Synchronize data from a self-managed SQL Server database to a PolarDB for MySQL cluster - Data Transmission Service

This topic describes how to synchronize data from a self-managed SQL Server database to a PolarDB for MySQL cluster by using Data Transmission Service (DTS).

Prerequisites

The version of the self-managed SQL Server database is supported by DTS. For more information, see Overview of data synchronization scenarios.
The destination PolarDB for MySQL cluster is created. For more information, see Purchase an Enterprise Edition cluster and Purchase a subscription cluster.
The available storage space of the PolarDB for MySQL cluster is larger than the total size of the data in the self-managed SQL Server database.
If the source ApsaraDB RDS for SQL Server instance meets one of the following conditions, we recommend that you split the synchronization task into multiple subtasks:
- 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.

Limits

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 the constraint check and cascade operations on foreign keys at the session level. If you perform the cascade update and delete operations on the source database during data synchronization, data inconsistency may occur.

Limit type	Description
Limits on the source database	The tables to be synchronized 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 be synchronized and you want to edit the tables in the destination database, such as renaming tables or columns, you can synchronize up to 5,000 tables in a single data synchronization task. If you run a task to synchronize more than 5,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the tables or configure a task to synchronize the entire database. A single data synchronization task can synchronize data from up to 10 databases. If you want to synchronize data from more than 10 databases, we recommend that you configure multiple tasks to synchronize the data. Otherwise, the performance and stability of your data synchronization task may be compromised. DTS uses the fn_log function to obtain logs of the source database. However, this function has performance bottlenecks. Therefore, we recommend that you do not clear the logs of the source database before the task is complete. Otherwise, the task may fail. 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 physical backup must be performed. If you perform only incremental data synchronization, the data logs of the source database must be retained 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 retained for at least seven days. Otherwise, DTS may fail to obtain the data logs and the task may fail. In some cases, data inconsistency or loss may even occur. After full data synchronization is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of data logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the service level agreement (SLA) of DTS may not be guaranteed. If CDC needs to be enabled for the tables to be synchronized from the source database, make sure that the tables meet the following requirements. Otherwise, the precheck fails. The value of the srvname field in the sys.sysservers view is the same as the return value of the SERVERPROPERTY function. If the source database is a self-managed SQL Server database, the database owner must be the sa user. If the source database is an ApsaraDB RDS for SQL Server database, the database owner must be the sqlsa user. If the source database is of the Enterprise edition, you must use SQL Server 2008 or later. If the source database is of the Standard edition, you must use SQL Server 2016 SP1 or later. If the source database is of the Standard or Enterprise edition and its version is SQL Server 2017, we recommend that you update the version. If the source database is a read-only instance, you cannot synchronize DDL operations. In hybrid log-based parsing mode, you cannot perform multiple operations to add columns to or remove columns from the source database within 10 minutes. For example, if you execute the following SQL statements within 10 minutes, an error is reported for the task. `ALTER TABLE test_table DROP COLUMN Flag; ALTER TABLE test_table ADD Remark nvarchar(50) not null default('');` During schema synchronization and full data synchronization, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data synchronization task fails. If the source database is an ApsaraDB RDS for SQL Server instance that runs SQL Server Web edition, you must set the SQL Server Incremental Synchronization Mode parameter to Incremental Synchronization Based on Logs of Source Database (Heap tables are not supported) when you configure the task.
