How to migrate between PolarDB for MySQL - Data Transmission Service

Note

Currently, you cannot upgrade a PolarDB for MySQL cluster to MySQL 8.0. Instead, you can create a new PolarDB for MySQL cluster that runs MySQL 8.0 and use this method to migrate data from your original cluster to the new cluster. For cross-version migrations, create a pay-as-you-go PolarDB for MySQL cluster to test compatibility. You can release the cluster after the test is complete.

Prerequisites

You have created the source and destination PolarDB for MySQL clusters. For more information, see Create a pay-as-you-go cluster and Create a subscription cluster.
Note
For information about the supported database versions, see Migration solutions.
The destination PolarDB for MySQL cluster must have more storage space than the source PolarDB for MySQL cluster uses.

Limitations

Note

During schema migration, DTS migrates foreign keys from the source database to the destination database.
During full data migration and incremental data migration, DTS temporarily disables constraint checks and foreign key cascade operations at the session level. Performing cascade update or delete operations on the source database while the task is running may cause data inconsistency.

Type	Description
Source database limits	Bandwidth requirements: The server that hosts the source database must have sufficient outbound bandwidth. Otherwise, the data migration speed is affected. The tables to be migrated must have a primary key or a UNIQUE constraint, and the fields in the key or constraint must be unique. Otherwise, duplicate data may appear in the destination database. If you migrate data at the table level and need to edit the tables, such as mapping column names, a single data migration task can migrate a maximum of 1,000 tables. If you exceed this limit, an error is reported after you submit the task. In this case, split the tables into multiple migration tasks or configure a task to migrate the entire database. If you perform incremental migration: You must enable binary logging and set the loose_polar_log_bin parameter to on. Otherwise, the precheck reports an error and the data migration task cannot start. For more information about how to enable binary logging and modify parameters, see Enable binary logging and Modify parameters. Note Enabling binary logging for a PolarDB for MySQL cluster consumes storage space and incurs storage fees. The binary logs of the PolarDB for MySQL cluster must be retained for at least 3 days. We recommend a retention period of 7 days. Otherwise, DTS may fail to obtain the binary logs, which can cause the task to fail. In extreme cases, this can lead to data inconsistency or data loss. Issues caused by a binary log retention period shorter than the DTS requirement are not covered by the DTS Service-Level Agreement (SLA). Note For more information about how to set the retention period for binary logs of a PolarDB for MySQL cluster, see Modify the retention period. Operational limits for the source database: During schema migration and full data migration, do not perform DDL operations that change the database or table schema. Otherwise, the data migration task fails. Note During full data migration, DTS queries the source database. This creates a metadata lock, which may block DDL operations on the source database. If you perform only full data migration, do not write new data to the source instance. Otherwise, data inconsistency occurs between the source and destination. To maintain real-time data consistency, select schema migration, full data migration, and incremental data migration.
Other limits	We recommend that the source and destination PolarDB for MySQL instances run the same MySQL version to ensure compatibility. DTS does not support migrating parsers defined using comment syntax. DTS does not support the migration of read-only nodes of the source PolarDB for MySQL instance. DTS does not support the migration of OSS external tables from the source PolarDB for MySQL instance. DTS does not support the migration of INDEX and PARTITION. DTS does not support primary/standby switchover scenarios for the database instance during full data migration. In such a scenario, reconfigure the migration task promptly. If your source database uses temporary-table-mode online DDL operations—including multi-table merge scenarios—or adds function-based indexes to unique key columns, data loss or task failure may occur on the destination database. If a primary key or unique key conflict occurs during migration: If the table schemas are consistent and a record in the destination database has the same primary key value as a record in the source database: During full migration, DTS keeps the record in the destination database. The record from the source database is not migrated. During incremental migration, DTS does not keep the record in the destination database. The record from the source database overwrites the record in the destination database. If the table schemas are inconsistent, only some columns of data may be migrated, or the migration may fail. Proceed with caution. If your data includes four-byte characters—such as rare Chinese characters or emojis—the destination database and table must use the utf8mb4 charset. Note If you use DTS to migrate schemas, set the instance-level parameter `character_set_server` to utf8mb4 in the destination database. Before you perform data migration, evaluate the performance of the source and destination databases. We also recommend that you perform data migration during off-peak hours. Otherwise, DTS consumes read and write resources on both the source and destination databases during full data migration, which may increase the database load. Because full data migration involves concurrent INSERT operations, table fragmentation occurs in the destination database. As a result, the table storage space in the destination database is larger than that in the source instance after full migration is complete. Confirm whether the migration precision for columns of the FLOAT or DOUBLE data type meets your business requirements. DTS reads the values of these columns using `ROUND(COLUMN,PRECISION)`. If you do not explicitly define the precision, DTS uses a default precision of 38 for FLOAT and 308 for DOUBLE. DTS attempts to recover failed tasks within seven days. Therefore, before you switch your business to the destination instance, you must end or release the task. Alternatively, revoke the write permissions of the database account that DTS uses to access the destination instance using the `revoke` command. This prevents the source data from overwriting the data in the destination instance if the task is automatically recovered. If a DDL statement fails to be written to the destination database, the DTS task continues to run. You need to check the task logs for the failed DDL statement. For more information about how to view task logs, see Query task logs. To migrate database accounts from the source, meet the required prerequisites and review related considerations. For more information, see Migrate database accounts. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.
Other notes	DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset.

