Apsara PolarDB is a next-generation relational database service developed by Alibaba Cloud. It is compatible with MySQL, PostgreSQL, and Oracle database engines. Apsara PolarDB provides superior performance in storage and computing to meet diverse requirements of enterprises. This topic describes how to synchronize data between PolarDB clusters compatible with Oracle by using Data Transmission Service (DTS).

Prerequisites

The tables to be synchronized contain primary keys or UNIQUE NOT NULL indexes.

Precautions

  • DTS uses read and write resources of the source and destination databases during initial full data synchronization. This may increase the database load. If the database performance is unfavorable, the specification is low, or the data volume is large, database services may become unavailable. For example, DTS occupies a large amount of read and write resources in the following cases: a large number of slow SQL queries are performed on the source database, the tables have no primary keys, or a deadlock occurs in the destination database. Before synchronizing data, you must evaluate the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. For example, you can synchronize data when the CPU usage of the source and destination databases is less than 30%.
  • A single data synchronization task can synchronize data from only one database. To synchronize data from multiple databases, you must create a data synchronization task for each database.
  • To ensure that the delay time of data synchronization is accurate, DTS adds a heartbeat table named dts_postgres_heartbeat to the source database. The following figure shows the schema of the heartbeat table.Schema of a heartbeat table

SQL operations that can be synchronized

  • Data manipulation language (DML) operations: INSERT, UPDATE, and DELETE
  • Data definition language (DDL) operations: CREATE TABLE (The CREATE TABLE operations to create partition tables or tables that contain functions cannot be synchronized.), ALTER TABLE, DROP TABLE, RENAME TABLE, CREATE INDEX, and ADD INDEX

Preparation

Set the value of the wal_level parameter to logical for the source PolarDB cluster. This setting ensures that logical decoding is supported in write-ahead logging (WAL). For more information, see Configure cluster parameters.

Warning If you change the value of the wal_level parameter, the source PolarDB cluster is restarted. Perform this operation with caution.

Procedure

  1. Purchase a data synchronization instance. For more information, see Purchase procedure.
    Note On the buy page, set both Source Instance and Target Instance to PolarDB, and set Synchronization Topology to One-Way Synchronization.
  2. Log on to the DTS console.
  3. In the left-side navigation pane, click Data Synchronization.
  4. At the top of the Synchronization Tasks page, select the region where the destination instance resides.
    Select a region
  5. Find the data synchronization instance and click Configure Synchronization Channel in the Actions column.
  6. Configure the source and destination instances.
    Configure the source and destination instances
    Section Parameter Description
    N/A Synchronization Task Name DTS automatically generates a task name. We recommend that you specify an informative name for easy identification. You do not need to use a unique task name.
    Source Instance Details Instance Type The value of this parameter is set to PolarDB Instance and cannot be changed.
    Instance Region The region of the source instance. The region is the same as the source region that you selected when you purchased the data synchronization instance. You cannot change the value of this parameter.
    PolarDB Instance ID Select the ID of the source PolarDB cluster.
    Database Name Enter the name of the source database.
    Database Account Enter the privileged account of the source PolarDB cluster. For more information about how to create a privileged database account, see Create a database account.
    Database Password Enter the password of the source database account.
    Destination Instance Details Instance Type The value of this parameter is set to PolarDB Instance and cannot be changed.
    Instance Region The region of the destination instance. The region is the same as the destination region that you selected when you purchased the data synchronization instance. You cannot change the value of this parameter.
    PolarDB Instance ID Select the ID of the destination PolarDB cluster.
    Database Name Enter the name of the destination database.
    Database Account Enter the database account of the destination PolarDB cluster. The account must have the owner permission on the database.
    Notice You can specify the database owner when you create a database.
    Database Password Enter the password of the destination database account.
  7. In the lower-right corner of the page, click Set Whitelist and Next.
    Note DTS adds the CIDR blocks of DTS servers to the whitelists of the source and destination PolarDB clusters. This ensures that DTS servers can connect to the source and destination PolarDB clusters.
  8. Configure the processing mode in existing destination tables and the objects to be synchronized.
    Select the objects to be synchronized
    Parameter Description
    Processing Mode In Existed Target Table
    • Pre-check and Intercept: checks whether the destination database contains tables that have the same names as tables in the source database. If the destination database does not contain tables that have the same names as tables in the source database, the precheck is passed. Otherwise, an error is returned during precheck and the data synchronization task cannot be started.
      Note If tables in the destination database have the same names as tables in the source database, and cannot be deleted or renamed, you can use the object name mapping feature. For more information, see Specify the name of an object in the destination instance.
    • Ignore: skips the precheck for identical table names in the source and destination databases.
      Warning If you select Ignore, data consistency is not guaranteed and your business may be exposed to potential risks.
      • DTS does not synchronize data records that have the same primary keys as data records in the destination database during initial data synchronization. This occurs if the source and destination databases have the same schema. However, DTS synchronizes these data records during incremental data synchronization.
      • If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only some columns are synchronized or the data synchronization task fails.
    Objects

    Select objects from the Available section and click the Right arrow icon to move the objects to the Selected section.

    You can select tables and databases as the objects to be synchronized.

    Note
    • If you select a database as the object to be synchronized, all schema changes in the database are synchronized to the destination database.
    • After an object is synchronized to the destination database, the name of the object remains unchanged. You can change the name of an object in the destination PolarDB cluster by using the object name mapping feature. For more information about how to use this feature, see Specify the name of an object in the destination instance.
  9. In the lower-right corner of the page, click Next.
  10. Configure initial synchronization.
    Configure initial synchronization
    Initial synchronization type Description
    Initial schema synchronization DTS synchronizes the schemas of the required objects to the destination PolarDB cluster. DTS supports initial schema synchronization for the following types of objects: table, view, synonym, trigger, stored procedure, function, package, and user-defined type.
    Notice However, if an object contains triggers, data will become inconsistent between the source and destination databases. For more information about how to solve this issue, see Configure a data synchronization task for a source database that contains a trigger.
    Initial full data synchronization DTS synchronizes historical data of the required objects from the source PolarDB cluster to the destination PolarDB cluster.
    Notice During initial schema synchronization and initial full data synchronization, do not perform DDL operations on the objects to be synchronized. Otherwise, the objects may fail to be synchronized.
  11. In the lower-right corner of the page, click Precheck.
    Note
    • Before you can start the data synchronization task, a precheck is performed. You can start the data synchronization task only after the task passes the precheck.
    • If the task fails to pass the precheck, click the Info icon icon next to each failed item to view details. Troubleshoot the issues based on the causes and run the precheck again.
  12. Close the Precheck dialog box after the following message is displayed: The precheck is passed.
  13. Wait until the initial synchronization is complete and the data synchronization task is in the Synchronizing state.
    On the Synchronization Tasks page, view the status of the data synchronization task.View the status of a data synchronization task.