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Data synchronization task

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The Data Synchronization feature helps you synchronize data between data sources in real time. It is suitable for various business scenarios, such as active geo-redundancy, geo-disaster recovery, on-premises data disaster recovery, cross-border data synchronization, cloud BI, and real-time data warehousing. This topic uses an example of synchronizing data from ApsaraDB RDS for MySQL to ApsaraDB RDS for MySQL to describe how to configure a data synchronization task in a DTS dedicated cluster.

Prerequisites

Usage notes

Note

  • During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.

  • During full data synchronization and incremental data synchronization, DTS temporarily disables constraint checks and foreign key cascade operations at the session level. Data inconsistency may occur if cascade update or delete operations are performed on the source database while the task is running.

Type

Description

Source database limits

  • The tables to synchronize must have a primary key or a unique constraint, and the fields must be unique. Otherwise, duplicate data may appear in the destination database.

  • If you synchronize at the table level and need to edit mappings (such as column name mapping), each synchronization task supports up to 1,000 tables. If you exceed this limit, the task fails with an error. To fix this, split the tables across multiple tasks or configure a full-database synchronization task.

  • Binary logs:

    • ApsaraDB RDS for MySQL enables binary logging by default. Ensure that the binlog_row_image parameter is set to full. Otherwise, the precheck fails and the synchronization task cannot start. For instructions, see Configure instance parameters.

      Important

      • If your source instance is a self-managed MySQL database, enable binary logging and set binlog_format to row and binlog_row_image to full.

      • If your self-managed MySQL database is a dual-primary cluster (where both nodes act as primary and secondary), enable the log_slave_updates parameter so DTS can capture all binary log events. For instructions, see Create an account and configure binary logging for a self-managed MySQL database.

    • The local binary logs for an ApsaraDB RDS for MySQL instance must be retained for at least three days (seven days is recommended). For a self-managed MySQL database, retain local binary logs for at least seven days. Otherwise, DTS may fail to retrieve binary logs, causing the task to fail. In extreme cases, this may cause data inconsistency or data loss. Issues caused by binary log retention periods shorter than DTS requires are not covered under the DTS SLA.

      Note

      To configure the retention period for local binary logs on an ApsaraDB RDS for MySQL instance, see Automatically delete local logs.

  • Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails.

    Note

    During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database.

  • Data generated by changes that do not write to binary logs—such as data restored from physical backups or created by cascade operations—is not synchronized to the destination database.

    Note

    If this occurs, remove the affected database or table from the synchronization objects. Then add it back. You can do this only if your business allows it. For more information, see Modify synchronization objects.

  • If your source database is MySQL 8.0.23 or later and contains invisible hidden columns, DTS may not read those columns. This may cause data loss.

    Note

    Run the ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE; command to make the hidden column visible. For more information, see Invisible Columns.

Other limits

  • Use the same version for the source and destination databases, or upgrade from a lower version to a higher version to ensure compatibility. Synchronizing from a higher version to a lower version may cause compatibility issues.

  • If your source database uses online DDL operations in temporary table mode—including but not limited to multi-table merge scenarios—or adds function-based indexes to unique key columns, data loss or task failure may occur in the destination database.

  • Resolvers defined by comment syntax do not support synchronous use.

  • If a primary key or unique key conflict occurs while the task is running:

    • If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database:

      • During full data synchronization, DTS retains the destination record and skips the source record.

      • During incremental synchronization, DTS overwrites the destination record with the source record.

    • If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.

  • If your destination database is MySQL 8.0.23 or later and contains invisible hidden columns in the receiving columns, DTS may fail to locate the target column. This may cause task failure or data loss.

    Note

    Run the ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE; command to make the hidden column visible. For more information, see Invisible Columns.

  • If you do not use DTS to synchronize table schemas, ensure field compatibility yourself. Otherwise, the task may fail or data may be lost. For example, if the source table uses the text data type and the destination column uses varchar(255), large fields in the source table may be truncated.

  • If your data includes four-byte characters—such as rare Chinese characters or emojis—the destination database and table must use the utf8mb4 charset.

