This topic describes how to synchronize data between PolarDB for MySQL clusters by using Data Transmission Service (DTS).

Prerequisites

  • The source and destination PolarDB for MySQL clusters are created. For more information, see Purchase a pay-as-you-go cluster and Purchase a subscription cluster.
  • The available storage space of the destination PolarDB for MySQL cluster is larger than the total size of the data in the source PolarDB for MySQL cluster.

Limits

Category Description
Limits on the source database
  • The tables to be synchronized must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records.
  • If you select tables as the objects to be synchronized and you need to edit tables (such as rename tables or columns), up to 1,000 tables can be synchronized in a single data synchronization task. If you run a task to synchronize more than 1,000 tables, a request error occurs. In this case, we recommend that you split the tables to be synchronized, configure multiple tasks to synchronize the tables, or call DTS API operations to configure tasks.
  • The following requirements for binary logs must be met:
    • The binary logging feature must be enabled. The value of the binlog_format parameter must be set to row. The value of the binlog_row_image parameter must be set to full. Otherwise, error messages are returned during precheck and the data synchronization task cannot be started.
    • Binary logs are retained for at least 7 days during full data synchronization. You can wait until full data synchronization is complete, and then clear the binary logs generated in the source database after the DTS task is run.
      Note To ensure data security, DTS stores only 50 GB of binary logs or the binary logs for the last 24 hours. If the limit is exceeded, DTS automatically clears the cached logs.
      Warning If you clear the binary logs of the source database during full data synchronization, the data synchronization task may fail. For example, full data synchronization takes more than 24 hours due to the large data volume in the source database and abnormal writing in the destination database. In this case, if the binary logs of the source database are cleared during full data synchronization, DTS cannot obtain the binary logs generated 24 hours ago. Therefore, the data synchronization task may fail.
  • During data synchronization, do not perform DDL operations to add comments, for example, ALTER TABLE table_name COMMENT='Table comment';. Otherwise, the data synchronization task fails.
Other limits
  • Read-only nodes of the source PolarDB for MySQL cluster cannot be synchronized.
  • 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. During full data synchronization, DTS uses read and write resources of the source and destination databases. This may increase the loads of the database servers.
  • During full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data synchronization is complete, the tablespace of the destination database is larger than that of the source database.
  • We recommend that you do not use gh-ost or pt-online-schema-change to perform DDL operations on source tables during data synchronization. Otherwise, data synchronization may fail.
  • If you use only DTS to write data to the destination database, you can use DMS to perform online DDL operations on source tables during data synchronization. For more information, see Change schemas without locking tables.
    Warning If you use tools other than DTS to write data to the destination database, we recommend that you do not use DMS to perform online DDL operations. Otherwise, data loss may occur in the destination database.
Special cases If you want to configure two-way data synchronization between PolarDB for MySQL clusters, take note of the following limits:
Note You cannot create two-way data synchronization tasks in the new DTS console. To create two-way data synchronization tasks, you can use the previous version of the DTS console.
  • DTS supports two-way data synchronization only between two PolarDB for MySQL clusters. DTS does not support two-way data synchronization between multiple PolarDB for MySQL clusters.
  • Limits on DDL synchronization direction: To ensure data consistency and the stability of two-way data synchronization, you can synchronize DDL operations only in the forward direction.
  • When DTS runs a two-way data synchronization task, DTS creates a database named dts in the destination database to prevent circular synchronization. When the task is running, do not modify the dts database.

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.

SQL operations that can be synchronized

Operation type SQL statements
DML INSERT, UPDATE, and DELETE
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

Procedure

  1. Log on to the DTS console.
  2. In the left-side navigation pane, click Data Synchronization.
  3. At the top of the Synchronization Tasks page, select the region where the destination instance resides.
    Select a region
  4. In the upper-right corner of the page, click New Console.
  5. On the Create Task page, configure the source and destination databases.
    Warning After you select the source and destination instances, we recommend that you read the limits displayed at the top of the page. This ensures that you can create and run the data synchronization task.
    Configure the source and destination databases
    Section Parameter Description
    N/A 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 Database Database Type Select PolarDB for MySQL.
    Access Method Select Alibaba Cloud Instance.
    Instance Region Select the region where the source PolarDB for MySQL cluster resides.
    PolarDB Cluster ID Select the ID of the source PolarDB for MySQL cluster.
    Note The source and destination PolarDB for MySQL clusters can be the same or different. You can use DTS to synchronize data between two PolarDB for MySQL clusters or within a PolarDB for MySQL cluster.
    Database Account Enter the database account of the source PolarDB for MySQL cluster. The account must have the read permissions on the objects to be synchronized.
    Database Password

    Enter the password of the database account.

