Configure one-way data synchronization between PolarDB-O clusters

Precautions

• DTS uses read and write resources of the source and destination databases during initial full data synchronization. This may increase the loads of the database servers. 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 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. For example, you can synchronize data when the CPU utilization 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.
• If you select a schema as the object to be synchronized and create a table in the schema or run the RENAME command to rename the table, you must run the ALTER TABLE schema.table REPLICA IDENTITY FULL; command before you write data to the table.
  Note Replace the schema and table in the preceding sample command with the actual schema name and table name.

SQL operations that can be synchronized

INSERT, UPDATE, and DELETE

Procedure

1. Purchase a data synchronization instance. For more information, see Purchase a DTS instance.
   Note On the buy page, set both Source Instance and Destination 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.
   
5. Find the data synchronization instance and click Configure Synchronization Channel in the Actions column.
6. 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 source region that you selected on the buy page. You cannot change the value of this parameter.
   	PolarDB Instance ID	Select the ID of the source PolarDB-O cluster.
   	Database Name	Enter the name of the source database.
   	Database Account	Enter the privileged account of the source PolarDB-O cluster. For more information about how to create a privileged database account, see Create database accounts.
   	Database Password	Enter the password of the database account.
   Destination Instance Details	Instance Type	The value of this parameter is set to PolarDB and cannot be changed.
   	Instance Region	The destination region that you selected on the buy page. You cannot change the value of this parameter.
   	PolarDB Instance ID	Select the ID of the destination PolarDB-O cluster.
   	Database Name	Enter the name of the destination database.
   	Database Account	Enter the database account of the destination PolarDB-O cluster. The account must have the permissions of the database owner. Notice You can specify the database owner when you create a database.
   	Database Password	Enter the password of the 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-O clusters. This ensures that DTS servers can connect to the source and destination PolarDB-O clusters.
8. Select the processing mode of conflicting tables, and the objects to be synchronized.
   

   Setting	Description
   Select the processing mode of conflicting tables	Pre-check and Intercept: 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 Rename an object to be synchronized. Ignore Errors and Proceed: 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. During initial data synchronization, DTS does not synchronize the data records that have the same primary keys as the data records in the destination database. 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.
   Select the objects to be synchronized	Select one or more objects from the Available section and click the icon to move the objects to the Selected section. You can select tables or 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. By default, after an object is synchronized to the destination cluster, the name of the object remains unchanged. You can use the object name mapping feature to rename the objects that are synchronized to the destination cluster. For more information, see Rename an object to be synchronized.

9. In the lower-right corner of the page, click Next.
10. Select the initial synchronization types.
    

    Initial synchronization type	Description
    Initial schema synchronization	DTS synchronizes the schemas of the required objects to the destination PolarDB-O 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-O cluster to the destination PolarDB-O cluster. Notice During data synchronization, do not perform data definition language (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, DTS performs a precheck. The data synchronization task can be started only after it passes the precheck.
    • If the task fails to pass the precheck, you can click the 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.
12. Close the Precheck dialog box after the following message is displayed: The precheck is passed. Then, the data synchronization task starts.
13. Wait until the initial synchronization is complete and the data synchronization task is in the Synchronizing state.
    You can view the status of the data synchronization task on the Synchronization Tasks page.