    Note

    If you use DTS to synchronize table schemas, set the character_set_server parameter at the instance level in the destination database to utf8mb4.

  • Assess the performance of both the source and destination databases before starting synchronization. Run synchronization during off-peak hours. Otherwise, full initialization consumes read and write resources on both databases and may increase database load.

  • Full initialization runs INSERT operations concurrently. This fragments destination tables. After full initialization, the tablespace of the destination instance is larger than that of the source instance.

  • If you synchronize one or more tables—not a full database—do not use tools like pt-online-schema-change to perform online DDL operations on the source tables. Otherwise, synchronization fails.

    You can use Data Management (DMS) to perform online DDL operations. For more information, see Online DDL without locking tables.

  • Do not write data to the destination database except through DTS while synchronization is running. Otherwise, data inconsistency may occur between the source and destination databases. For example, if you use DMS to perform online DDL operations while other data is written to the destination database, data loss may occur.

  • If a DDL operation fails in the destination database, the DTS task continues to run. Check the failed DDL in the task logs. For instructions, see View task logs.

  • If you write columns with names that differ only in case to the same table in the destination MySQL database, unexpected results may occur because MySQL column names are case-insensitive.

  • After data synchronization completes (the instance's Status is Completed), you should use the ANALYZE TABLE <table_name> command to confirm that all data is written to the target table. For example, after the HA failover mechanism is triggered in the target MySQL database, data might be written only to memory, which can cause data loss.

  • If your ApsaraDB RDS for MySQL instance has Always-Encrypted enabled, full data synchronization is not supported.

    Note

    ApsaraDB RDS for MySQL instances with Transparent Data Encryption (TDE) enabled support schema synchronization, full data synchronization, and incremental data synchronization.

  • To synchronize accounts from the source database, meet the prerequisites and review related considerations. For more information, see Migrate database accounts.

  • If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters.

    Note

    Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.

Other considerations

  • For a self-managed MySQL source database:

    • If a primary/secondary switchover occurs in the source database during synchronization, the task fails.

    • DTS calculates latency by comparing the timestamp of the last synchronized record with the current time. If no DML operations run for a long time in the source database, latency reporting may become inaccurate. If latency appears too high, run a DML operation in the source database to update the latency.

      Note

      If you select a full database for synchronization, create a heartbeat table. Update or write to this table every second.

    • DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command in the source database to advance the binary log offset.

    • If your source database is Amazon Aurora MySQL or another clustered MySQL instance, ensure the domain name or IP address used in the task configuration—and its DNS resolution—always points to a read/write (RW) node. Otherwise, synchronization may fail.

  • For an ApsaraDB RDS for MySQL source database:

    • Read-only instances—such as ApsaraDB RDS for MySQL 5.6 read-only instances—that do not record transaction logs cannot serve as source databases.

    • DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command in the source database to advance the binary log offset.

Synchronization topologies

  • One-way one-to-one synchronization

  • One-way one-to-many synchronization

  • One-way cascade synchronization

  • One-way many-to-one synchronization

  • Two-way one-to-one synchronization

For details on each data synchronization topology and its usage notes, see Data Synchronization Topologies.

Supported SQL operations

Operation type

SQL statement

DML

INSERT, UPDATE, DELETE

DDL

  • ALTER TABLE, ALTER VIEW

  • CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, CREATE VIEW

  • DROP INDEX, DROP TABLE

  • RENAME TABLE

    Important

    The RENAME TABLE operation may cause data inconsistency. For example, if the synchronization object is only a specific table and you rename the table in the source instance during synchronization, the data of the table is not synchronized to the destination database. To prevent this issue, select the entire database to which the table belongs as the synchronization object when you configure the data synchronization task. Make sure that the databases to which the table belongs before and after the RENAME TABLE operation are both included in the synchronization object.

  • TRUNCATE TABLE

Procedure

  1. Go to the DTS dedicated cluster page.

  2. On the right side of the workbench, select the region where the DTS dedicated cluster is located.

    Note

    If you are logged in to the Data Management (DMS) console, select the region of the dedicated cluster to the right of Cluster Task.

