Data Transmission Service (DTS) supports two-way data synchronization between PolarDB for MySQL clusters. This feature is suitable for scenarios such as active geo-redundancy and geo-disaster recovery. This topic describes how to configure two-way data synchronization between PolarDB for MySQL clusters.

Prerequisites

  • The source and destination PolarDB for MySQL clusters are created. For more information, see Create a PolarDB for MySQL cluster.
  • The binary logging feature is enabled for the source and destination PolarDB for MySQL clusters. For more information, see Enable binary logging.

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 the initial full data synchronization is completed, 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.
  • If the source or destination instance is located in a region outside the Chinese mainland, two-way data synchronization is supported only between instances located within the same region. For example, two-way data synchronization is supported between instances within the Japan (Tokyo) region. Two-way data synchronization between an instance in the Japan (Tokyo) region and another instance in the Germany (Frankfurt) region is not supported.

Limits

  • DTS supports two-way data synchronization between two PolarDB clusters, but not between multiple PolarDB clusters.
  • Incompatibility with triggers

    If you select a database as the object to synchronize 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 DDL operations 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

Operation type SQL statements
DML INSERT, UPDATE, DELETE, and REPLACE
DDL
  • ALTER TABLE and ALTER VIEW
  • CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, and CREATE VIEW
  • DROP INDEX and DROP TABLE
  • RENAME TABLE
  • TRUNCATE TABLE

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 a single 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 on the other node.

  • Inconsistent records caused by UPDATE operations
    • If the records to update 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 insert may conflict with those of existing records in the destination instance.
  • Non-existent records to be deleted

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

Notice
  • During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS does not ensure that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
  • DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.

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 Destination 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 Task 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 Task in the Actions column.
      Two-way data synchronization tasks
    2. Configure the source and destination clusters.
      Configure the source and destination instances
      Section Parameter Description
      None Synchronization Task Name The task name that DTS generates. We recommend that you specify a descriptive name that makes it easy to identify. You do not need to use a unique task name.
      Source Instance Details Instance Type The instance type of the source instance. The value of 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 The ID of the source PolarDB for MySQL cluster.
      Notice When you configure the data synchronization task in the reverse direction, select the ID of the destination PolarDB cluster of the forward synchronization.
      Database Account The database account of the source PolarDB cluster. The account must have the read and write permissions on the objects to synchronize.
      Database Password The password of the database account.
      Destination Instance Details Instance Type The instance type of the destination instance. The value of 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 The ID of the destination PolarDB for MySQL cluster.
      Notice When you configure the data synchronization task in the reverse direction, select the ID of the source PolarDB cluster of the forward synchronization.
      Database Account The database account of the destination PolarDB cluster. The account must have the read and write permissions on the objects to synchronize.
      Database Password The password of the database account.
    3. 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 completed, we recommend that you remove the CIDR blocks of DTS servers from the whitelists or security groups.
    4. Select the synchronization policy and the objects to synchronize.
      Select the synchronization policy and the objects to synchronize
      Setting Parameter Description
      Select the synchronization policy Exclude DDL Statements
      • To exclude DDL operations, select Yes.
      • To include DDL operations, select No.
        Notice Limits on DDL synchronization direction: To ensure the stability of two-way data synchronization, you can synchronize DDL operations only in the forward 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.

      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 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 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 specific columns are synchronized or the data synchronization task fails.
      Select the objects to be synchronized N/A

      Select one or more objects (tables or a database) from the Available section and click the Rightwards arrow 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 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 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 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.
    5. In the lower-right corner of the page, click Next.
    6. Select the initial synchronization types.
      Two-way data synchronization between PolarDB for MySQL clusters: Advanced Settings
      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. 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 Info icon 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: Precheck Passed. Then, the data synchronization task in the forward direction starts.
  6. Wait until initial synchronization is completed and the data synchronization task is in the Synchronizing state.
    You can view the state 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 Task in the Actions column.
      Configure the data synchronization task in the reverse direction
    2. Repeat substeps ii to viii that are described in Step 5.

Result

After a period of time, both data synchronization tasks are in the Synchronizing state. Two-way data synchronization between PolarDB clusters