Other limits	Requirements for the objects to be synchronized: DTS supports initial schema synchronization for the following types of objects: database, schema, and table. DTS does not synchronize the following types of data: CURSOR, ROWVERSION, SQL_VARIANT, HIERARCHYID, POLYGON, GEOMETRY, and GEOGRAPHY. If you set the SQL Server Incremental Synchronization Mode parameter to Incremental Synchronization Based on Logs of Source Database (Heap tables are not supported) in the Select Objects step, the tables to be synchronized must have clustered indexes that contain primary key columns. The tables to be synchronized cannot be heap tables, tables without primary keys, compressed tables, or tables with computed columns. Ignore the preceding limits in the hybrid log-based parsing mode. Note For more information about how to view table information, see the How do I view the information about heap tables, tables without primary keys, compressed tables, and tables that contain computed columns in an SQL Server database? section of the FAQ topic. If you set the SQL Server Incremental Synchronization Mode parameter to Log-based Parsing for Non-heap Tables and CDC-based Incremental Synchronization for Heap Tables (Hybrid Log-based Parsing) in the Select Objects step, DTS uses the CDC component to synchronize incremental data. Make sure that the CDC job in the source database runs as expected. Otherwise, the DTS task fails. If you set the SQL Server Incremental Synchronization Mode parameter to Polling and querying CDC instances for incremental synchronization in the Select Objects step, the following limits apply: The source database account used by the DTS instance must have the permissions to enable the CDC feature. To enable database-level CDC, you must use an account that is assigned the sysadmin role. To enable table-level CDC, you must use a privileged account. Note A server administrator account of a database in Microsoft Azure SQL Database has the required permissions. CDC can be enabled for all databases that are purchased in Azure SQL Database based on the vCore model. CDC can be enabled for databases that are purchased in Azure SQL Database based on the database transaction unit (DTU) model only if the databases have a service tier of S3 or greater. A privileged account of an Amazon RDS for SQL Server instance has the required permissions. CDC can be enabled for stored procedures at the database level. DTS obtains incremental data by performing round-robin queries on the CDC instance of each table in the source database. Therefore, the number of tables to be migrated from the source database cannot exceed 1,000. Otherwise, the data migration task may be delayed or unstable. You cannot execute a DDL statement to add or remove columns more than twice within a minute. Otherwise, the data migration task may fail. During data migration, you cannot modify the CDC instances of the source database. Otherwise, the data migration task may fail or data loss may occur. DTS does not support synchronization of complicated DDL operations. In Incremental Synchronization Based on Logs of Source Database mode, DTS creates a trigger named dts_cdc_sync_ddl, a heartbeat table named dts_sync_progress, and a DDL history table named dts_cdc_ddl_history in the source database to ensure that the latency of data synchronization is accurate. In hybrid log-based parsing incremental synchronization mode, DTS creates a trigger named dts_cdc_sync_ddl, a heartbeat table named dts_sync_progress, and a DDL history table named dts_cdc_ddl_history and enables CDC for the source database and specific tables. We recommend that you set the maximum number of records per second to 1,000 for the tables for which CDC is enabled in the source database. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. During full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data synchronization is complete, the size of the used tablespace of the destination database is larger than that of the source database. Data inconsistency between the source and destination databases occurs if data from other sources is written to the destination database during data synchronization. For example, if you use DMS to execute online DDL statements while data from other sources is written to the destination database, data loss may occur in the destination database. If the data synchronization task involves incremental data synchronization, DTS does not allow you to perform the reindexing operation. If you perform the reindexing operation, the data synchronization task may fail and data loss may occur. Note DTS cannot synchronize DDL operations related to the primary key of a table for which change data capture (CDC) is enabled. If the number of tables for which CDC is enabled is greater than 1,000 in a data synchronization task, the precheck fails. If DDL statements fail to be written to the destination database, the DTS task continues to run. You can view the DDL statements that fail to be executed in task logs. For more information about how to view the task logs, see View task logs. When you modify the objects to be synchronized, you cannot add or remove databases.
Special cases	If the source instance is an ApsaraDB RDS for SQL Server instance, DTS automatically creates an account named `rdsdt_dtsacct` on the ApsaraDB RDS for SQL Server instance. This account is used for data synchronization. Do not delete this account or change the password of this account when your data synchronization task is running. Otherwise, the task may fail. For more information, see System accounts.