  3. In the Operation column of the target dedicated cluster, choose Configure Task > Configure Data Synchronization Task.

  4. Configure the Source Database and Destination Database.

    Warning

    After you select the source and destination instances, review the Limits at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

    Section

    Parameter

    Description

    N/A

    Task Name

    DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.

    Source Database

    Select Existing Connection

    You can choose whether to use an existing instance, as needed.

    • If you use an existing instance, the database information below is automatically filled in. You do not need to enter it again.

    • If you do not use an existing instance, you must enter the database information below.

    Database Type

    Select MySQL.

    Connection Type

    Select Alibaba Cloud Instance.

    Instance Region

    The region of the source ApsaraDB RDS for MySQL instance. This is specified when you create the dedicated cluster and cannot be changed.

    Replicate Data Across Alibaba Cloud Accounts

    For this example, select No, as the database instance belongs to the current Alibaba Cloud account.

    RDS Instance ID

    Select the ID of the source ApsaraDB RDS for MySQL instance.

    Note

    The source and destination can be the same ApsaraDB RDS for MySQL instance or different instances. DTS can synchronize data between two instances or within a single instance.

    Database Account

    Enter the database account of the source ApsaraDB RDS for MySQL instance. The account must have read permissions on the objects to be synchronized.

    Database Password

    Enter the password for the specified database account.

    Connection method

    Select Non-encrypted or SSL-encrypted as needed. If you set this to SSL-encrypted, you must enable SSL encryption for the RDS for MySQL instance beforehand. For more information, see Use a cloud certificate to quickly enable SSL link encryption.

    Destination Database

    Select Existing Connection

    You can choose whether to use an existing instance, as needed.

    • If you use an existing instance, the database information is automatically filled in. You do not need to enter it again.

    • If you do not use an existing instance, you must enter the database information below.

    Database Type

    Select MySQL.

    Connection Type

    Select Alibaba Cloud Instance.

    Instance Region

    Select the region where the destination ApsaraDB RDS for MySQL instance resides.

    Replicate Data Across Alibaba Cloud Accounts

    For this example, select No, as the database instance belongs to the current Alibaba Cloud account.

    RDS Instance ID

    Select the ID of the destination ApsaraDB RDS for MySQL instance.

    Note

    The source and destination can be the same ApsaraDB RDS for MySQL instance or different instances. DTS can synchronize data between two instances or within a single instance.

    Database Account

    Enter the database account of the destination ApsaraDB RDS for MySQL instance. The account must have read and write permissions on the destination database.

    Database Password

    Enter the password for the specified database account.

    Connection method

    Select Non-encrypted or SSL-encrypted as needed. If you set this to SSL-encrypted, you must enable SSL encryption for the RDS for MySQL instance beforehand. For more information, see Use a cloud certificate to quickly enable SSL link encryption.

  5. In the lower part of the page, click Test Connectivity and Proceed.

    If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must ensure that the ECS instance can access the database. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database. For more information, see DTS server IP whitelist.

    Warning

    If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database or instance, or to the ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

  6. Configure the task objects.

    1. On the Configure Objects page, specify the objects to synchronize.

      Parameter

      Description

      Synchronization Types

      DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.

      Method to Migrate Triggers in Source Database

      Select a method to synchronize triggers. If the objects to be synchronized do not involve triggers, you do not need to configure this parameter. For more information, see Configure a method to synchronize or migrate triggers.

      Note

      This parameter is available only when Synchronization Types is selected for Schema Synchronization.

      Enable Migration Assessment

      Assess whether the schemas of the source and destination databases meet the requirements. This includes aspects like index length, stored procedures, and dependent tables. You can select Yes or No.

      Note

      • This parameter is available only when Synchronization Types is selected for Schema Synchronization.

      • If you select Yes, the precheck may take longer. You can view the Assessment Result during the precheck phase. The assessment result does not affect the precheck result.

      Synchronization Topology

      Select One-way Synchronization.

      Processing Mode of Conflicting Tables

      • Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful.

