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

This topic describes how to use Data Transmission Service (DTS) to migrate 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 to which the source database belongs must have sufficient outbound bandwidth (greater than or equal to 100 Mbit/s). Otherwise, the data migration speed is affected. The source database does not support read-only instances of the Enterprise Edition of PolarDB-X 2.0. Requirements for the objects to migrate: Tables whose names contain uppercase letters cannot be migrated. INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FK cannot be migrated. TABLEGROUP and databases or tables that contain the Locality attribute cannot be migrated. 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 be migrated and you need to modify the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables in a single data migration task. If you run a task to migrate more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to migrate the tables in batches or configure a task to migrate the entire database. Binary logging is enabled for PolarDB-X 2.0 instances by default. If you need to migrate incremental data, make sure that the binlog_row_image parameter is set to full. Otherwise, error messages are returned during precheck and the data migration task cannot be started. For more information, see Parameter settings. Limits on operations to be performed on the source database: During schema migration and full data migration, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data migration task fails. If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency between the source and destination databases occurs. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types. If the network information of the PolarDB-X 2.0 instance changes, the migration 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 binary log position. If you select Yes for Whether to delete SQL operations on heartbeat tables of forward and reverse tasks when you configure the migration instance (or if the source database account used by the migration instance does not have the permission to create databases), and no DML operations are performed in the source database for a long time, the latency information may be inaccurate. Note If the migration instance displays a large latency, you can perform a DML operation in the source database to update the latency information. Currently, data can be migrated only 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 migrated. 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 migration instance has been retried. The migration instance has been restarted. After the migration instance starts running, the same data to be migrated 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. 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. During data migration, do not create clusters in the destination ApsaraDB for SelectDB instance. Otherwise, the data migration task fails. You can restart the data migration instance to resume the data migration 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 migrate 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 migrate, 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 execute DDL statements to modify multiple columns at a time or modify the same table in succession when you run the migration instance. During data migration, do not add backend nodes to the ApsaraDB for SelectDB database. Otherwise, the task fails. You can restart the data migration instance to resume the task that failed. In a PolarDB-X 2.0 instance, M in `VARCHAR(M)` indicates the character length. In SelectDB, N in `VARCHAR(N)` indicates the byte length. If you do not use the schema migration 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 full data migration, DTS will consume certain read and write resources from both the source and destination databases, which might increase the database load. Therefore, we recommend that you assess the performance of the source and destination databases before executing data migration, and perform data migration during off-peak hours (for example, when the CPU load of the source and destination databases is below 30%). Because full data migration concurrently performs INSERT operations, tables in the destination database become fragmented. Therefore, after full data migration is complete, the tables in the destination database occupy more storage space than those in the source database. During the migration instance operation, if there are data sources other than DTS writing data to the destination database, it may cause data inconsistency between the source database and the destination database, or even cause the migration instance to 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

Migration type	Task configuration fee	Data transfer fee
Schema migration and full data migration	Free of charge.	Free of charge in this example. Note When Access Method of the destination database is Public IP Address, you are charged for the data transfer fee. For more information, see Billing overview.
Incremental data migration	Charged. For more information, see Billing overview.

SQL operations that can be synchronized during incremental migration

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	ADD COLUMN DROP COLUMN and DROP TABLE TRUNCATE TABLE RENAME TABLE Important RENAME TABLE operations may cause data inconsistency between the source and destination databases. For example, if you select a table as the object to be migrated and rename the table during data migration, the data of this table is not migrated to the destination database. To prevent this situation, you can select the database to which this table belongs as the object to be migrated when you configure the data migration 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 migrated.

Permissions required for database accounts

Database	Schema migration	Full migration	Incremental migration
Source PolarDB-X 2.0	The SELECT permission.	The SELECT permission.	The REPLICATION SLAVE permission, the REPLICATION CLIENT permission, and the SELECT permission on the objects to migrate.
Destination SelectDB	The cluster access permission (Usage_priv) and the read and write permissions on the database (Select_priv, Load_priv, Alter_priv, Create_priv, and Drop_priv).

