Configure two-way data synchronization between Apsara 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 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%.
• 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.
• 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.
• Incompatibility with triggers

  If the object to be synchronized is a database and the database contains a trigger that updates a synchronized 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 a table to be synchronized is renamed during data synchronization, the data of this table is not synchronized to the destination database. To avoid 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 a two-way synchronization channel, you can synchronize DDL updates of 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 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 uniqueness 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 instance.
• 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 have specified.

Procedure

1. Purchase a data synchronization instance. For more information, see Purchase procedure.
   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 one 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 configure a channel 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 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 region of the source instance. The region is the same as the source region that you selected when you purchased the data synchronization instance. 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 opposite 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/write permissions on the required objects.
      	Database Password	Enter the password of the source database account.
      Destination Instance Details	Instance Type	The value of this parameter is set to PolarDB Instance and cannot be changed.
      	Instance Region	The region of the destination instance. The region is the same as the destination region that you selected when you purchased the data synchronization instance. 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 opposite 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/write permissions on the required objects.
      	Database Password	Enter the password of the destination 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. Configure the synchronization policy and objects.
      

      Section	Parameter	Description
      Synchronization policy	Exclude DDL Statements	To exclude DDL operations, select Yes. To include DDL operations, select No. Notice After you select No, the table does not support synchronizing DDL operations in the opposite direction.
      	DML Statements for Synchronization	Select the type of DML operations to be synchronized. 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 in case of a synchronization conflict. 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 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 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 change the names of the tables to be synchronized by using the object name mapping feature. You can use this feature if the source and destination databases contain identical table names and 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. DTS does not synchronize data records that have the same primary keys as data records in the destination database during initial data synchronization. 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.
      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. If you select a table as the object to be synchronized, only schema changes in this table 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 names of the objects that are synchronized to the destination database by using the object name mapping feature. For more information about how to use this feature, see Specify the name of an object in the destination instance.

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

      Notice

      • During initial synchronization, DTS synchronizes the schemas and data of the required 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 required objects have been synchronized from the source cluster to the destination cluster, the data synchronization task in the opposite 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, 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.
   8. Close the Precheck dialog box after the following message is displayed: The precheck is passed. Then, the first data synchronization task starts.
6. Wait until the initial synchronization is complete and the first 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 opposite 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.