    Destination Database Database Type Select PolarDB for MySQL.
    Access Method Select Alibaba Cloud Instance.
    Instance Region Select the region where the destination PolarDB for MySQL cluster resides.
    PolarDB Cluster ID Select the ID of the destination PolarDB for MySQL cluster.
    Database Account Enter the database account of the destination PolarDB for MySQL cluster. The account must have the read and write permissions.
    Database Password

    Enter the password of the database account.

  6. In the lower part of the page, click Test Connectivity and Proceed.
  7. Select objects for the task and configure advanced settings.
    • Basic SettingsBasic Settings
      Parameter Description
      Task Stages

      By default, you must select both Schema Synchronization and Full Data Synchronization. After the precheck, DTS synchronizes the historical data of the selected objects from the source instance to the destination cluster. The historical data is the basis for subsequent incremental synchronization.

      Processing Mode of Conflicting Tables
      • Precheck and Report Errors: 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.
        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 Rename an object to be synchronized.
      • Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.
        Warning If you select Ignore Errors and Proceed, data consistency is not guaranteed and your business may be exposed to potential risks.
        • If the source and destination databases have the same schema, DTS does not synchronize data records that have the same primary keys as data records in the destination database.
        • If the source and destination databases have different schemas, data may fail to be initialized. In this case, only some columns are synchronized or the data synchronization task fails.
      Select Objects

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

      Note You can select columns, tables, or databases as the objects to be synchronized.
      Rename Databases and Tables
      • To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see Rename an object to be synchronized.
      • To rename multiple objects at a time, click in the upper-right corner of the Selected Objects section. For more information, see Rename an object to be synchronized.
      Filter data

      You can specify WHERE conditions to filter data. For more information, see Use SQL conditions to filter data.

      Select the SQL operations to be synchronized In the Selected Objects section, right-click an object. In the dialog box that appears, select the DML and DDL operations that you want to synchronize. For more information, see SQL operations that can be synchronized.
    • Advanced SettingsAdvanced Settings
      Parameter Description
      Set Alerts
      Specify whether to set alerts for the data synchronization task. If the task fails or the synchronization latency exceeds the threshold, the alert contacts will receive notifications.
      • Select No if you do not want to set alerts.
      • Select Yes to set alerts. In this case, you must also set the alert threshold and alert contacts.
      Replicate Temporary Tables When DMS Performs DDL Operations
      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
      Specify the retry time for failed connections. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the retry time to more than 30 minutes. 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
      • If multiple DTS instances have the same source or destination database, the lowest value takes effect. For example, the retry time is set to 30 minutes for Instance A and 60 minutes for Instance B, DTS retries failed connections at an interval of 30 minutes.
      • 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.
  8. Click Next: Save Task Settings and Precheck in the lower part of the page.
    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.
  9. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  10. On the Purchase Instance page, specify the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.
    Section Parameter Description
    Parameters Billing Method
    • Subscription: You can pay for your subscription when you create an instance. We recommend that you select the subscription billing method for long-term use because it is more cost-effective than the pay-as-you-go billing method. You can save more costs with longer subscription periods.
    • Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. We recommend that you select the pay-as-you-go billing method for short-term use. If you no longer require your pay-as-you-go instance, you can release it to reduce costs.
    Instance Class DTS provides several instance classes that have different performance in synchronization speed. You can select an instance class based on your business scenario. For more information, see Specifications of data synchronization channels.
    Subscription Length If you select the subscription billing method, set the subscription length and the number of instances that you want to create. The subscription length can be one to nine months or one to three years
    Note This parameter is available only if you select the subscription billing method.
  11. Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
  12. Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.