Billing

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

Supported synchronization topologies

One-way one-to-one synchronization
One-way one-to-many synchronization
One-way many-to-one synchronization

For more information about the synchronization topologies that are supported by DTS, 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 operation creates a partitioned table or a table that contains functions, DTS does not synchronize the operation. ADD COLUMN and DROP COLUMN DROP TABLE CREATE INDEX and DROP INDEX Note DTS does not synchronize DDL operations that contain user-defined types. DTS does not synchronize online DDL operations. Transactional DDL statements are not supported to be synchronized.

Preparations

Before you configure a data synchronization task, configure log settings and create clustered indexes on the self-managed SQL Server database.

Important If you need to synchronize data from multiple databases, repeat Steps 1 to 3 for each database. Otherwise, data inconsistency may occur.

Execute the following statement on the self-managed SQL Server database to change the recovery model to full. You can also change the recovery model by using SQL Server Management Studio (SSMS). For more information, see View or Change the Recovery Model of a Database (SQL Server).
```
use master;
GO
ALTER DATABASE <database_name> SET RECOVERY FULL WITH ROLLBACK IMMEDIATE;
GO
```
Parameter:
<database_name>: the name of the source database.
Example:
```
use master;
GO
ALTER DATABASE mytestdata SET RECOVERY FULL WITH ROLLBACK IMMEDIATE;
GO
```
Execute the following statement to create a logical backup for the source database. Skip this step if you have already created a logical backup.
```
BACKUP DATABASE <database_name> TO DISK='<physical_backup_device_name>';
GO
```
Parameter:
- <database_name>: the name of the source database.
- <physical_backup_device_name>: the storage path and file name of the backup file.
Example:
```
BACKUP DATABASE mytestdata TO DISK='D:\backup\dbdata.bak';
GO
```
Execute the following statement to create a log backup for the source database.
```
BACKUP LOG <database_name> to DISK='<physical_backup_device_name>' WITH init;
GO
```
Parameter:
- <database_name>: the name of the source database.
- <physical_backup_device_name>: the storage path and file name of the backup file.
Example:
```
BACKUP LOG mytestdata TO DISK='D:\backup\dblog.bak' WITH init;
GO
```

Procedure

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.
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.

Click Create Task. In the Create Task wizard, configure the source and destination databases. The following table describes the parameters.

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 Express Connect, VPN Gateway, or Smart Access Gateway.
	Instance Region	The region in which the self-managed SQL Server database resides. Note If your source database is a self-managed database, you must deploy the network environment for the database. For more information, see Preparation overview.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to migrate data across Alibaba Cloud accounts. In this example, No is selected.
	Connected VPC	The ID of the virtual private cloud (VPC) in which the self-managed SQL Server database is deployed.
	IP address	The server IP address of the self-managed SQL Server database.
	Port Number	The service port number of the self-managed SQL Server database. Default value: 1433.
	Database Account	The account that is used to log on to the self-managed SQL Server database. The account must have permissions of the sysadmin role. For more information, see CREATE USER and GRANT (Transact-SQL).
	Database Password	The password that is used to access the database instance.
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 PolarDB for MySQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination PolarDB for MySQL cluster resides.
	PolarDB Cluster ID	The ID of the destination PolarDB for MySQL cluster.
	Database Account	The account that is used to log on to the destination PolarDB for MySQL cluster. The account must have read and write permissions on the destination database.
	Database Password	The password that is used to access the database instance.

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.

Configure the objects for change tracking and advanced settings.

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.
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.
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.
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 In this scenario, data synchronization is performed between heterogeneous databases. Therefore, only tables can be synchronized. Other objects such as views, triggers, or stored procedures are not synchronized 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 the SQL operations performed on a specific database or table, right-click an object to be synchronized in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, see SQL operations that can be synchronized. 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 about how to specify the conditions, see Specify filter conditions. If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be synchronized.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Monitoring and Alerting	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 the 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 The wait time before a retry when other issues occur in the source and destination databases parameter must be smaller than the value of the Retry Time for Failed Connection 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.

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

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.

Read and select the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the dialog box that appears, click OK.
You can view the progress of the task in the task list.