Data Transmission Service:Synchronize data from a PolarDB-X 2.0 instance to a MaxCompute project

Limits

Billing

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 Configure the DMS console based on your business requirements.
   • You can also go to the Data Synchronization Tasks page of the new DTS console.
2. From the drop-down list to the right 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 page that appears, configure the source and destination databases.

   Section	Parameter	Description
   N/A	Task Name	The name of the 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 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 select 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 PolarDB-X 2.0.
   	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the PolarDB-X instance resides.
   	Database Account	The database account of the source PolarDB-X instance. The account must have the SELECT permission on the objects to be synchronized, and the REPLICATION CLIENT and REPLICATION SLAVE permissions. Note For more information about how to grant the permissions to the database account, see Data synchronization tools for PolarDB-X.
   	Database Password	The password of the database account.
   Destination Database	Select an existing DMS database instance	The database instance that you want to use. You can choose whether to select 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 MaxCompute.
   	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the destination MaxCompute project resides.
   	Project	The name of the destination MaxCompute project. You can search for a project on the Workspaces page in the DataWorks console.
   	accessKeyId	The AccessKey ID of the account that you use to connect to the destination MaxCompute project. For more information about how to obtain the AccessKey ID, see Obtain an AccessKey pair.
   	accessSecret	The AccessKey secret of the account that you use to connect to the destination MaxCompute project. For more information about how to obtain the AccessKey secret, see Obtain an AccessKey pair.

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 ensure that the ECS instance can access the database. 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 to the security settings of on-premises databases.

   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. Click OK to grant permissions to your MaxCompute account.

6. Configure objects to synchronize and advanced settings.

   Parameter	Description
   Partition Definition of Incremental Data Table	Select the partition names based on your business requirements. For more information about partitions, see Partition.
   Synchronization Type	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 You can use the object name mapping feature to rename the tables that are synchronized to the destination database. You can use this feature if the source and destination databases contain identical table names and the tables in the destination database cannot be deleted or renamed. 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 schemas, and a data record has the same primary key value as an existing data record in the destination 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, only some columns are synchronized, or the data synchronization task fails. Operate with caution.
   Naming Rules of Additional Columns	After DTS synchronizes data to MaxCompute, DTS adds additional columns to the destination table. If the names of additional columns are the same as the names of existing columns in the destination table, data synchronization fails. Select New Rule or Previous Rule based on your business requirements. Warning Before you specify this parameter, check whether additional columns and existing columns in the destination table have name conflicts. For more information about the naming rules and definitions of additional columns, see Naming rules for additional columns.
   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 columns, tables, or databases as the objects to be synchronized. If you select tables or columns as the objects to be synchronized, DTS does not synchronize other objects such as views, triggers, or 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 Map the name of a single object. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time. Note To select the SQL operations performed on a specific database or table, perform the following steps: In the Selected Objects section, right-click an object. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information about the SQL operations that can be synchronized, 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 Use SQL conditions to filter data.

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

   Parameter	Description
   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 configure 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.
   Specify the retry time range 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 the 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 set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set 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 configure 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.
   Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Specifies whether to write SQL operations on heartbeat tables to the source database while the DTS instance is running. Yes: does not write SQL operations on heartbeat tables. In this case, a latency of the DTS instance may be displayed. No: writes SQL operations on heartbeat tables. In this case, specific features such as physical backup and cloning of the source database may be affected.

8. 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 data synchronization task, DTS performs a precheck. You can start the data synchronization 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 causes, run a precheck again.
   • 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 issues. 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 Purchase Instance 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 the instance 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. Pay-as-you-go: A pay-as-you-go instance is charged 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 instance, you can release the pay-as-you-go instance to reduce costs.
    	Resource Group	The resource group on which the instance is run. Default value: default resource group. For more information, see What is Resource Management?.
    	Instance Class	DTS provides various synchronization specifications that provide different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business scenario. For more information, see Specifications of data synchronization instances.
    	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.