Billing

Migration type	Instance configuration fee	Internet traffic fee
Schema migration and full data migration	Free of charge.	When the Access Method parameter of the destination database is set to Public IP Address, you are charged for Internet traffic. For more information, see Billing overview.
Incremental data migration	Charged. For more information, see Billing overview.

Migration types

Schema migration

Data Transmission Service (DTS) migrates the schema definitions of the migration objects from the source database to the destination database.
- DTS supports schema migration for tables, views, triggers, stored procedures, and functions.
  
  Note
  The routine_body of stored procedures, the routine_body of functions, and the select_statement of views are not modified.
- During schema migration, DTS changes the `DEFINER` to `INVOKER` for the views, stored procedures, and functions to be migrated. This action changes the value of `SQL SECURITY` to `INVOKER`. DTS also sets the `DEFINER` to the destination database account that is used for the migration task.
  
  Note
  The security authentication method and definer of the source database are not modified.
- Because DTS does not migrate user information, you must grant read and write permissions to the invoker to call views, stored procedures, and functions in the destination database.
Full migration

DTS migrates all historical data of the specified migration objects from the source database to the destination database.
Incremental migration

After a full migration is complete, DTS migrates incremental data updates from the source database to the destination database. Incremental migration lets you smoothly migrate data without interrupting your self-managed applications.

SQL operations for incremental migration

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	ALTER TABLE, ALTER VIEW CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, CREATE VIEW DROP INDEX, DROP TABLE RENAME TABLE Important A RENAME TABLE operation may cause data inconsistency. For example, if you select only one table as the migration object and rename the table in the source instance during migration, the data of this table is not migrated to the destination database. To prevent this issue, select the entire database to which the table belongs as the migration object 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 both included in the migration objects. TRUNCATE TABLE

Database account permissions

Database	Permissions
Source PolarDB for MySQL cluster	Read permissions on the objects to be migrated
Destination PolarDB for MySQL cluster	Read and write permissions on the destination database

For PolarDB for MySQL clusters, see Create and manage database accounts.

Procedure

Navigate to the migration task list page for the destination region using one of the following methods.
From the DTS console
1. Log on to the Data Transmission Service (DTS) console.
2. In the navigation pane on the left, click Data Migration.
3. In the upper-left corner of the page, select the region where the migration instance is located.
From the DMS console

Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.
1. Log on to the Data Management (DMS) console.
2. In the top menu bar, choose Data + AI > Data Transmission (DTS) > Data Migration.
3. To the right of Data Migration Tasks, select the region where the migration instance is located.
Click Create Task to navigate to the task configuration page.

Configure the source and destination databases.

Warning

After you select the source and destination instances, we recommend that you carefully read the limits displayed at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

Category	Configuration	Description
None	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured. Note In the DMS console, this parameter is named Select a DMS database instance.. If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB for MySQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB for MySQL cluster resides.
	PolarDB Cluster ID	Enter the ID of the source PolarDB for MySQL cluster.
	Database Account	Enter the database account of the source PolarDB for MySQL cluster. For information about the required permissions, see Database account permissions.
	Database Password	Enter the password for the database account.
	Encryption	Select a connection type as needed. For more information about the SSL encryption feature, see Enable SSL encryption.
Destination Database	Select Existing Connection	To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured. Note In the DMS console, this parameter is named Select a DMS database instance.. If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB for MySQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination PolarDB for MySQL cluster resides.
	PolarDB Cluster ID	Select the ID of the destination PolarDB for MySQL cluster. Note If the destination is a Multi-master Cluster (Limitless) in Read/Write (Automatic Read/Write Splitting) mode, you must add the primary node for the destination database and table to the read/write endpoint, or select all primary nodes. This is required because Multi-master Clusters use a database and table-based routing mechanism. Evaluate the routing impact before selecting nodes. For more information, see Configure a database proxy.
	Database Account	Enter the database account of the destination PolarDB for MySQL cluster. For information about the required permissions, see Database account permissions. Important Use a privileged account.
	Database Password	Enter the password for the database account.
	Encryption	Select a connection type as needed. For more information about the SSL encryption feature, see Enable SSL encryption.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that the IP address segment of the DTS service is 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 DTS server IP addresses to a whitelist.
- If the source or destination database is a self-managed database (the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box that appears.

