Track data changes from a PolarDB-X 1.0 instance - Data Transmission Service

This topic describes how to create a change tracking task to track data changes from a PolarDB-X 1.0 instance by using Data Transmission Service (DTS).

Prerequisites

A PolarDB-X 1.0 instance is created. For more information, see Create a PolarDB-X 1.0 instance and Create a database.
Note The storage type of the PolarDB-X 1.0 instance must be ApsaraDB RDS for MySQL, such as custom ApsaraDB RDS instance or purchased ApsaraDB RDS instance. PolarDB for MySQL cannot be used as the storage type.
If the ApsaraDB RDS for MySQL instance is of the classic network type, an internal endpoint is configured for the ApsaraDB RDS for MySQL instance.

Limits

Category	Description
Limits on the source database	The source tables must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, DTS may track duplicate data changes. DTS does not allow you to track the schema updates of tables that have only UNIQUE constraints. We recommend that you select the tables that have PRIMARY KEY constraints. If you select tables as the objects to be tracked, up to 500 tables can be tracked in a single change tracking task. If you run a change tracking task to track more than 500 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to track the tables in batches or configure a change tracking task for the entire database. The following requirements for the binary logs of the ApsaraDB RDS for MySQL instances attached to a PolarDB-X 1.0 instance must be met: The binary logging feature must be enabled. The value of the binlog_row_image parameter must be set to full. Otherwise, error messages are returned during the precheck, and the change tracking task cannot be started. The binary logs of the source database must be stored for more than 24 hours. Otherwise, DTS may fail to obtain the binary logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. Make sure that you set the retention period of binary logs based on the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability or performance. If the source instance is a read-only or temporary instance, make sure that the instance records transaction logs.
Other limits	When DTS tracks data changes from a PolarDB-X 1.0 instance, data is distributed across the attached ApsaraDB RDS for MySQL instances. DTS runs a subtask for each ApsaraDB RDS for MySQL instance. The status of the subtask is displayed on the Task Topology page. A PolarDB-X 1.0 instance supports database and table sharding. If a DDL operation is performed on the source PolarDB-X 1.0 instance, the DDL operation is performed on all table shards. The progress of each subtask may be inconsistent with that of other subtasks. Therefore, the data tracked by DTS may contain duplicate DDL operations. If you consume these DDL operations multiple times, an exception may occur on the change tracking client. We recommend that you handle the exception as appropriate. Limits on the objects for change tracking: You can track data changes only at the table level. After the change tracking task is configured, the objects for change tracking cannot be reselected. If you want to track the data changes of tables that are not included in the selected objects, you must create another change tracking task for the tables. If you perform a primary/secondary switchover on the source database when the change tracking task is running, the task fails. When the change tracking task is running, do not scale in or out the source instance, migrate frequently-accessed tables, change shards, or perform DDL operations. Otherwise, the change tracking task fails or data inconsistency occurs. If the source database is used in another task such as a running data migration task, DTS may track the data changes of other objects. In this case, you must manually filter the tracked data on the change tracking client. Make sure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the `ROUND(COLUMN,PRECISION)` function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for columns of the FLOAT data type to 38 digits and the precision for columns of the DOUBLE data type to 308 digits. DTS does not track the DDL operations that are performed by using pt-online-schema-change. Therefore, the change tracking client may fail to write the consumed data to the destination tables due to schema conflicts.

Procedure

Go to the Change Tracking 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) > Change Tracking.
Note
- If you log on to the DMS console and click the Enter Simple Mode icon in the upper-right corner, you can move the pointer over the icon in the upper-left corner, and then choose All functions > DTS > Change Tracking. For more information, see Customize the layout and style of the DMS console.
- You can also configure the settings by using the new DTS console.
To the right of Change Tracking Tasks, select the region in which you want to create the change tracking task.
Note If you use the new DTS console, you must select the region from the drop-down list to the right of Workbench on the Change Tracking Tasks page of the DTS console.

Click Create Task. On the page that appears, specify the source database instance and the consumer network type.

Warning

After you specify the source database instance, we recommend that you read the Limits that are displayed in the upper part of the page. Otherwise, the task may fail or the tracked data cannot be consumed.