11. Read and select the check box to agree to the Data Transmission Service (Pay-as-you-go) Service Terms.
12. Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.

Schema of an incremental data table

DTS synchronizes incremental data that is generated in the source PolarDB-X 2.0 instance to the incremental data table in MaxCompute. The incremental data table stores incremental data and specific metadata. The following figure shows the schema of an incremental data table.

Schema of an incremental data table

Additional information about the before_flag and after_flag fields

The before_flag and after_flag fields of an incremental log entry are defined depending on the operation type.

• INSERT

  For an INSERT operation, the column values are the newly inserted record values (post-update values). The value of the before_flag field is N and the value of the 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 record_id, operation_flag, and utc_timestamp fields.

  The first log entry records the pre-update values, so the value of the before_flag field is Y and the value of the after_flag field is N. The second log entry records the post-update values, so the value of the before_flag field is N and the value of the 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 before_flag field is Y and the value of the after_flag field is N.

  

Merge a full baseline table and an incremental data table

After a data synchronization task is started, DTS creates a full baseline table and an incremental data table in MaxCompute. You can use SQL statements to merge the two tables. This allows you to obtain the full data at a specific time point.

This section describes how to merge data for a table named customer. The following figure shows the schema of the customer table.

1. Create a table in MaxCompute based on the schema of the source table. The table is used to store the merged data.

   For example, you can obtain full data of the customer table at the 1565944878 time point. Run the following SQL statements to create the required table:

   CREATE TABLE `customer_1565944878` (
       `id` bigint NULL,
       `register_time` datetime NULL,
       `address` string);

   Note

   • You can use the ad-hoc query feature to run SQL statements. For more information, see Use the ad-hoc query feature to execute SQL statements (optional).
   • For more information about the data types that are supported by MaxCompute, see Data type editions.
2. Run the following SQL statements in MaxCompute to merge the full baseline table and the incremental data table and obtain full data at a specific time point:

   set odps.sql.allow.fullscan=true;
   insert overwrite table <result_storage_table>
   select <col1>,
          <col2>,
          <colN>
     from(
   select row_number() over(partition by t.<primary_key_column>
    order by record_id desc, after_flag desc) as row_number, record_id, operation_flag, after_flag, <col1>, <col2>, <colN>
     from(
   select incr.record_id, incr.operation_flag, incr.after_flag, incr.<col1>, incr.<col2>,incr.<colN>
     from <table_log> incr
    where utc_timestamp< <timestamp>
    union all
   select 0 as record_id, 'I' as operation_flag, 'Y' as after_flag, base.<col1>, base.<col2>,base.<colN>
     from <table_base> base) t) gt
   where row_number=1 
     and after_flag='Y'

   Note

   • <result_storage_table>: the name of the table that stores the merged data.
   • <col1>/<col2>/<colN>: the names of the columns in the table to be merged.
   • <primary_key_column>: the name of the primary key column in the table to be merged.
   • <table_log>: the name of the incremental data table.
   • <table_base>: the name of the full baseline table.
   • <timestamp>: the timestamp that is generated when full data is obtained.

   Run the following SQL statements to obtain full data of the customer table at the 1565944878 time point:

   set odps.sql.allow.fullscan=true;
   insert overwrite table customer_1565944878
   select id,
          register_time,
          address
     from(
   select row_number() over(partition by t.id
    order by record_id desc, after_flag desc) as row_number, record_id, operation_flag, after_flag, id, register_time, address
     from(
   select incr.record_id, incr.operation_flag, incr.after_flag, incr.id, incr.register_time, incr.address
     from customer_log incr
    where utc_timestamp< 1565944878
    union all
   select 0 as record_id, 'I' as operation_flag, 'Y' as after_flag, base.id, base.register_time, base.address
     from customer_base base) t) gt
    where gt.row_number= 1
      and gt.after_flag= 'Y';

3. Query the merged data from the customer_1565944878 table.