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

Prerequisites

Limits

Note By default, DTS disables FOREIGN KEY constraints for the destination database in a data synchronization task. Therefore, the cascade and delete operations of the source database are not synchronized to the destination database.
Category Description
Limits on the source database
  • The tables to synchronize 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 synchronize and you want to edit tables (such as renaming 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, configure multiple tasks to synchronize the tables, or configure a task to synchronize the entire database.
  • A single data synchronization task can synchronize up to 10 databases. If you want to synchronize more than 10 databases, we recommend that you split the tables to synchronize or configure multiple tasks to synchronize the databases. Otherwise, the performance and stability of your data synchronization task may be compromised.
  • The following requirements for data logs must be met:
    • The data logging feature must be enabled. The backup mode must be set to Full, and full logical backup must be performed.
    • If you perform only incremental data synchronization, the data logs of the source database must be stored for more than 24 hours. If you perform both full data synchronization and incremental data synchronization, the data logs of the source database must be stored for at least seven days. After the full data synchronization is complete, you can set the retention period to more than 24 hours. Otherwise, DTS may fail to obtain the data logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. Make sure that you set the retention period of data logs in accordance with the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not ensure service reliability and performance.

Other limits
  • Requirements for the objects to synchronize:
    • DTS supports initial schema synchronization for the following types of objects: database, schema, and table.
    • DTS does not synchronize data of the following types: TIMESTAMP, CURSOR, ROWVERSION, HIERACHYID, SQL_VARIANT, SPATIAL GEOMETRY, SPATIAL GEOGRAPHY, and TABLE.
  • If you set SQL Server Incremental Synchronization Mode to Incremental Synchronization Based on Logs of Source Database in the Configure Objects and Advanced Settings step, the tables to synchronize must have clustered indexes that contain primary key columns. In addition, the table to synchronize cannot be compressed tables and cannot contain computed columns. Ignore the preceding limits in the mixed log-based parsing mode.
  • Complex synchronous DDL operations are not supported at present.
  • In the Incremental Synchronization Based on Logs of Source Database mode, DTS adds a heartbeat table named dts_log_heart_beat to the source database to ensure that the latency of data synchronization is accurate. In the mixed log-based parsing incremental synchronization mode, DTS creates a trigger named dts_cdc_sync_ddl, a heartbeat table named dts_sync_progress, and a storage table named dts_cdc_ddl_history by using DDL and enables Change Data Capture (CDC) for the database and some tables.
  • 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. Therefore, after the 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.
  • During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases. If you use tools other than DTS to write data to the destination database, data loss may occur in the destination database when you use DMS to perform online DDL operations.

Supported synchronization topologies

  • One-way one-to-one synchronization
  • One-way one-to-many synchronization
  • One-way many-to-one synchronization
For more information about synchronization topologies, see Synchronization topologies.

SQL operations that can be synchronized

Operation type SQL statement
DML INSERT, UPDATE, and DELETE
DDL
  • CREATE TABLE
    Note If a CREATE TABLE statement contains partitioning clauses, subpartitioning clauses, or functions, DTS does not synchronize the operation.
  • ADD COLUMN and DROP COLUMN
  • DROP TABLE
  • CREATE INDEX and DROP INDEX

Procedure

  1. Go to the Data Synchronization page of the new DTS console.
    Note You can also log on to the Data Management (DMS) console. In the top navigation bar, choose DTS > Data Synchronization.
  2. In the upper-left corner of the page, select the region where the data synchronization instance resides.
    Select a region
  3. Click Create Task. On the page that appears, configure the source and destination databases.
    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 to identify the task. You do not need to use a unique task name.

    Source Database Database Type Select SQL Server.
    Access Method Select Cloud Instance.
    Instance Region The region where the source ApsaraDB RDS for SQL Server instance resides.
    RDS Instance ID The ID of the source ApsaraDB RDS for SQL Server instance.
    Database Account The database account of the source ApsaraDB RDS for SQL Server instance. The account must have the owner permissions on objects to synchronize. A privileged account has the required permissions.
    Database Password

    Enter the password of the database account.

    Destination Database Database Type Select MySQL.
    Access Method Select Cloud Instance.
    Instance Region The region where the destination ApsaraDB RDS for MySQL instance resides.
    RDS Instance ID The ID of the destination ApsaraDB RDS for MySQL instance.
    Database Account 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 of the database account.

    Encryption

    Select Non-encrypted or SSL-encrypted. If you want to select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data synchronization task. For more information, see Configure SSL encryption for an ApsaraDB RDS for MySQL instance.

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

      Incremental Data Synchronization is selected by default. You must also select 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 Map object names.
      • 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, and a data record has the same primary key as an existing data record in the destination database:
          • During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained.
          • During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten.
        • 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.
      Schema Mapping Mode of Source and Destination Databases
      Select a schema mapping mode based on your business requirements.
      Warning If you do not use the schema names of the source database, the source tables in different schemas must have unique names.
      SQL Server Incremental Synchronization Mode
      • Log-based Parsing for Non-heap Tables and CDC-based Incremental Synchronization for Heap Tables:
        • Advantages:
          • Supports heap tables and tables without primary keys.
          • Prevents log truncation in the source database from interrupting DTS tasks.
        • Disadvantages:
          • DTS creates Change Data Capture (CDC) records in the source databases and tables.
          • Causes higher latency than log-based parsing mode.
      • Incremental Synchronization Based on Logs of Source Database (Heap tables are not supported):
        • Advantages:
          • Provides low latency and good performance.
          • Provides no intrusion to the source database.
        • Disadvantages:
          • Does not support heap tables or tables without primary keys.
          • DTS tasks may be interrupted due to large volumes of data written to the source database and frequent log truncation or archiving.
      Note The Log-based Parsing for Non-heap Tables and CDC-based Incremental Synchronization for Heap Tables mode supports compressed tables, but the Incremental Synchronization Based on Logs of Source Database (Heap tables are not supported) mode does not.
      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.

      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 Map the name of a single object.
      • To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time.
      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 (Capitalization of Object Names in Destination Instance)
      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.
      Capitalization of Object Names in Destination Instance

      Specify the capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to make sure that the capitalization of object names is consistent with that of the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.

      Retry Time for Failed Connection
      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.
  6. 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.
  7. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  8. 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 instances.
    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.
  9. Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
  10. Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.