Configure the task objects.

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

Configuration	Description
Migration Types	If you only need to perform a full migration, select both Schema Migration and Full Data Migration. To perform a migration with no downtime, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you do not select Schema Migration, you must ensure that a database and tables to receive the data exist in the destination database. You can also use the object name mapping feature in the Selected Objects box as needed. If you do not select Incremental Data Migration, do not write new data to the source instance during data migration to ensure data consistency.
Method to Migrate Triggers in Source Database	Select a method to migrate triggers as needed. If the objects to migrate do not involve triggers, you do not need to configure this parameter. For more information, see Configure a method to synchronize or migrate triggers. Note This parameter is available only when you select Schema Migration for Migration Types.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks whether tables with the same names exist in the destination database. If no tables with the same names exist, the precheck is passed. If tables with the same names exist, an error is reported during the precheck, and the data migration task does not start. Note If a table in the destination database has the same name but cannot be easily deleted or renamed, you can change the name of the table in the destination database. For more information, see Object name mapping. Ignore Errors and Proceed: Skips the check for tables with the same names. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and business risks. For example: If the table schemas are consistent and a record in the destination database has the same primary key value as a record in the source database: During full migration, DTS keeps the record in the destination database. The record from the source database is not migrated. During incremental migration, DTS does not keep the record in the destination database. The record from the source database overwrites the record in the destination database. If the table schemas are inconsistent, only some columns of data may be migrated, or the migration may fail. Proceed with caution.
Whether to migrate Event	Select whether to migrate events from the source database based on your requirements. If you select Yes, you must also follow the relevant requirements and perform additional steps. For more information, see Synchronize or migrate events.
Source Objects	Select one or more objects from the Source Objects section. Click the icon and add the objects to the Selected Objects section. Note The granularity for selecting migration objects is schema, table, and column. If you select only tables or columns as migration objects, other objects such as views, triggers, and stored procedures are not migrated to the destination database.
Selected Objects	To change the name of a single migration object in the target instance, right-click the object in the Selected Objects box. For more information, see Map individual schema, table, and column names. To change the names of multiple migration objects in the target instance, click Selected Objects in the upper-right corner of the Batch Edit box. For more information, see Map multiple schema, table, and column names. Note If you use the object name mapping feature, the migration of other objects that depend on the mapped object may fail. To filter data by using a WHERE clause, right-click the table to migrate in the Selected Objects box and set the filter condition in the dialog box that appears. For more information about how to set the condition, see Set filter conditions. To select the SQL operations to migrate at the database or table level, right-click the object to migrate in the Selected Objects box and select the desired SQL operations in the dialog box that appears. For information about the supported operations, see SQL operations supported for incremental data migration.

