Data Transmission Service：Synchronize data between ApsaraDB RDS for SQL Server instances

Prerequisites

• The source and destination ApsaraDB RDS for SQL Server instances are created. For more information about supported database versions, see Overview of data synchronization scenarios. For more information about how to create an ApsaraDB RDS for SQL Server instance, see Create an ApsaraDB RDS for SQL Server instance.

  Important

  If the SQL Server Incremental Synchronization Mode parameter is set to Log-based Parsing for Non-heap Tables and CDC-based Incremental Synchronization for Heap Tables, data is synchronized in the hybrid log-based parsing mode and the following versions of source databases are supported:

  • Self-managed SQL Server or ApsaraDB RDS for SQL Server database of the Enterprise or Enterprise Evaluation edition: version 2012, 2014, 2016, or 2019.

  • Self-managed SQL Server or ApsaraDB RDS for SQL Server database of the Standard edition: version 2016 or 2019.

• The available storage space of the destination ApsaraDB RDS for SQL Server instance is larger than the total size of the data in the source ApsaraDB RDS for SQL Server instance.

• If you synchronize data from a self-managed SQL Server database to an ApsaraDB RDS for SQL Server instance and one of the following conditions is met, we recommend that you synchronize data by using the backup feature of ApsaraDB RDS for SQL Server. For more information, see Migrate data from a self-managed database to an ApsaraDB RDS for SQL Server instance.

  • The source instance contains more than 10 databases.

  • A single database of the source instance backs up its logs at an interval of less than 1 hour.

  • A single database of the source instance executes more than 100 DDL statements each hour.

  • Logs are written at a rate of 20 MB/s for a single database of the source instance.

  • The change data capture (CDC) feature needs to be enabled for more than 1,000 tables in the source ApsaraDB RDS for SQL Server instance.

  • The logs of a database in the source ApsaraDB RDS for SQL Server instance involve heap tables, tables without primary keys, compressed tables, or tables with computed columns. You can execute the following SQL statements to check whether the source database contains these tables.

    1. Execute the following SQL statement to check for heap tables:

       SELECT s.name AS schema_name, t.name AS table_name FROM sys.schemas s INNER JOIN sys.tables t ON s.schema_id = t.schema_id AND t.type = 'U' AND s.name NOT IN ('cdc', 'sys') AND t.name NOT IN ('systranschemas') AND t.object_id IN (SELECT object_id FROM sys.indexes WHERE index_id = 0);

    2. Execute the following SQL statement to check for tables without primary keys:

       SELECT s.name AS schema_name, t.name AS table_name FROM sys.schemas s INNER JOIN sys.tables t ON s.schema_id = t.schema_id AND t.type = 'U' AND s.name NOT IN ('cdc', 'sys') AND t.name NOT IN ('systranschemas') AND t.object_id NOT IN (SELECT parent_object_id FROM sys.objects WHERE type = 'PK');

    3. Execute the following SQL statement to check for primary key columns that are not contained in clustered index columns:

       SELECT s.name AS schema_name, t.name AS table_name FROM sys.schemas s INNER JOIN sys.tables t ON s.schema_id= t.schema_id WHERE t.type= 'U' AND s.name NOT IN('cdc', 'sys')  AND t.name NOT IN('systranschemas')  AND t.object_id IN (SELECT pk_columns.object_id AS object_id FROM (select sic.object_id object_id, sic.column_id FROM sys.index_columns sic, sys.indexes sis WHERE sic.object_id= sis.object_id AND sic.index_id= sis.index_id AND sis.is_primary_key= 'true') pk_columns LEFT JOIN (SELECT sic.object_id object_id, sic.column_id FROM sys.index_columns sic, sys.indexes sis WHERE sic.object_id= sis.object_id AND sic.index_id= sis.index_id AND sis.index_id= 1) cluster_colums ON pk_columns.object_id= cluster_colums.object_id WHERE pk_columns.column_id != cluster_colums.column_id);

