Synchronize data from an ApsaraDB RDS for MySQL instance to a user-created MySQL database connected over Express Connect, VPN Gateway, or Smart Access Gateway

Notes

• 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%.
• You cannot synchronize data from an ApsaraDB RDS for MySQL instance that resides in Zone A of the China (Hong Kong) region.
• The source ApsaraDB RDS for MySQL instance must have an internal endpoint.
• During initial full data synchronization, concurrent INSERT operations cause segments in the tables of the destination instance. After initial full data synchronization, the tablespace of the destination instance is larger than that of the source instance.

Limits

• If you have selected one or more tables (not a database) for synchronization, do not use gh-ost or pt-online-schema-change to modify the tables during data synchronization. Otherwise, data synchronization may fail.
  Notice To avoid synchronization failure, you can use Data Management (DMS) to perform online DDL schema changes during data synchronization. For more information, see Change the table schema without locking.
• If the source database does not have primary keys or UNIQUE constraints, and fields are not required to be unique, duplicate data may exist in the destination database.

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
• Two-way one-to-one synchronization

For more information about synchronization topologies, see Synchronization topologies.

SQL operations that can be synchronized

Limits

• Incompatibility of triggers

  If the object you want to synchronize is a database and the database contains a trigger that updates the synchronized table, the synchronized data may be inconsistent. For example, the source database contains Table A and Table B. If a data record is inserted into Table A, a trigger inserts a data record into Table B. In this case, after an INSERT operation is performed on Table A in the source instance, the data in Table B becomes inconsistent between the source and destination instances.

  To avoid this situation, you must delete the trigger that is synchronized to the destination instance and select Table B as the object to be synchronized. For more information, see Configure synchronization when triggers exist.

• Limits on RENAME TABLE operations

  RENAME TABLE operations may cause data inconsistency between the source and destination databases. For example, if only Table A needs to be synchronized and it is renamed Table B, Table B cannot be synchronized to the destination database. To avoid this situation, you can select the database to which Table A and Table B belong as the object when configuring the data synchronization task.

Procedure

1. Create a data synchronization task. For more information, see Purchase procedure.
   Note Select MySQL for both the source instance and the destination instance. Select One-Way Synchronization as the synchronization topology.
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 use an informative name for easy identification. You do not need to use a unique task name.
   Source Instance Details	Instance Type	Select RDS Instance.
   	Instance Region	The region of the source instance that you select when purchasing the data synchronization instance. You cannot change the value of this parameter.
   	Instance ID	Select the ID of the source RDS instance.
   	Database Account	Enter the database account of the source RDS instance. Note If the database type of the source RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.
   	Database Password	Enter the password for the database account.
   	Encryption	Select Non-encrypted or SSL-encrypted. If you want to select SSL-encrypted, you must enable SSL encryption for the RDS instance before configuring the data synchronization task. For more information, see Configure SSL encryption for an RDS for MySQL instance. Note The Encryption parameter is available only in mainland China and Hong Kong(China).
   Destination Instance Details	Instance Type	Select User-Created Database Connected over Express Connect, VPN Gateway, or Smart Access Gateway.
   	Instance Region	The region of the destination instance that you select when purchasing the data synchronization instance. You cannot change the value of this parameter.
   	Peer VPC	Select the ID of the VPC that is connected to the user-created MySQL database.
   	Database Type	This parameter is set to MySQL and cannot be modified.
   	IP Address	Enter the IP address of the user-created MySQL database.
   	Port Number	Enter the service port number of the user-created MySQL database. The default port number is 3306.
   	Database Account	Enter the account for the user-created MySQL database. Note The account must have the REPLICATION SLAVE permission, the REPLICATION CLIENT permission, the SHOW VIEW permission, and the permission to perform SELECT operations on the required objects.
   	Database Password	Enter the password for the database account.

7. In the lower-right corner of the page, click Set Whitelist and Next.
8. Configure the synchronization policy and objects.
   

   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. If the source and destination databases have the same schema, and the primary key of a record in the destination database is the same as that in the source database, the record remains unchanged during initial data synchronization. However, the record is overwritten 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 to be synchronized	Select objects from the Available section and click the 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 instance by using the object name mapping feature provided by DTS. 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.
    

    Note

    • During initial synchronization, DTS synchronizes the schemas and data of the required objects from the source instance to the destination instance. The schemas and data are the basis for subsequent incremental synchronization.
    • Initial synchronization includes initial schema synchronization and initial full data synchronization. You must select both Initial Schema Synchronization and Initial Full Data Synchronization in most cases.
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 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.

    You can view the status of the data synchronization task on the Data Synchronization page.