Synchronize data from PolarDB-X 2.0 to SelectDB - Data Transmission Service

This topic describes how to use Data Transmission Service (DTS) to synchronize data from a PolarDB-X 2.0 instance to a SelectDB instance.

Important

This feature is in the grayscale testing phase and is available only to some users.

Prerequisites

You have created a source PolarDB-X 2.0 instance and a destination SelectDB instance.

Precautions

Type	Description
Limits on source database	Bandwidth requirements: The server on which the source database is deployed must have sufficient outbound bandwidth (greater than or equal to 100 Mbit/s). Otherwise, the data synchronization speed is affected. The source database does not support Enterprise Edition PolarDB-X 2.0 read-only instances. Requirements for the objects to be synchronized: Tables whose names contain uppercase letters cannot be synchronized. INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FK cannot be synchronized. The data synchronization instance does not support the TABLEGROUP and the database or schema that contains Locality attribute. The tables to be migrated 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 synchronize and you need to edit tables (such as renaming tables or columns) in the destination database, 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 in batches or configure a task to synchronize the entire database. Binary logging is enabled for PolarDB-X 2.0 instances by default. You must make sure that the binlog_row_image parameter is set to full. Otherwise, error messages are returned during the precheck, and the data synchronization task cannot be started. For more information, see Parameter settings. 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 network information of the PolarDB-X 2.0 instance changes, the synchronization instance may experience latency for a period of time.
Other limits	DTS periodically attempts to execute the CREATE DATABASE IF NOT EXISTS `test` command in the source database to create the `test` database and write heartbeat data to advance the binlog position. If you select Yes for Whether to delete SQL operations on heartbeat tables of forward and reverse tasks when you configure the synchronization instance (or the source database account used by the synchronization instance does not have the permission to create databases), and no DML operations are performed on the source database for a long time, the latency information may be inaccurate. Note If the synchronization instance displays a large latency, you can perform a DML operation on the source database to update the latency information. You can specify only the `bucket_count` parameter in the Selected Objects section. Note The `bucket_count` parameter value must be a positive integer. Default value: auto. Currently, data can only be synchronized to Unique or Duplicate engine tables in the SelectDB instance. Destination table is a Unique engine table If the destination table is a Unique engine table, make sure that all unique keys in the destination table exist in the source table and are included in the objects to be synchronized. Otherwise, data inconsistency may occur. Destination table is a Duplicate engine table If the destination table is a Duplicate engine table, duplicate data may appear in the destination database in the following scenarios. You can perform deduplication operations based on the additional columns (_is_deleted, _version, and _record_id): The synchronization instance has been retried. The synchronization instance has been restarted. After the synchronization instance starts running, the same data to be synchronized has been modified by DML operations twice or more. Note When the destination table is a Duplicate engine table, DTS converts UPDATE or DELETE statements to INSERT statements. During data synchronization, do not create clusters in the destination ApsaraDB for SelectDB instance. Otherwise, the data synchronization task fails. You can restart the data synchronization instance to resume the data synchronization task that failed. ApsaraDB for SelectDB instances support only database names and table names that start with letters. If the name of a database or table that you want to synchronize does not start with a letter, you must use the object name mapping feature to rename the database or table. If the name of the object that you want to synchronize, such as a database, a table, or a column, contains Chinese characters, you must use the object name mapping feature to rename the object. For example, you can change the name from Chinese to English. Otherwise, the task may fail. You cannot modify DDL operations on multiple columns at a time or modify DDL operations on a table consecutively. During data synchronization, do not add backend nodes to the ApsaraDB for SelectDB database. Otherwise, the task fails. You can restart the data synchronization instance to resume the data synchronization task that failed. In a PolarDB-X 2.0 instance, M in `VARCHAR(M)` represents the character length. In SelectDB, N in `VARCHAR(N)` represents the byte length. If you do not use the schema synchronization feature provided by DTS, we recommend that you set the length of VARCHAR fields in SelectDB to four times the length of VARCHAR fields in the PolarDB-X 2.0 instance. During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. Before you synchronize data, assess the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours, such as when the CPU usage of the source and destination databases is lower than 30%. Because concurrent INSERT operations are performed during full initialization, the tables in the destination database may have fragments. Therefore, after full initialization is complete, the tables in the destination database occupy more storage space than those in the source database. During the synchronization instance operation, if data sources other than DTS write data to the destination database, data inconsistency may occur between the source and destination databases, and the synchronization instance may fail. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

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.

SQL operations that can be synchronized during incremental synchronization

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	ADD COLUMN DROP COLUMN and DROP TABLE TRUNCATE TABLE RENAME TABLE Important The RENAME TABLE operation may cause data inconsistency between the source and destination databases. For example, if you select a table as the object to be synchronized and rename the table during data synchronization, the data of this table is not synchronized to the destination database. To prevent this situation, select the database to which this table belongs as the object to be synchronized when you configure the data synchronization task. Make sure that the databases to which the table belongs before and after the RENAME TABLE operation are added to the objects to be synchronized.

