Data Transmission Service：Synchronize data from a PolarDB for MySQL cluster to a DataHub project

Prerequisites

• A PolarDB for MySQL cluster is created. For more information, see Purchase an Enterprise Edition cluster and Purchase a subscription cluster.

• DataHub is activated, and a project is created to receive the data to be synchronized. For more information, see Get started with DataHub and Manage projects.

• The binary logging feature is enabled for the PolarDB for MySQL cluster. For more information, see Enable binary logging.

Limits

Billing

Supported synchronization topologies

• One-way one-to-one synchronization

• One-way one-to-many synchronization

• One-way many-to-one synchronization

• One-way cascade synchronization

For more information about the synchronization topologies that are supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

INSERT, UPDATE, and DELETE

Permissions required for database accounts

The database account of the source PolarDB for MySQL cluster must have at least the read permissions on the objects to 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 Data Synchronization Task page, 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 configure the following database information.
   	Database Type	The type of the source database. Select PolarDB for MySQL.
   	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the source PolarDB for MySQL cluster resides.
   	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
   	PolarDB Cluster ID	The ID of the source PolarDB for MySQL cluster.
   	Database Account	The database account of the source PolarDB for MySQL cluster. For more information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
   	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 configure the following database information.
   	Database Type	The type of the destination database. Select DataHub.
   	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the destination DataHub project resides.
   	Project	The destination DataHub project to which data is to be synchronized.

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 the CIDR blocks of DTS servers section of the Add the CIDR blocks of DTS servers topic.

   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.

   Parameter	Description
   Synchronization Types	By default, Incremental Data Synchronization is selected. You can select only Schema Synchronization. You cannot select Full Data Synchronization. Note During initial schema synchronization, DTS synchronizes the schemas of the selected objects such as tables from the source database to the destination DataHub project.
   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 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. Proceed with caution.
   Naming Rules of Additional Columns	When DTS synchronizes data to a DataHub project, DTS adds additional columns to the destination topic. If the names of additional columns are the same as the names of existing columns in the destination topic, the data synchronization task fails. You can select Yes or No to specify whether to enable the new naming rules for additional columns based on your business requirements. Warning Before you set this parameter, check whether additional columns have name conflicts with existing columns in the destination topic. Otherwise, the data synchronization task may fail or data may be lost. For more information, see the Naming rules for additional columns section of the Modify the naming rules for additional columns topic.
   Capitalization of Object Names in Destination Instance	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 ensure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
   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 tables or databases as the objects to be synchronized.
   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.

6. 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 notification settings. For more information, see the Configure monitoring and alerting when you create a DTS task section of the Configure monitoring and alerting topic.
   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.
   Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. Select Yes or No based on your business requirements. If you select Yes, you must enter domain-specific language (DSL) statements in the code editor. For more information, see Configure ETL in a data migration or synchronization task.

7. Optional: In the Selected Objects section, move the pointer over the name of a topic to be synchronized and right-click it. In the dialog box that appears, modify the name of a table or database and set the shard key for partitioning based on your business requirements.

8. 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.

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

10. 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.

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

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

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

Schema of a DataHub topic

The following table describes the additional columns in the DataHub topic.

Additional information about the dts_before_flag and dts_after_flag fields

The values of the dts_before_flag and dts_after_flag fields in an incremental log entry vary based on operation types:

• INSERT

  For an INSERT operation, the column values are the newly inserted record values (post-update values). The value of the dts_before_flag field is N, and the value of the dts_after_flag field is Y.

  

• UPDATE

  DTS generates two incremental log entries for an UPDATE operation. The two incremental log entries have the same values for the dts_record_id, dts_operation_flag, and dts_utc_timestamp fields.

  The first log entry records the pre-update values. Therefore, the value of the dts_before_flag field is Y, and the value of the dts_after_flag field is N. The second log entry records the post-update values. Therefore, the value of the dts_before_flag field is N, and the value of the dts_after_flag field is Y.

  

• DELETE

  For a DELETE operation, the column values are the deleted record values (pre-update values). The value of the dts_before_flag field is Y, and the value of the dts_after_flag field is N.

  

What to do next

After you configure the data synchronization task, you can use Alibaba Cloud Realtime Compute for Apache Flink to analyze the data that is synchronized to the DataHub project. For more information, see What is Alibaba Cloud Realtime Compute for Apache Flink?