Data synchronization between RDS for SQL Server instances

Prerequisites

• You have created the source and target ApsaraDB RDS for SQL Server instances. For supported versions, see Synchronization solution overview. To create an instance, see Create an ApsaraDB RDS for SQL Server instance.

  Important

  The following source database versions and editions are supported in hybrid log-based parsing mode (when 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):

  • Enterprise or Enterprise Evaluation edition: 2012, 2014, 2016, 2019, or 2022.

  • Standard edition: 2016, 2019, or 2022.

• The target ApsaraDB RDS for SQL Server instance must have more storage space than the source ApsaraDB RDS for SQL Server instance.

• When synchronizing data from a self-managed SQL Server to an ApsaraDB RDS for SQL Server instance, we recommend using the database backup feature of ApsaraDB RDS for SQL Server if any of the following conditions apply. For more information, see Migrate data from a self-managed database to an ApsaraDB RDS for SQL Server instance.

  • The source instance has more than 10 databases.

  • Log backups are performed more than once per hour for a single database.

  • More than 100 Data Definition Language (DDL) operations are performed per hour on a single database.

  • The log generation rate for a single database exceeds 20 MB/s.

  • You must enable change data capture (CDC) on more than 1,000 tables.

  • The source database contains heap tables, tables without a primary key, compressed tables, tables with computed columns, or tables with sparse columns. You can run the following SQL statements to check the source database for these types of tables:

    1. Check for heap tables in the source database:

       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. Check for tables without a primary key:

       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. Check for primary key columns that are not included in the clustered index:

       SELECT s.name schema_name, t.name 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_colums_counter.object_id AS object_id FROM (select pk_colums.object_id, sum(pk_colums.column_id) column_id_counter 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_colums group by object_id) pk_colums_counter inner JOIN ( select cluster_colums.object_id, sum(cluster_colums.column_id) column_id_counter 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.index_id = 1) cluster_colums group by object_id ) cluster_colums_counter ON pk_colums_counter.object_id = cluster_colums_counter.object_id and pk_colums_counter.column_id_counter != cluster_colums_counter.column_id_counter);

    4. Check for compressed tables in the source database:

       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. Check for tables that contain 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);

    6. Check for tables that contain sparse 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_sparse = 1);

Usage notes

Billing

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 a description of each synchronization topology and its use cases, see Synchronization topologies.

Supported SQL operations

Procedure

1. Go to the data synchronization task list page in the destination region. You can do this in one of two ways.

   DTS console

   1. Log on to the DTS console.

   2. In the navigation pane on the left, click Data Synchronization.

   3. In the upper-left corner of the page, select the region where the synchronization instance is located.

   DMS console

   Note

   The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.

   1. Log on to the DMS console.

   2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.

   3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.

2. Click Create Task to open the task configuration page.

3. Configure the source and destination databases.

   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	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
   	Database Type	Select SQL Server.
   	Connection Type	Select Alibaba Cloud Instance.
   	Instance Region	Select the region of the source ApsaraDB RDS for SQL Server instance.
   	Instance ID	Select the ID of the source ApsaraDB RDS for SQL Server instance.
   	Database Account	Enter the database account of the source ApsaraDB RDS for SQL Server instance. The account must have owner permissions on the objects to be synchronized. A privileged account meets this requirement.
   	Database Password	Enter the password for the specified database account.
   	Encryption	Select Non-encrypted or SSL-encrypted. 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. DTS trusts the server certificate by default.
   Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
   	Database Type	Select SQL Server.
   	Connection Type	Select Alibaba Cloud Instance.
   	Instance Region	Select the region of the destination ApsaraDB RDS for SQL Server instance.
   	Instance ID	Select the ID of the destination ApsaraDB RDS for SQL Server instance.
   	Database Account	Enter the database account of the destination ApsaraDB RDS for SQL Server instance. The account must have owner permissions on the objects to be synchronized.
   	Database Password	Enter the password for the specified database account.
   	Encryption	Select Non-encrypted or SSL-encrypted. 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. DTS trusts the server certificate by default.

4. After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.

   Note

   • Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.

   • If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

5. 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 based on your requirements. If you are not synchronizing any triggers, you do not need to configure this parameter. For more information, see Configure how to synchronize or migrate triggers. Note This parameter is available only when the Schema Synchronization parameter is set to Synchronization Types.
      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: Supports scenarios with source database heap tables, tables without primary keys, compressed tables, and tables with computed columns. High link stability. This mode can obtain complete DDL statements and supports a wide range of DDL scenarios. 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. It also enables database-level Change Data Capture (CDC) and partial table CDC. You cannot execute SELECT INTO, TRUNCATE, and RENAME COLUMN statements on tables for which CDC is 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: This mode is non-intrusive to the source database. Disadvantages: Does not support scenarios with source database heap tables, tables without primary keys, compressed tables, or tables with computed columns. Polling and querying CDC instances for incremental synchronization: Advantages: Supports full and incremental synchronization when the source database is Amazon RDS for SQL Server, Azure SQL Database, Azure SQL Managed Instance, Azure SQL Server on Virtual Machine, or Google Cloud SQL for SQL Server. Uses the native SQL Server CDC component to obtain incremental data, which makes incremental synchronization more stable and uses less network bandwidth. Disadvantages: The source database account used by the DTS instance must have the permission to enable CDC. Incremental data synchronization has a latency of about 10 seconds. In scenarios involving synchronization of multiple databases and tables, there may be risks of stability and performance issues.
      The maximum number of tables for which CDC is enabled that DTS supports.	This setting limits the number of tables on which CDC can be enabled within the current synchronization instance. The default value is 1,000. Note This parameter is not available if you set SQL Server Incremental Synchronization Mode to Incremental Synchronization Based on Logs of Source Database (Heap tables are not supported).
      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.
      Source Objects	In the Source Objects box, click the objects, and then click 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 an object in the Selected Objects box and select the desired SQL operations in the dialog box that appears. To filter data by using WHERE clauses, right-click a table in the Selected Objects box and specify the filter conditions in the dialog box that appears. For more information, see Set filter conditions. If you use the object name mapping feature, other objects that depend on a renamed object may fail to synchronize.

   2. Click Next: Advanced Settings.

      Parameter	Description
      Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
      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 This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
      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. You can skip this setting for this example.
      Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
      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. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert contact. For more information, see Configure monitoring and alerting during task configuration.

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

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

6. 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.

7. Purchase the instance.

   1. When the Success Rate reaches 100%, click Next: Purchase Instance.

   2. On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

      Category	Parameter	Description
      New Instance Class	Billing Method	Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks. Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.
      	Resource Group Settings	The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.
      	Instance Class	DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.
      	Subscription Duration	In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years. Note This option appears only when the billing method is Subscription.

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

   4. Click Buy and Start, and then click OK in the OK dialog box.

      You can monitor the task progress on the data synchronization page.

Check CDC status

• To check the CDC status of the database:

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

• 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>');

• To verify that CDC is working correctly:

  • Check the log space usage.

    SELECT * FROM sys.dm_db_log_space_usage;

  • Verify that the SQL Server Agent service is working correctly. For details, see SQL Server Agent.

  • Verify that CDC is scanning the log correctly. If the log scan fails, DTS may retry or terminate the task.

    SELECT * FROM sys.dm_cdc_log_scan_sessions;

• To disable CDC for a database:

  USE [<your db name>];
  
  DROP TRIGGER [dts_cdc_sync_ddl] ON database; 
  # You must drop this trigger to disable CDC. Ignore this statement if the trigger does not exist.
  
  EXECUTE [sys].[sp_cdc_disable_db];