Permissions required for database accounts

Database	Required permissions	Creation and authorization methods
Source PolarDB-X 2.0	REPLICATION SLAVE, REPLICATION CLIENT, and the SELECT permission on the objects to be synchronized.	Manage database accounts and Account permission issues during data synchronization.
Destination SelectDB	Cluster access permission (Usage_priv) and database read and write permissions (Select_priv, Load_priv, Alter_priv, Create_priv, Drop_priv).	Cluster permission management and Basic permission management.

Procedure

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization task resides.
DMS console
Note
The actual 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.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.

Configure the source and destination databases. The following table describes the parameters.

Category	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 Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select PolarDB-X 2.0.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB-X 2.0 instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Instance ID	Select the ID of the source PolarDB-X 2.0 instance.
	Database Account	Enter the database account of the source PolarDB-X 2.0 instance. For information about the permissions required for the account, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select SelectDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination SelectDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Instance ID	Select the ID of the destination SelectDB instance.
	Database Account	Enter the database account of the destination SelectDB instance. For information about the permissions required for the account, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.

In the lower part of the page, click Test Connectivity and Proceed.
Note
Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.

Configure the objects to be synchronized.

In the Configure Objects step, configure the objects that you want to synchronize.

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. Important When data is synchronized from a PolarDB-X 2.0 instance to SelectDB, type conversion is performed. If you do not select Schema Synchronization, you need to create Unique or Duplicate model tables with corresponding structures in the destination SelectDB in advance. For more information, see Data type mapping, Additional column information, and Data model.
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 instance fails. Proceed with caution.
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. If you select Schema Synchronization for the Synchronization Types parameter, you can select only tables as the objects to be synchronized and the `bucket_count` parameter must be set. Right-click a table to be migrated in the Selected Objects section, select Yes for the Enable Parameter Settings parameter in the Parameter Settings section that appears, specify Value based on your business requirements, and click OK. Note To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, 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
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization instances, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
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 this 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 Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note You can configure this parameter only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
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. Valid values: 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, features such as physical backup and cloning of the source database may be affected.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is selected.
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.
Monitoring and Alerting	Specifies whether to configure alerting for the data synchronization instance. 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.

Optional. In the lower part of the page, click Next: Configure Database and Table Fields. In the dialog box that appears, specify Primary Key Column, Distribution Key, and Engine parameters for the tables that you want to synchronize.
Note
- This step is available only if you select Schema Synchronization for the Synchronization Types parameter. You can set Definition Status to All and modify the database and table fields.
- You can select multiple columns for the Primary Key Column parameter from the drop-down list. One or more columns specified for the Primary Key Column parameter can be selected for the Distribution Key parameter.
- If the table does not have primary keys or UNIQUE constraints, select duplicate for the Engine parameter. Otherwise, the data synchronization task may fail or data loss may occur.

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.

Purchase an instance.

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 task. 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 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.

Data type mapping

Category	Data type in PolarDB-X 2.0	Data type in SelectDB
Numeric	TINYINT	TINYINT
	TINYINT UNSIGNED	SMALLINT
	SMALLINT	SMALLINT
	SMALLINT UNSIGNED	INT
	MEDIUMINT	INT
	MEDIUMINT UNSIGNED	BIGINT
	INT	INT
	INT UNSIGNED	BIGINT
	BIGINT	BIGINT
	BIGINT UNSIGNED	LARGEINT
	BIT(M)	INT
	Decimal	Decimal Note zerofill is not supported.
	Numeric	Decimal
	Float	Float
	Double	DOUBLE
	BOOL BOOLEAN	BOOLEAN
DATE AND TIME	DATE	DATEV2
	DATETIME[(fsp)]	DATETIMEV2
	Timestamp[(fsp)]	DATETIMEV2
	Time[(fsp)]	VARCHAR
	YEAR[(4)]	INT
STRING	CHAR VARCHAR	VARCHAR Important To avoid data loss, CHAR and VARCHAR(n) data types are converted to VARCHAR(4*n) when synchronized to SelectDB. If the data length is not specified, the default value VARCHAR(65533) of SelectDB is used. If the data length exceeds 65533, the data is converted to STRING when synchronized to SelectDB.
	BINARY VARBINARY	STRING
	TINYTEXT TEXT MEDIUMTEXT LONGTEXT	STRING
	TINYBLOB BLOB MEDIUMBOLB LONGBLOB	STRING
	ENUM	STRING
	SET	STRING
	JSON	STRING

Additional column information

Note

The following table describes the additional columns that DTS automatically adds or that you need to manually add to the destination Duplicate model table.

Name	Data type	Default value	Description
_is_deleted	Int	0	Indicates whether the data is deleted. Insert: The value is 0. Update: The value is 0. Delete: The value is 1.
_version	Bigint	0	Data synchronized during full synchronization: The value is 0. Data synchronized during incremental synchronization: The timestamp (in seconds) in the source database binlog.
_record_id	Bigint	0	Data synchronized during full synchronization: The value is 0. Data synchronized during incremental synchronization: The record ID of the incremental log, which is the unique identifier of the log. Note The ID value is unique and incremental.

Prerequisites

Precautions

Destination table is a Unique engine table

Destination table is a Duplicate engine table