Data Transmission Service:Configure two-way synchronization between MySQL databases

Prerequisites

The source and destination ApsaraDB RDS for MySQL instances are created. For more information, see Create an ApsaraDB RDS for MySQL instance.

Usage notes

• 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 or specifications are unfavorable, 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 database servers is less than 30%.

• We recommend that you do not use gh-ost or pt-online-schema-change to perform DDL operations on objects during data synchronization. Otherwise, data synchronization may fail.

• If you use only DTS to write data to the destination database, you can use Data Management (DMS) to perform online DDL operations during data synchronization. For more information, see Perform lock-free DDL operations.

• If the destination database is an ApsaraDB RDS for MySQL instance that runs MySQL 5.7 or 8.0, DTS creates the dtssyncwriter database account that has write permissions and is exclusive to DTS in the destination database.

Billing

Supported synchronization topologies

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

Supported databases

The following table lists the types of MySQL databases that are supported for two-way data synchronization. This topic uses ApsaraDB RDS for MySQL instances as the data sources. You can also follow the procedure to configure two-way data synchronization for other types of MySQL databases.

SQL operations that can be synchronized

Supported 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 on 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 insert 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.

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 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 prevent this situation, before you synchronize data in Table B from the source instance, delete the trigger that is synchronized to the destination instance. For more information, see Configure a data synchronization or migration 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.

• Limits on DDL synchronization direction

  To ensure the stability of two-way data synchronization, you can synchronize DDL operations only in one direction. If DDL synchronization in a direction is configured, DDL synchronization in the reverse direction is not supported. Only DML operations can be synchronized in the reverse direction.

Procedure

1. Purchase an instance for two-way data synchronization. For more information, see Purchase a data synchronization instance.

   Important

   On the buy page, set both the Source Instance and Destination Instance parameters to MySQL and the Synchronization Topology parameter to Two-way Synchronization.

2. Log on to the DTS console.

   Note

   • If you are navigated to the DMS console from the DTS console, you can move the pointer over the icon in the lower-right corner and click the icon to return to the DTS console.

   • If the new version of the DTS console appears after the logon, you can click the icon in the lower-right corner to return to the previous version.

3. In the left-side navigation pane, click Data Synchronization.

4. In the upper part of the Data Synchronization Tasks page, select the region in which the destination instance resides.

5. Find the data synchronization instance and click Configure Task in the Actions column of the first data synchronization task.

   Important

   A two-way data synchronization instance contains two data synchronization tasks. You must configure parameters for each task.

   

6. Configure the source and destination instances.

   

   Setting	Parameter	Description
   N/A	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 Database	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.
   	Instance ID	The ID of the source ApsaraDB RDS instance.
   	Database Account	The database account of the source ApsaraDB RDS instance. Important If the database engine of the source ApsaraDB RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the Database Account or Database Password parameter.
   	Database Password	The password of the database account.
   	Encryption	Specifies whether to encrypt the connection to the source instance. Select Non-encrypted or SSL-encrypted. If you select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS instance before you configure the data synchronization task. For more information, see Use a cloud certificate to enable SSL encryption. Important The Encryption parameter is available only within regions in the Chinese mainland and the China (Hong Kong) region.
   Destination Database	Instance Type	Select RDS Instance.
   	Instance Region	The destination region that you selected on the buy page. The value of this parameter cannot be changed.
   	Instance ID	The ID of the destination ApsaraDB RDS instance.
   	Database Account	The database account of the destination ApsaraDB RDS instance. Important If the database engine of the destination ApsaraDB RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the Database Account or Database Password parameter.
   	Database Password	The password of the database account.
   	Encryption	Specifies whether to encrypt the connection to the destination instance. Select Non-encrypted or SSL-encrypted. If you select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS instance before you configure the data synchronization task. For more information, see Use a cloud certificate to enable SSL encryption. Important The Encryption parameter is available only within regions in the Chinese mainland and the China (Hong Kong) region.

7. In the lower-right corner of the page, click Test Connectivity and Proceed.

   • 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. Configure the synchronization policy and the objects to be synchronized.

   Setting	Parameter	Description
   Set the synchronization policy	Exclude DDL Operations	Yes: does not synchronize DDL operations. No: synchronizes DDL operations. Important Limits on DDL synchronization directions: To ensure the stability of two-way data synchronization, you can synchronize DDL operations only in the forward direction.
   	DML statements for synchronization	The types of DML operations that you want to synchronize. By default, the INSERT, UPDATE, and DELETE operations are selected. You can select the DML operation types based on your business requirements.
   	Conflict Resolution Policy	The resolution policy for synchronization conflicts. By default, TaskFailed is selected. You can select a conflict resolution policy based on your business requirements. TaskFailed (When a conflict occurs, an error is reported and the task is terminated.) The default conflict resolution policy. If a conflict occurs during data synchronization, the synchronization task reports an error and exits. The task enters a failed state and you must manually resolve the conflict. Ignore (When 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 conflicting records in the destination database are used. Overwrite (Conflicting records in the destination instance are overwritten.) If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten.
   Select the objects to be synchronized	N/A	Select one or more objects (tables or a database) from the Source Objects section and click the icon to add the objects to the Selected Objects section. Important 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 instance, 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 instance. For more information, see Rename an object to be synchronized.
   Rename Databases and Tables	N/A	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	N/A	If you use 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	N/A	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.

    During initial synchronization, DTS synchronizes the schemas and data of the selected 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.

    Important

    If tables to be synchronized in one direction are also included in the objects to be synchronized in the reverse direction, DTS does not synchronize these tables during initial synchronization.

11. In the lower-right corner of the page, click Precheck.

    Important

    • 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, you can click the icon next to each failed item to view details.

      • After you troubleshoot the issues based on the causes, you can 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 The precheck is passed. message is displayed in the Precheck dialog box. Then, the data synchronization task starts.

13. Wait until 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.

14. Find the second data synchronization task and click Configure Task in the Actions column. Configure the task by performing Steps 5 to 12 of this topic.

15. After the second data synchronization task is configured, wait until both tasks are in the Synchronizing state. The two-way data synchronization tasks are configured.