Click Next: Advanced Settings to configure advanced parameters.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules tasks on a shared cluster. You do not need to select one. If you want more stable tasks, you can purchase a dedicated cluster to run DTS migration tasks.
Select the engine type of the destination database	Select an engine type for the destination database as needed. InnoDB: The default storage engine. X-Engine: A storage engine for On-Line Transaction Processing (OLTP) databases.
Copy the temporary table of the Online DDL tool that is generated in the source table to the destination database.	If you use Data Management (DMS) or gh-ost to perform online DDL changes in the source database, you can choose whether to migrate the data from the temporary tables generated by the online DDL changes. Important DTS tasks do not support using tools such as pt-online-schema-change to perform online DDL changes. Otherwise, the DTS task fails. The processing methods for each phase are as follows: The Schema Migration and Full Data Migration phases do not allow DDL operations that change the database or table structure. Therefore, they are not controlled by the online DDL policy. Schema Migration: Not controlled by the online DDL policy. Related temporary tables are created. Full Data Migration: Not controlled by the online DDL policy. The migration of temporary tables is not included in the full migration objects. All tables whose names match the regular expression (`^_(.+)_(?:gho\|new)$` or `^_(.+)_(?:ghc\|del\|old)$`) are filtered out. Incremental Data Migration: Controlled by the online DDL policy. Yes: Migrates data changes from temporary tables (for example, `_table_name_gho`) generated by online DDL operations. No, Adapt to DMS Online DDL and No, Adapt to gh-ost: Filters out data changes from temporary tables (for example, `_table_name_gho`) generated by tools such as gh-ost based on regular expression rules. Yes: Migrates the data from the temporary tables generated by online DDL changes. Note If online DDL changes generate a large amount of data in temporary tables, it may cause task latency. No, Adapt to DMS Online DDL: Does not migrate the data from the temporary tables generated by online DDL changes. It only migrates the original DDL statements executed using Data Management (DMS). Note This option causes tables in the destination database to be locked. No, Adapt to gh-ost: Does not migrate the data from the temporary tables generated by online DDL changes. It supports custom filtering rules. DTS filters out data changes from temporary tables (for example, `_table_name_gho`) generated by tools such as gh-ost based on regular expression rules. You can modify the default regular expressions used to match shadow and useless tables as needed: Shadow table: `^_(.+)_(?:gho\|new)$` Useless table: `^_(.+)_(?:ghc\|del\|old)$` Note This option causes tables in the destination database to be locked.
Whether to Migrate Accounts	Select whether to migrate account information from the source database as needed. If you select Yes, you must also select the accounts to migrate and confirm their permissions. For more information about authorization methods, see Migrate database accounts.
Retry Time for Failed Connections	After the migration task starts, if the connection to the source or destination database fails, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1440 minutes. We recommend that you set the duration to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified duration, the migration task automatically resumes. Otherwise, the task fails. Note For multiple DTS instances that share the same source or destination, the network retry time is determined by the setting of the last created task. Because you are charged for the task during the connection retry period, we recommend that you customize the retry time based on your business needs, or release the DTS instance as soon as possible after the source and destination database instances are released.
Retry Time for Other Issues	After the migration task starts, if a non-connectivity issue, such as a DDL or DML execution exception, occurs in the source or destination database, DTS reports an error and immediately begins to retry the operation. The default retry duration is 10 minutes. You can customize the retry time to a value from 1 to 1440 minutes. We recommend that you set the duration to more than 10 minutes. If the related operations succeed within the specified retry duration, the migration task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than the value of Retry Time for Failed Connections.
Enable Throttling for Full Data Migration	During full migration, DTS consumes read and write resources on the source and destination databases, which may increase the database load. If required, you can enable throttling for the full migration task. You can set Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) to reduce the load on the destination database. Note This configuration item is available only if you select Full Data Migration for Migration Types. You can also adjust the full migration speed after the migration instance is running.
Enable Throttling for Incremental Data Migration	If required, you can also choose to set speed limits for the incremental migration task. You can set RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) to reduce the load on the destination database. Note This configuration item is available only if you select Incremental Data Migration for Migration Types. You can also adjust the incremental migration speed after the migration instance is running.
Environment Tag	Select an environment tag to identify the instance as needed. For this example, you do not need to select a tag.
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Choose whether DTS writes heartbeat SQL information to the source database while the instance is running. Yes: Does not write heartbeat SQL information to the source database. The DTS instance may display latency. No: Writes heartbeat SQL information to the source database. This may interfere with source database operations like physical backups and cloning.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Select whether to set alerts and receive alert notifications based on your business needs. No: Does not set an alert. Yes: Configure alerts by setting an alert threshold and an alert notifications. If a migration fails or the latency exceeds the threshold, the system sends an alert notification.

Click Next: Data Validation to configure a data validation task.

For more information about the data validation feature, see Configure data validation.

Save the task and run a precheck.
- To view the parameters for configuring this instance when you call the API operation, move the pointer over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the bubble that appears.
- If you do not need to view or have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before the migration task starts, DTS performs a precheck. The task starts only after it passes the precheck.
- If the precheck fails, click View Details next to the failed check item, fix the issue based on the prompt, and then run the precheck again.
- If a warning is reported during the precheck:
  
  For check items that cannot be ignored, click View Details next to the failed item, fix the issue based on the prompt, and then run the precheck again.
  
  For check items that can be ignored, you can click Confirm Alert Details, Ignore, OK, and Precheck Again to skip the alert item and run the precheck again. If you choose to ignore a warning, it may cause issues such as data inconsistency and pose risks to your business.

Purchase an instance.

When the Success Rate is 100%, click Next: Purchase Instance.

On the Purchase page, select the link specification for the data migration instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Resource Group Settings	Select the resource group to which the instance belongs. The default value is default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides migration specifications with different performance levels. The link specification affects the migration speed. You can select a specification based on your business scenario. For more information, see Data migration link specifications.

After the configuration is complete, read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the OK dialog box that appears, click OK.

You can view the progress of the migration task on the Data Migration Tasks list page.
Note
- If the migration task does not include incremental migration, it stops automatically after the full migration is complete. After the task stops, its Status changes to Completed.
- If the migration task includes incremental migration, it does not stop automatically. The incremental migration task continues to run. While the incremental migration task is running, the Status of the task is Running.