Data Transmission Service (DTS) supports two-way data synchronization between two MySQL databases. This feature is suitable for 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. You can also follow the procedure to configure data synchronization tasks for self-managed MySQL databases.

Prerequisites

The source and destination ApsaraDB RDS for MySQL instances are created. For more information, see Create an ApsaraDB 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 loads of the database servers. If the database performance or specifications are unfavorable, 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%.
  • We recommend that you do not use gh-ost or pt-online-schema-change to perform DDL operations on objects during data synchronization. Otherwise, data synchronization may fail.
  • If you use only DTS to write data to the destination database, you can use Data Management (DMS) to perform online DDL operations during data synchronization. For more information, see Perform lock-free DDL operations.
    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.
  • If the source or destination database of a two-way data synchronization task resides in a region outside the Chinese mainland, data can be synchronized only within this region. Cross-region two-way synchronization is not supported. For example, if the source instance resides in the Japan (Tokyo) region, data can be synchronized only within the Japan (Tokyo) region and cannot be synchronized to the Germany (Frankfurt) region.
  • If the destination database is an ApsaraDB RDS for MySQL instance that runs MySQL 5.7 or 8.0, DTS creates the dtssyncwriter database account that has write permissions and is exclusive to DTS in the destination database.

Billing

Synchronization type Task configuration fee
Schema synchronization and full data synchronization Free of charge.
Incremental data synchronization Charged. For more information, see Billing overview.

Supported synchronization topologies

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 databases

The following table lists the types of 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 MySQL databases.

Source database Destination database
  • ApsaraDB RDS for MySQL instance
  • Self-managed database hosted on Elastic Compute Service (ECS)
  • Self-managed database connected over Express Connect, VPN Gateway, or Smart Access Gateway
  • Self-managed database connected over Database Gateway
  • Self-managed database connected over Cloud Enterprise Network (CEN)
  • ApsaraDB RDS for MySQL instance
  • Self-managed database hosted on Elastic Compute Service (ECS)
  • Self-managed database connected over Express Connect, VPN Gateway, or Smart Access Gateway
  • Self-managed database connected over Database Gateway
  • Self-managed database connected over Cloud Enterprise Network (CEN)

SQL operations that can be synchronized

Operation type SQL statement
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 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 specify 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 on 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 insert 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 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.

Limits

  • Incompatibility with triggers

    If you select a database as the object to be synchronized and the database contains a trigger that updates a table, data inconsistency may occur. For example, assume that 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 prevent 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 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 reverse direction is not supported. Only DML operations can be synchronized in the reverse 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 Destination Instance to MySQL and set Synchronization Topology to Two-way Synchronization.
  2. Go to the DTS console.
  3. In the left-side navigation pane, click Data Synchronization.
  4. In the upper part of the Data Synchronization Tasks page, select the region where the destination instance resides.
  5. Find the data synchronization instance and click ConfigureTask 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.
    Two-way data synchronization tasks
  6. Configure the source and destination instances. Configure the source and destination instances
    Section Parameter Description
    N/A Task Name The task name that DTS automatically generates. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to use a unique task name.
    Source Database Database Type Select MySQL.
    Instance Region The source region that you selected on the buy page. The value of this parameter cannot be changed.
    RDS Instance ID The ID of the source RDS instance.
    Database Account 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 parameter.
    Database Password The password of the database account.
    Encryption Specifies whether to encrypt the connection to the source instance. 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 ApsaraDB RDS for MySQL instance.
    Notice The Encryption parameter is available only within regions in the Chinese mainland and the China (Hong Kong) region.
    Destination Database Database Type Select MySQL.
    Instance Region The destination region that you selected on the buy page. The value of this parameter cannot be changed.
    RDS Instance ID The ID of the destination ApsaraDB RDS instance.
    Database Account 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 parameter.
    Database Password The password of the database account.
    Encryption Specifies whether to encrypt the connection to the source instance. 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 ApsaraDB RDS for MySQL instance.
    Notice The Encryption parameter is available only within regions in the Chinese mainland and the China (Hong Kong) region.
  7. In the lower-right corner of the page, click Test Connectivity and Proceed.
    • 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 complete, we recommend that you remove the CIDR blocks of DTS servers from the whitelists or security groups.
  8. Select the synchronization policy and the objects to be synchronized. Select the synchronization policy and the objects to be synchronized
    Section Parameter Description
    Set the synchronization policy Exclude DDL Operations
    • To exclude DDL operations, select Yes.
    • To include DDL operations, select No.
      Notice Limits on DDL synchronization directions: To ensure the stability of two-way data synchronization, you can synchronize DDL operations only in the forward direction.
    DML statements for synchronization 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 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 (Conflicting records in the destination instance are overwritten)

      If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten.

    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.
      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 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.
      • 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 N/A

    Select one or more objects (tables or a database) from the Available section and click the Rightwards arrow icon to add 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 database, 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.
  9. In the lower-right corner of the page, click Next.
  10. Select the initial synchronization types. Advanced Settings

    During initial synchronization, DTS synchronizes the schema and data of required objects from the source instance to the destination instance. The schema 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 reverse 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, you can click the Info icon icon next to each failed item to view details.
      • After you troubleshoot the issues based on the causes, you can 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 The precheck is passed. message is displayed in the Precheck dialog box. Then, the data synchronization task starts.
  13. 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 Data Synchronization Tasks page.

  14. Find the second data synchronization task and click Configure Task in the Actions column. Configure the task by performing Steps 5 to 12 of this topic. Configure data synchronization in the reverse direction
  15. After the second data synchronization task is configured, wait until both tasks are in the Synchronizing state. The two-way data synchronization tasks are configured. State of two-way data synchronization tasks