Configure two-way data synchronization between PolarDB for MySQL 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%.
• 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 cluster.
• The source cluster must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination cluster may contain duplicate data records.

Limits

• DTS supports two-way data synchronization only between two PolarDB clusters. DTS does not support two-way data synchronization between multiple PolarDB clusters.
• 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.

• Limits on DDL synchronization direction

  To ensure the stability of two-way data synchronization, you can synchronize the DDL operations on a single table only in one direction. If DDL synchronization in a direction is configured, DDL synchronization in the opposite direction is not supported. Only data manipulation language (DML) operations can be synchronized in the opposite direction.

SQL operations that can be synchronized

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 PolarDB cluster. If data records are updated on both two PolarDB clusters, a synchronization conflict occurs.

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 UNIQUE constraint cannot be synchronized. For example, if data records with the same primary key are inserted into 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 already exists in the other node.

• Inconsistent records caused by UPDATE operations
  • If the records to be updated do not exist in the destination cluster, DTS converts the UPDATE operation into an INSERT operation. However, uniqueness conflicts may occur.
  • The primary keys or unique keys of the records to be inserted may conflict with those of existing records in the destination cluster.
• Non-existent records to be deleted

  The records to be deleted do not exist in the destination cluster. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

Procedure

1. Purchase a data synchronization instance. For more information, see Purchase a DTS instance.
   Notice On the buy page, set both Source Instance and Target Instance to PolarDB, and set Synchronization Topology to Two-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. Configure the data synchronization task in the forward direction.
   1. Find the data synchronization instance, and click Configure Synchronization Channel in the Actions column of the first data synchronization task.
      Notice A two-way data synchronization instance contains two data synchronization tasks. You must set parameters for each task. When you configure the second data synchronization task, find the task and click Configure Synchronization Channel in the Actions column.

      
   2. Configure the source and destination clusters.
      

      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	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 cluster. Notice When you configure the data synchronization task in the reverse direction, select the ID of the destination PolarDB cluster.
      	Database Account	Enter the database account of the source PolarDB cluster. The account must have the read and write permissions on the objects to be synchronized.
      	Database Password	Enter the password of the database account.
      Destination Instance Details	Instance Type	This parameter is set to PolarDB Instance 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 cluster. Notice When you configure the data synchronization task in the reverse direction, select the ID of the source PolarDB cluster.
      	Database Account	Enter the database account of the destination PolarDB cluster. The account must have the read and write permissions on the objects to be synchronized.
      	Database Password	Enter the password of the database account.

   3. In the lower-right corner of the page, click Set Whitelist and Next.
      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.
   4. Select the synchronization policy and the objects to be synchronized.
      

      Setting	Parameter	Description
      Select the synchronization policy	Exclude DDL Statements	To exclude DDL operations, select Yes. To include DDL operations, select No. Notice To ensure the stability of two-way data synchronization, you can synchronize DDL operations only in one direction.
      	DML Statements for Synchronization	Select 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	Select the resolution policy for synchronization conflicts. By default, TaskFailed is selected. You can select a conflict resolution policy based on your business requirements. TaskFailed The default conflict resolution policy. If a conflict occurs during data synchronization, the synchronization task reports an error and exits the process. The task enters a failed state and you must manually resolve the conflict. Ignore If a conflict occurs during data synchronization, the synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used. Overwrite If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten.
      	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. Notice You can use the object name mapping feature to change the names of 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 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. 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	N/A	Select objects (tables or a database) from the Available section and click the icon to move the objects to the Selected section. Notice 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 change the names of the objects that are synchronized to the destination instance. For more information, see Specify the name of an object in the destination instance.
      	N/A
      	N/A
      	N/A

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

      Notice

      • During initial synchronization, DTS synchronizes the schemas and data of the selected objects from the source cluster to the destination cluster. 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.
      • If all the selected objects have been synchronized from the source cluster to the destination cluster, the data synchronization task in the reverse direction synchronizes only incremental data.
   7. 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.
   8. Close the Precheck dialog box after the following message is displayed: The precheck is passed. Then, the data synchronization task in the forward direction starts.
6. 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.
7. Configure the data synchronization task in the reverse direction.
   1. Find the second data synchronization task, and click Configure Synchronization Channel in the Actions column.
      
   2. Repeat substeps ii to viii that are described in Step 5.