Synchronize data from an ApsaraDB RDS for MySQL instance to a PolarDB for MySQL cluster

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%.
• During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination cluster. After initial full data synchronization, the tablespace of the destination cluster is larger than that of the source database.
• The source database must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination cluster may contain duplicate data records.

SQL operations that can be synchronized

Supported synchronization topologies

• One-way one-to-one synchronization
• One-way one-to-many synchronization
• One-way cascade synchronization
• One-way many-to-one synchronization

For more information, see Synchronization topologies.

Limits

• Incompatibility with triggers

  If you select a database as the object to be synchronized and the database contains a trigger that updates a table, data inconsistency may occur. For more information about how to solve this issue, see Configure a data synchronization task for a source database that contains a trigger.

• Limits on RENAME TABLE operations

  RENAME TABLE operations may cause data inconsistency between the source and destination databases. For example, if you select a table as the object and rename the table during data synchronization, the data of this table is not synchronized to the destination database. To prevent this situation, you can select the database to which this table belongs as the object when you configure the data synchronization task.

Procedure

1. Purchase a data synchronization instance. For more information, see Purchase a DTS instance.
   Note On the buy page, set Source Instance to MySQL, set 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.
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
   None	Synchronization Task Name	The task name that DTS automatically generates. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to use a unique task name.
   Source Instance Details	Instance Type	Select RDS Instance.
   	Instance Region	The source region that you selected on the buy page. The value of this parameter cannot be changed.
   	Database Account	The database account of the source ApsaraDB RDS for MySQL instance. Note The account must have the SELECT permission on the objects to synchronize and the REPLICATION CLIENT, REPLICATION SLAVE, and SHOW VIEW permissions. If the source ApsaraDB RDS for MySQL instance runs MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.
   	Database Password	The password of the database account.
   	Encryption	Select Non-encrypted or SSL-encrypted. If you want to select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data synchronization task. For more information, see Configure SSL encryption for an ApsaraDB RDS for MySQL instance. Notice The Encryption parameter is available only for regions in the Chinese mainland and the China (Hong Kong) region.
   Destination Instance Details	Instance Type	This parameter is set to PolarDB and cannot be changed.
   	Instance Region	The destination region that you selected on the buy page. The value of this parameter cannot be changed.
   	PolarDB Instance ID	The ID of the destination PolarDB for MySQL cluster.
   	Database Account	The database account of the source PolarDB cluster. Note The database account must have the ALL permission on the objects to synchronize.
   	Database Password	The password of the database account.

7. In the lower-right corner of the page, click Set Whitelist and Next.
   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 complete, we recommend that you remove the CIDR blocks of DTS servers from the whitelists or security groups.
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 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 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: 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.
   Rename Databases and Tables	You can use the object name mapping feature to rename the objects that are synchronized to the destination instance. For more information, see Object name mapping.
   Replicate Temporary Tables When DMS Performs DDL Operations	If you use Data Management (DMS) to perform online DDL operations on the source database, you can specify whether to synchronize temporary tables generated by online DDL operations. Yes: DTS synchronizes the data of temporary tables generated by online DDL operations. Note If online DDL operations generate a large amount of data, the data synchronization task may be delayed. No: DTS does not synchronize the data of temporary tables generated by online DDL operations. Only the original DDL data of the source database is synchronized. Note If you select No, the tables in the destination database may be locked.
   Retry Time for Failed Connections	By default, if DTS fails to connect to the source or destination database, DTS retries within the next 720 minutes (12 hours). You can specify the retry time based on your needs. 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 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.

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

    Note Initial synchronization includes initial schema synchronization and initial full data synchronization. If you select both Initial Schema Synchronization and Initial Full Data Synchronization, DTS synchronizes the schemas and historical data of the required objects before DTS synchronizes incremental data.
11. In the lower-right corner of the page, click 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 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.