historical data of the selected objects from the source instance 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 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.
      Conflict Resolution Policy If you encounter the conflicts described in Conflict detection, select a conflict resolution policy based on your business needs.
      • TaskFailed (If a conflict occurs, an error is reported and the task is stopped)

        If a conflict occurs during data synchronization, the synchronization task reports an error and exits the process. The task enters a failed state and you must manually resolve the conflict.

      • Ignore (If a conflict occurs, the conflicting record in the destination instance is used)

        If a conflict occurs during data synchronization, the synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used.

      • Overwrite (If a conflict occurs, the conflicting record in the destination instance is overwritten)

        If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten.

      Select 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.

      Note You can select columns, tables, or schemas as the objects to be synchronized.
      Rename Databases and Tables
      • 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.
      Filter data

      You can specify WHERE conditions to filter data. For more information, see Use SQL conditions to filter data.

      Select the SQL operations to be synchronized In the Selected Objects section, right-click an object. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, see SQL operations that can be synchronized.
    • Advanced SettingsAdvanced Settings
      Parameter Description
      Capitalization of Object Names in Destination Instance This parameter is set to DTS default policy. In this scenario, you cannot change the capitalization of object names in the destination database. For two-way data synchronization between PostgreSQL databases, all object names in the destination database are in lowercase. For more information, see Specify the capitalization of object names in the destination instance.
      Retry Time for Failed Connection
      Specify the retry time for failed connections. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the retry time to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified time, 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 takes effect. For example, the retry time is set to 30 minutes for Instance A and 60 minutes for Instance B, DTS retries failed connections at an interval of 30 minutes.
      • When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time based on your business needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
  8. Click Next: Save Task Settings and Precheck in the lower part of the page.
    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.
  9. Wait until the Success Rate becomes 100%. Then, click Back.
  10. The data synchronization task in the forward direction starts. You can view the progress of the task in the task list.
  11. Wait until initial synchronization is completed and the data synchronization task in the forward direction is in the Running state. You can view the status of the data synchronization task on the Data Synchronization page.
  12. Find the data synchronization task in the reverse direction and click Configure Task.
  13. On the Create Task page, configure the source and destination databases, as described in Step 5.
    Notice When you configure the data synchronization task in the reverse direction, you must select the correct source and destination instances. The source instance in the reverse direction corresponds to the destination instance in the forward direction. The destination instance in the reverse direction corresponds to the source instance in the forward direction. You must also make sure that the parameter settings (such as the database name, account, and password) are consistent.
  14. Select objects for the task and configure advanced settings., as described in Step 7. We recommend that you specify the same settings. The following table provides additional description.
    Basic Settings
    Parameter Description
    Processing Mode of Conflicting Tables When DTS checks for conflicting tables in the reverse direction, the tables that have been synchronized to the destination instance are ignored.
    Select Objects We recommend that you select the same objects for the forward and reverse directions.

    You can also add or delete objects based on your business requirements.

    Rename Databases and Tables We recommend that you do not use this feature when you configure the task in the reverse direction. Otherwise, data inconsistency may occur.
  15. Click Next: Save Task Settings and Precheck in the lower part of the page.
    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.
  16. Wait until the Success Rate becomes 100%. Then, click Back.
  17. After the second data synchronization task is configured, wait until both tasks are in the Running state. The two-way data synchronization tasks are configured.