This topic describes how to synchronize data from a PolarDB for MySQL cluster to an ApsaraDB RDS for MySQL instance by using Data Transmission Service (DTS).

Prerequisites

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 database.
  • The source database must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination cluster may contain duplicate data records.

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

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

For more information, see Synchronization topologies.

Limits

  • Incompatibility with triggers

    If you select a database as the object 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 during synchronization. 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 prevent this situation, you can select the entire database where Table A is located as the object when you configure the data synchronization task.

Procedure

  1. Purchase a data synchronization instance. For more information, see Purchase a DTS instance.
    Note On the buy page, set Source Instance to PolarDB, set Target Instance to MySQL, and set Synchronization Topology to One-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.
    Select a region
  5. Find the data synchronization instance and click Configure Synchronization Channel in the Actions column.
  6. Configure the source and destination instances.
    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 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.
    Database Account Enter the database account of the source PolarDB cluster.
    Database Password Enter the password of the database account.
    Destination Instance Details Instance Type Select RDS Instance.
    Instance Region The destination region that you selected on the buy page. You cannot change the value of this parameter.
    Database Account Enter the database account of the destination RDS instance.
    Database Password Enter the password of the database account.
    Note If the database engine of the destination RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to specify the database account or database password.
    Encryption Select Non-encrypted or SSL-encrypted. If you want to select SSL-encrypted, you must enable SSL encryption for the RDS instance before you configure the data synchronization task. For more information, see Configure SSL encryption on an ApsaraDB RDS for MySQL instance.
    Notice The Encryption parameter is available only for regions in mainland China and the China (Hong Kong) region.
  7. In the lower-right corner of the page, click Set Whitelist and Next.
    Note DTS adds the CIDR blocks of DTS servers to the whitelists of the source PolarDB cluster and the destination RDS instance. This ensures that DTS servers can connect to the source PolarDB cluster and the destination RDS instance.
  8. Select the processing mode of conflicting tables, and the objects to be synchronized.
    Select the processing mode and the objects to be synchronized
    Setting Description
    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 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 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 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

    Select one or more objects from the Available section and click the Rightwards arrow icon to move the objects to the Selected section.

    You can select tables or 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.
    • By default, after an object is synchronized to the destination cluster, 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 cluster. For more information, see Specify the name of an object in the destination instance.
  9. In the lower-right corner of the page, click Next.
  10. Select the initial synchronization types.
    Advanced Settings
    Note Initial synchronization includes initial schema synchronization and initial full data synchronization. If you select both Initial Schema Synchronization and Initial Full Data Synchronization, DTS synchronizes the schemas and historical data of the required objects before DTS synchronizes incremental data.
  11. 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 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.
  12. Close the Precheck dialog box after the following message is displayed: The precheck is passed. Then, the data synchronization task starts.
  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 Synchronization Tasks page. View the status of a data synchronization task