Section	Parameter	Description
N/A	Task Name	The name of the change tracking task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to use a unique task name. The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you use an existing instance, DTS automatically populates the parameters for the instance. Note To modify the connection settings of a template, edit the template when you configure the Select an existing database connection parameter in the Source Database section. To edit the template name, click Edit Template in the lower part of the Source Database section. The modified connection template takes effect the next time you select the connection template. The database instance that you have configured by using the template is not affected. If you do not use an existing instance, you must configure the following parameters.
Source Database	Select an existing database connection
	Database Type	The type of the source database. Select PolarDB-X 1.0.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the PolarDB-X 1.0 instance resides.
	Instance ID	The ID of the PolarDB-X 1.0 instance.
	Database Account	The database account of the PolarDB-X 1.0 instance. The account must have read permissions on the objects for change tracking.
	Database Password	The password that is used to access the database instance.
	Save as Instance or Edit Template	This parameter must be specified based on whether you select an existing instance for the Select an existing database connection parameter. If you select an existing instance, you can click Edit Template to specify a custom template name. Note The modified connection template takes effect the next time you select the connection template. The database instance that you have configured by using the template is not affected. If you do not select an existing instance, click Save as Instance. In the dialog box that appears, set the name of the database connection and click OK. The connection settings of the database instance are saved as a template. Note We recommend that you specify a descriptive name that makes it easy to identify the database connection. You do not need to use a unique name.
Consumer Network Type	Network Type	The value of the Network Type parameter is fixed to VPC. You must select a VPC and a vSwitch. For more information, see VPCs. Note After a change tracking task is configured, you cannot change the settings in the Consumer Network Type section. If your change tracking client is deployed in a VPC, we recommend that you select the same VPC and vSwitch connected to the client. If you track data changes over internal networks, the network latency is minimal.

In the lower part of the page, click Test Connectivity and Proceed.
If the source database instance 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 in the corresponding region to the whitelist of the instance. If the source database instance is a self-managed database hosted on an ECS instance, DTS automatically adds the CIDR blocks of DTS servers in the corresponding region to the security group rules of the ECS instance. To allow DTS to access the database, you must also manually add the CIDR blocks of DTS servers in the corresponding region to the security settings of the database. If the source database instance 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 in the corresponding region to the security settings 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 public CIDR blocks of DTS servers are automatically or manually added to the whitelist of a database instance or to the security group rules of an ECS instance, security risks may arise. Therefore, before you use DTS to track data changes, 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 security group rules and forbidding unauthorized CIDR blocks, or connecting the database instance to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects for change tracking and advanced settings.

Basic Settings

Parameter	Description
Data Change Types	Data Update DTS tracks data updates of the selected objects, including the INSERT, DELETE, and UPDATE operations. Schema Update DTS tracks the create, delete, and modify operations that are performed on all object schemas of the source instance. You must use the change tracking client to filter the data to be tracked.
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 track data changes only at the table level. After the change tracking task is configured, the objects for change tracking cannot be reselected. If you want to track the data changes of tables that are not included in the selected objects, you must create another change tracking task for the tables.

Advanced Settings

Parameter	Description
Monitoring and Alerting	Specifies whether to enable alerting for the change tracking task. If alerting is configured and the task fails or the latency exceeds the threshold, alert notifications are sent. Valid values: No: does not enable alerting. Yes: enables alerting. In this case, you must also configure the alert threshold and alert notification settings. 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 change tracking task fails, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS is reconnected to the source database instance within the specified time range, DTS resumes the change tracking task. Otherwise, the change tracking task fails. Note If multiple change tracking tasks are configured for a database instance, the shortest retry time range takes precedence. For example, Task A and Task B are configured for the same database instance. Task A is configured with a retry time range of 30 minutes, and Task B is configured with a retry time range of 60 minutes. In this case, the retry time range of 30 minutes takes precedence. When DTS retries a connection, fees are charged. We recommend that you specify the retry time range based on your business requirements, or release the DTS instance at the earliest opportunity after the source database instance is released.

Click Next: Save Task Settings and Precheck in the lower part of the page.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameter settings of the API operation that is called to configure the instance.
Note
- Before you can start the change tracking task, DTS performs a precheck. You can start the change tracking task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you troubleshoot the issues based on the error message, you can run a precheck again.
- If an alert is generated for an item during the precheck, perform the following operations based on the scenario:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
  If the 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 Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase page, specify the billing method of the change tracking instance. The following table describes the parameters.

Parameter	Description
Billing method	Subscription: You pay for your subscription when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. You are offered lower prices for longer subscription durations. Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. We recommend that you select the pay-as-you-go billing method for short-term use. If you no longer require a pay-as-you-go instance, you can release the instance to reduce costs.
Resource Group Settings	The resource group to which the instance belongs. Default value: default resource group. For more information, see What is Resource Management?
Subscription Duration	If you select the subscription billing method, set the subscription duration and the number of 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 to start the change tracking task. You can view the progress of the task in the task list.

What to do next

When the change tracking task is running, you can create consumer groups based on the downstream client to consume the tracked data.

For more information about how to create and manage consumer groups, see Create consumer groups.
Use one of the following methods to consume the tracked data: