Data Transmission Service (DTS) supports real-time two-way data synchronization between two MySQL databases. This feature is applicable to scenarios such as active geo-redundancy (unit-based) and geo-disaster recovery. This topic describes how to configure two-way data synchronization between ApsaraDB RDS for MySQL instances.

Prerequisites

The source and destination ApsaraDB RDS for MySQL instances are created. For more information, see Create an RDS for MySQL instance.

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 you synchronize data, 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%.

Supported synchronization topology

DTS supports two-way data synchronization only between two MySQL databases. DTS does not support two-way data synchronization between multiple MySQL databases.

Two-way data synchronization

Supported data sources

The following table describes the MySQL databases that are supported by two-way data synchronization. This topic uses ApsaraDB RDS for MySQL instances as the data sources. You can also follow the procedure to configure two-way data synchronization for other types of databases.

Source database Destination database
  • ApsaraDB RDS for MySQL instance
  • User-created database hosted on ECS
  • User-created database connected over Express Connect, VPN Gateway, or Smart Access Gateway
  • User-created database connected over a database gateway
  • Self built database accessed through Cloud Enterprise Network(CEN)

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 one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you have specified for the data synchronization task.

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 a record with the same primary key value is inserted into the 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 value already exists in the other node.

  • Inconsistent records caused by UPDATE operations
    • If the records to be updated do not exist in the destination instance, 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 instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you have specified.

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 guarantee that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that data 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 a two-way data synchronization task.

Limits

  • 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 the synchronized table, the synchronized data may be inconsistent. For example, the source database contains Table A and Table B. If a data record is inserted into Table A, a trigger inserts a data record into Table B. In this case, after an INSERT operation is performed on Table A in the source instance, the data in Table B becomes inconsistent between the source and destination instances.

    To avoid this situation, before you synchronize data in Table B from the source instance, delete the trigger that is synchronized to the destination instance. For more information, see Configure synchronization when triggers exist.

  • 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.

Procedure

  1. Purchase an instance for two-way data synchronization. For more information, see Purchase a data synchronization instance.
    Notice On the buy page, set both Source Instance and Target Instance to MySQL 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.Select a region
  5. 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.
    Two-way data synchronization task
  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 Select RDS Instance.
    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.
    Instance ID Select the ID of the source RDS instance.
    Database Account Enter the database account of the source RDS instance.
    Notice If the database engine of the source RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.
    Database Password Enter the password for the source database account.
    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 for an RDS for MySQL instance.
    Notice The Encryption parameter is available only for regions in mainland China and the Hong Kong (China) region.
    Destination Instance Details Instance Type Select RDS Instance.
    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.
    Instance ID Select the ID of the destination RDS instance.
    Database Account Enter the database account of the destination RDS instance.
    Notice If the database engine of the destination RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.
    Database Password Enter the password for the destination database account.
    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 for an RDS for MySQL instance.
    Notice The Encryption parameter is available only for regions in mainland China and the Hong Kong (China) region.
  7. In the lower-right corner of the page, click Set Whitelist and Next.
  8. Configure synchronization policy and objects.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 Right 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. 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.
  9. In the lower-right corner of the page, click Next.
  10. Configure initial synchronization.Advanced settings

    During initial synchronization, DTS synchronizes the schemas and data of the required objects from the source instance to the destination instance. 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.

    Notice If tables to be synchronized in one direction are also included in the objects to be synchronized in the opposite direction, DTS does not synchronize these tables during initial synchronization.
  11. In the lower-right corner of the page, click Precheck.
    Notice
    • 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 Info icon icon next to each failed item to view details. Troubleshoot the issues based on the causes and run the 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.

  14. Find the second data synchronization task and click Configure Synchronization Channel in the Actions column. Configure data synchronization by following step 5 to step 12.Configure data synchronization in the opposite direction
  15. After the second data synchronization task is configured, wait until both tasks are in the Synchronizing state. The two-way data synchronization task is configured.Status of the two-way data synchronization task