        Note

        If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Database Table Column Name Mapping.

      • Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database.

        Warning

        Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example:

        • If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database:

          • During full data synchronization, DTS retains the destination record and skips the source record.

          • During incremental synchronization, DTS overwrites the destination record with the source record.

        • If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.

      Capitalization of Object Names in Destination Instance

      Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.

      Source Objects

      In the Source Objects box, click the objects, and then click 向右 to move them to the Selected Objects box.

      Note

      You can select objects at the database, table, or column level. If you select only tables or columns, DTS does not synchronize other object types (such as views, triggers, and stored procedures).

      Selected Objects

      • To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name.

      • To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk.

      Note

      • To select the SQL operations to synchronize at the database or table level, right-click the object to be synchronized in the Selected Objects box and select the desired SQL operations in the dialog box that appears.

      • To filter data by using a WHERE clause, right-click the table to be synchronized in the Selected Objects box and set the filter condition in the dialog box that appears. For more information, see Set filter conditions.

      • If you use object name mapping, synchronization of dependent objects might fail.

    2. Click Next: Advanced Settings.

      Parameter

      Description

      Dedicated Cluster for Task Scheduling

      This parameter is fixed to the current dedicated cluster and cannot be changed.

      Copy the temporary table of the Online DDL tool that is generated in the source table to the destination database.

      If the source database uses Data Management (DMS) or gh-ost for online DDL changes, choose whether to synchronize the temporary tables generated during these operations.

      Important

      DTS tasks do not currently support online DDL changes performed by tools like pt-online-schema-change. Using such tools will cause the DTS task to fail.

      • Yes: Synchronizes the temporary tables generated by online DDL changes.

        Note

        If the data of temporary tables generated by online DDL changes is too large, it may cause synchronization latency.

      • No, Adapt to DMS Online DDL: Does not synchronize temporary tables generated by online DDL changes. Instead, it synchronizes only the original DDL statements executed in Data Management (DMS).

        Note

        This approach will cause table locks on the destination database.

      • No, Adapt to gh-ost: Does not the synchronize temporary tables generated by online DDL changes. Instead, it synchronizes only the original DDL statements executed by gh-ost. You can use default or custom regular expressions for gh-ost shadow and trash tables.

        Note

        This approach will cause table locks on the destination database.

      Whether to Migrate Accounts

      Select whether to synchronize the account information of the source database. If you select Yes, you also need to select the accounts to be synchronized and confirm their permissions. For more information, see Migrate database accounts.

      Retry Time for Failed Connections

      If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails.

      Note

      • If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances.

      • DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.

      Retry Time for Other Issues

      If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails.

      Important

      The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.

      Enable Throttling for Full Data Synchronization

      During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s).

      Note

      Enable Throttling for Incremental Data Synchronization

      You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).

      Environment Tag

      You can select an environment tag to identify the instance. No selection is required in this example.

      Whether to delete SQL operations on heartbeat tables of forward and reverse tasks

      Choose whether DTS writes heartbeat SQL information to the source database while the instance is running.

      • Yes: Does not write heartbeat SQL information to the source database. The DTS instance may display latency.

      • No: Writes heartbeat SQL information to the source database. This may interfere with source database operations like physical backups and cloning.

      Monitoring and Alerting

      Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts.

    3. Click Data Verification to configure a data verification task.

      To use the data verification feature, see Configure data verification.

  7. Save the task and perform a precheck.

    • To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.

    • If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.

    Note

    • Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.

    • If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.

    • If the precheck generates warnings:

      • For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.

      • For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

  8. When the precheck success rate reaches 100%, click Next: Select DTS Instance Type.

  9. In the New Instance Class section, set the Instance Class for the task. You can specify a value from 1 DU up to the maximum available DUs in the cluster.

  10. After you complete the configuration, read and select the Data Transmission Service (Pay-as-you-go) Service Terms checkbox.

  11. Click Start Task. In the dialog box that appears, click OK to start the synchronization task.

    You can filter for the task in the Cluster Task List to monitor its progress.

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