    4. Execute the following SQL statement to check for compressed tables:

       SELECT s.name AS schema_name, t.name AS table_name FROM sys.objects t, sys.schemas s, sys.partitions p WHERE s.schema_id = t.schema_id AND t.type = 'U' AND s.name NOT IN ('cdc', 'sys') AND t.name NOT IN ('systranschemas') AND t.object_id = p.object_id AND p.data_compression != 0;

    5. Execute the following SQL statement to check for tables with computed columns:

       SELECT s.name AS schema_name, t.name AS table_name FROM sys.schemas s INNER JOIN sys.tables t ON s.schema_id = t.schema_id AND t.type = 'U' AND s.name NOT IN ('cdc', 'sys') AND t.name NOT IN ('systranschemas') AND t.object_id IN (SELECT object_id FROM sys.columns WHERE is_computed = 1);

Usage notes

Billing

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 about the synchronization topologies supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

Procedure

1. Go to the Data Synchronization Tasks page.

   1. Log on to the Data Management (DMS) console.

   2. In the top navigation bar, click DTS.

   3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.

   Note

   • Operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.

   • You can also go to the Data Synchronization Tasks page of the new DTS console.

2. On the right side of Data Synchronization Tasks, select the region in which the data synchronization instance resides.

   Note

   If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.

3. Click Create Task. On the Create Task page, configure the source and destination databases. The following table describes the parameters.

   Warning

   After you configure the source and destination databases, we recommend that you read the Limits that are displayed on the page. Otherwise, the task may fail or data inconsistency may occur.

   Section	Parameter	Description
   N/A	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
   Source Database	Select an existing DMS database instance	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
   	Database Type	The type of the source database. Select SQL Server.
   	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the source ApsaraDB RDS for SQL Server instance resides.
   	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 the objects to be synchronized. A privileged account has the required permissions.
   	Database Password	The password that is used to access the database instance.
   	Encryption	Specifies whether to encrypt the connection to the source database. Select Non-encrypted or SSL-encrypted based on your business requirements. If SSL encryption is not enabled for the source database, select Non-encrypted. If SSL encryption is enabled for the source database, select SSL-encrypted. By default, DTS trusts the server certificate.
   Destination Database	Select an existing DMS database instance	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
   	Database Type	The type of the destination database. Select SQL Server.
   	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the destination ApsaraDB RDS for SQL Server instance resides.
   	Instance ID	The ID of the destination ApsaraDB RDS for SQL Server instance.
   	Database Account	The database account of the destination ApsaraDB RDS for SQL Server instance. The account must have the owner permissions on the objects to be synchronized.
   	Database Password	The password that is used to access the database instance.
   	Encryption	Specifies whether to encrypt the connection to the destination database. Select Non-encrypted or SSL-encrypted based on your business requirements. If SSL encryption is not enabled for the destination database, select Non-encrypted. If SSL encryption is enabled for the destination database, select SSL-encrypted. By default, DTS trusts the server certificate.

4. 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 make sure that the ECS instance can access the database. If the database is deployed on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. 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 Add the CIDR blocks of DTS servers.

   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.