For more information about how to create a database account and grant permissions to the account, see the following topics:

PolarDB-X 2.0 instance: Manage database accounts and Account permissions during data synchronization.
SelectDB instance: Cluster permission management and Basic permission management

Procedure

Use one of the following methods to go to the Data Migration page and select the region in which the data migration instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Migration.
3. In the upper-left corner of the page, select the region in which the data migration instance resides.
DMS console
Note
The actual operation 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 > DTS (DTS) > Data Migration.
3. From the drop-down list to the right of Data Migration 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 an informative 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 instance 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 instance.
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 instance 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 instance.

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 migrated.

On the Configure Objects page, configure the objects that you want to migrate.

Parameter	Description
Migration Types	To perform only full data migration, select both Schema Migration and Full Data Migration. To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration. When data is migrated from a PolarDB-X 2.0 instance to SelectDB, type conversion is performed. If you do not select Schema Migration, 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. If you do not select Incremental Data Migration, to ensure data consistency, do not write new data to the source instance during data migration.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables that use 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 migration 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 migrated 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 the following potential risks: If the source and destination databases have the same schema, and a data record has the same primary key as an existing data record in the destination database, the following scenarios may occur: 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 migration, DTS migrates 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, only specific columns are migrated or the data migration task 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 make sure that the capitalization of object names is consistent with that of 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. Click the icon to add the objects to the Selected Objects section. Note You can select tables or databases as the objects that you want to migrate.
Selected Objects	To change a table name to be used in the destination project, right-click the table in the Selected Objects section. For more information, see Map object names. If you select Schema Migration for the Migration Types parameter, you can select only tables as the objects to be migrated 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 the object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to migrate. For more information, see the SQL operations that support incremental migration section of this topic. 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 migrated.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the data migration task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data migration tasks, 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 migration task is started, DTS immediately retries a connection within the retry time range. Valid values: 10 to 1,440. 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 and destination databases within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Note If you specify different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is specified later 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 the earliest opportunity after the source database and destination instance are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry 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 retry time range, DTS resumes the data migration task. Otherwise, the data migration 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 Migration	Specifies whether to enable throttling for full data migration. During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. You can enable throttling for full data migration based on your business requirements. To configure throttling, you must 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. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Full Data Migration for the Migration Types parameter.
Enable Throttling for Incremental Data Migration	Specifies whether to enable throttling for incremental data migration. To configure throttling, you must configure the RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) parameters. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Incremental Data Migration for the Migration Types parameter.
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 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 migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts receive notifications. Valid values: No: does not configure 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 for the tables that you want to migrate.
Note
- This step is available only if you select Schema Migration for the Migration Types parameter. You can set the Definition Status parameter 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 migration 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 migration task, DTS performs a precheck. You can start the data migration 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 analyze the causes based on the check results, troubleshoot the issues. Then, 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 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.

Purchase an instance.

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

On the Purchase Instance page, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Resource Group	The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Instance classes of data migration instances.

Read and agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.
Click Buy and Start. In the message that appears, click OK.
You can view the progress of the task on the Data Migration page.
Note
- If a data migration task cannot be used to migrate incremental data, the task automatically stops. The Completed is displayed in the Status section.
- If a data migration task can be used to migrate incremental data, the task does not automatically stop. The incremental data migration task never stops or completes. The Running is displayed in the Status section.

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 migrated to SelectDB. If the data length is not specified, the default value of SelectDB, VARCHAR(65533), is used. If the data length exceeds 65533, the data is converted to STRING when migrated 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 provides additional columns that are automatically created by DTS or you manually add in the destination tables in the Duplicate key model.

Column	Type	Default value	Description
_is_deleted	Int	0	Specify whether the data is deleted. Insert: The data record is inserted. Default value: 0. Update: The data record is updated. Default value: 0. Delete: The data record is deleted. Default value: 1.
_version	Bigint	0	Full data migration: Default value: 0. Incremental data migration: The timestamp (seconds) of the source binary log file.
_record_id	Bigint	0	Full data migration: Default value: 0. Incremental data migration: The unique ID of the incremental log entry. Note The ID auto-increments for each new log entry.

Prerequisites

Precautions

Destination table is a Unique engine table

Destination table is a Duplicate engine table