5. Configure the objects to be synchronized and advanced settings. The following table describes the parameters.

   Parameter	Description
   Synchronization Types	The synchronization types. By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.
   Method to Migrate Triggers in Source Database	The method used to synchronize triggers from the source database. You can select a synchronization method based on your business requirements. If no triggers are to be synchronized, you do not need to configure this parameter. For more information, see Synchronize or migrate triggers from the source database. Note This parameter is available only if you select Schema Synchronization for the Synchronization Types parameter.
   SQL Server Incremental Synchronization Mode	Log-based Parsing for Non-heap Tables and CDC-based Incremental Synchronization for Heap Tables (Hybrid Log-based Parsing): Advantages: This mode supports heap tables, tables without primary keys, compressed tables, and tables with computed columns. This mode provides higher stability and a variety of complete DDL statements. Disadvantages: DTS creates the trigger dts_cdc_sync_ddl, the heartbeat table dts_sync_progress, and the DDL storage table dts_cdc_ddl_history in the source database and enables Change Data Capture (CDC) for the source database and specific tables. You cannot execute the SELECT INTO or TRUNCATE statement on tables with CDC enabled in the source database. Triggers created by DTS in the source database cannot be manually deleted. Incremental Synchronization Based on Logs of Source Database (Heap tables are not supported): Advantages: Provides no intrusion to the source database. Disadvantages: This mode does not support heap tables, tables without primary keys, compressed tables, or tables with computed columns. Polling and querying CDC instances for incremental synchronization: Advantages: If the source database is an Amazon RDS for SQL Server instance, a Microsoft Azure SQL Server database, or a Google Cloud SQL for SQL Server instance, full and incremental data migration is supported. You can use a native CDC component of SQL Server to obtain incremental data. This improves the stability of incremental data migration and reduces bandwidth usage. Disadvantages: The account that DTS uses to access the source database must have the permission to enable the CDC feature. Incremental data migration has a latency of 10 seconds. If you migrate multiple tables in multiple databases, stability and performance issues may occur.
   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 tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck, and the data synchronization task cannot be started. Note If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are synchronized to the destination database. 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 inconsistency may occur and your business may be exposed to potential risks. If the source and destination databases have the same schema and a data record in the destination database has the same primary key value or unique key value as a data record in the source database: During full data migration, DTS does not migrate 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. Proceed with caution.
   Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select columns, tables, or databases as objects to synchronize. If you select tables or columns as the objects to be synchronized, DTS does not synchronize other objects such as views, triggers, and stored procedures to the destination database.
   Selected Objects	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 the Map the name of a single object section of the Map object names topic. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see the Map multiple object names at a time section of the Map object names topic. Note To select SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, see the SQL operations that can be synchronized section of this topic. To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Set filter conditions. If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be synchronized.

6. Click Next: Advanced Settings to configure advanced settings.

   • Data Verification Settings

     For more information about how to configure the data verification feature, see Configure data verification

   • Advanced Settings

     Parameter	Description
     Dedicated Cluster for Task Scheduling	By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. If you want to improve the stability of data migration tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
     Set Alerts	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values: No: does not enable alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert contacts. For more information, see Configure monitoring and alerting when you create a DTS task.
     Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set this parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note If you specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
     Retry Time for Other Issues	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
     Enable Throttling for Full Data Migration	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
     Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
     Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, you do not need to configure this parameter.
     Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.

7. Save the task settings and run a precheck.

   • To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.

   • If you do not need to view or have viewed the parameters, 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 data synchronization task fails the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, rerun the precheck.

   • If an alert is triggered for an item during the precheck:

     • If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issue. Then, run a precheck again.

     • If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.

8. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

9. On the buy page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

   Section	Parameter	Description
   New Instance Class	Billing Method	Subscription: You pay for a subscription when you create a data synchronization instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go data synchronization instance, you can release the instance to reduce costs.
   	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
   	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
   	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

10. Read and select the Data Transmission Service (Pay-as-you-go) Service Terms.

11. Click Buy and Start. In the dialog box that appears, click OK.

    You can view the progress of the task in the task list.

Check the status of CDC

• Execute the following SQL statement to check the status of CDC:

  SELECT name, is_cdc_enabled FROM sys.databases WHERE name = '<your db name>';

• Execute the following SQL statement to check the status of a CDC job:

  SELECT database_name(database_id), job_type FROM [msdb].[dbo].[cdc_jobs] WHERE database_id = DB_ID('<your db name>');

• Execute the following SQL statements to check whether CDC works as expected:

  • Execute the following SQL statement to check the usage of disk space:

    SELECT * FROM sys.dm_db_log_space_usage;

  • Check whether the SQL Server agent works as expected. For more information, see SQL Server Agent.

  • Check whether CDC scans data as expected. If data is not properly scanned by using CDC, the corresponding DTS task may be retried or stopped.

    SELECT * FROM sys.dm_cdc_log_scan_sessions;

• Execute the following SQL statements to disable CDC for a database:

  USE [<your db name>];
  
  DROP TRIGGER [dts_cdc_sync_ddl] ON database; 
  # If no trigger exists, ignore this statement. If a trigger exists, you must delete the trigger first.
  
  EXECUTE [sys].[sp_cdc_disable_db];