ApsaraDB for OceanBase:Migrate data from a PolarDB-X 2.0 database to a MySQL tenant of OceanBase Database

Background

PolarDB-X is a cloud-based high-performance distributed database service that is independently developed by Alibaba Group. PolarDB-X provides high throughput, large storage capacity, low latency, high scalability, and high availability to meet your business requirements in the cloud era. For more information, see What is PolarDB for Xscale?

Prerequisites

• The data transmission service has the privilege to access cloud resources. For more information, see Grant privileges to roles for data transmission.

• You have created dedicated database users for data migration in the PolarDB-X 2.0 database and the MySQL tenant of OceanBase Database, and granted required privileges to the users. For more information, see Manage database accounts for a cluster and Create a database user.

Limitations

• Limitations on the source database

  Do not perform DDL operations that modify database or table schemas during schema migration or full migration. Otherwise, the data migration task may be interrupted.

• The data transmission service supports PolarDB-X 2.0 databases of version 5.4.x.

• When you migrate data from a PolarDB-X 2.0 database to a MySQL tenant of OceanBase Database, you cannot migrate tables of the time to live (TTL) type.

• A table to be migrated must have a PRIMARY KEY or UNIQUE constraint and contain unique fields to avoid data inconsistency between the source and target databases.

• The data transmission service supports the migration of an object only when the following conditions are met: the database name, table name, and column name of the object are ASCII-encoded without special characters. The special characters are line breaks, spaces, and the following characters: . | " ' ` ( ) = ; / & \.

• The data transmission service does not support triggers in the target database. If triggers exist in the target database, the data migration may fail.

• OceanBase Database does not support using an auto-increment column as the partitioning column. If a source table contains an auto-increment column, manually modify the table creation statement.

Considerations

• When you migrate broadcast tables from a PolarDB-X 2.0 database, data loss may occur, and data accuracy cannot be ensured. ​####

• If the UTF-8 character set is used in the source, we recommend that you use a compatible character set, such as UTF-8 or UTF-16, in the target to avoid garbled characters.

• If the clocks between nodes or between the client and the server are out of synchronization, the latency may be inaccurate during incremental synchronization or reverse incremental synchronization.

  For example, if the clock is earlier than the standard time, the latency can be negative. If the clock is later than the standard time, the latency can be positive.

• Check whether the migration precision of the data transmission service for columns of data types such as DECIMAL, FLOAT, and DOUBLE is as expected. If the precision of the target field type is lower than that of the source field type, the value with a higher precision may be truncated. This may result in data inconsistency between the source and target fields.

• The default configuration lower_case_table_names = 1 of the target database is used. Database objects are created in lowercase in the target database.

• The server where the source PolarDB-X 2.0 database resides must have sufficient outbound bandwidth to ensure the data migration efficiency.

• During full migration and incremental synchronization, the data transmission service temporarily disables constraint checks and cascade operations on foreign keys at the session level. Cascade update or delete operations performed in the source database may cause data inconsistency between the source and target databases.

• If you switch the network type of the PolarDB-X 2.0 database during data migration, you must modify the network connection information of the data migration task after the switch.

• If only full migration is configured for the data migration task, to avoid data inconsistency between the source and target databases, do not write new data to the source instance during the migration. To ensure real-time data consistency, we recommend that you select schema migration, full migration, and incremental synchronization when you configure a data migration task.

• If you need to perform incremental synchronization, take note of the following points about binlogs:

  • The data transmission service supports only binlog consumption and subscription in a single flow in PolarDB-X 2.0. For more information, see Binary logs.

  • You must enable binlogs and set binlog_row_image to full. Otherwise, errors will be returned during the precheck and the data migration task cannot be started.

  • If you selected only Incremental Synchronization when you created the data migration task, the data transmission service requires that the local incremental logs of the source database be retained for at least 48 hours.

    If you selected Full Migration and Incremental Synchronization when you created the data migration task, the data transmission service requires that the local incremental logs of the source database be retained for at least 7 days. If the data transmission service cannot obtain incremental logs, the data migration task may fail or even the data between the source and target databases may be inconsistent after migration.

    Note

    After full migration is completed, you can set the retention period of local incremental logs to more than 48 hours.

• Take note of the following considerations if you want to aggregate multiple tables:

  • We recommend that you configure the mappings between the source and target databases by specifying matching rules.

  • We recommend that you manually create schemas at the target. If you create a schema by using the data transmission service, skip the failed objects in the schema migration step.

• A difference between the source and target table schemas may result in data consistency. Some known scenarios are described as follows:

  • When you manually create a table schema at the target, if the data type of any column is not supported by the data transmission service, implicit data type conversion may occur at the target, which causes inconsistent column types between the source and target databases.

  • If the length of a column at the target is shorter than that in the source database, the data of this column may be automatically truncated, which causes data inconsistency between the source and target databases.

• If the source or target database contains table objects that differ only in letter cases, the data migration results may not be as expected due to case insensitivity in the source or target database.

Supported source and target instance types

In the following table, OB_MySQL stands for the MySQL tenant of OceanBase Database.

Data type mappings

Procedure

1. Log on to the ApsaraDB for OceanBase console and purchase a data migration task.

   For more information, see Purchase a data migration task.

2. Choose Data Transmission > Data Migration. On the page that appears, click Configuration for the data migration task.

   

   If you want to reference the configurations of an existing task, click Reference Configuration. For more information, see Reference the configuration of a data migration task.

3. On the Select Source and Target page, configure the parameters.

   Parameter	Description
   Migration Task Name	We recommend that you set it to a combination of digits and letters. It must not contain any spaces and cannot exceed 64 characters in length.
   Source	If you have created a PolarDB-X 2.0 data source, select it from the drop-down list. Otherwise, click New Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information about parameters, see Create a PolarDB-X 2.0 data source.
   Target	If you have created a data source for the MySQL tenant of OceanBase Database, select it from the drop-down list. Otherwise, click New Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information about the parameters, see Create an OceanBase data source.
   Tag (Optional)	Select a target tag from the drop-down list. You can also click Manage Tags to create, modify, and delete tags. For more information, see Use tags to manage data migration tasks.

4. Click Next. On the Select Migration Type page, specify migration types for the current data migration task.

   Supported migration types are schema migration, full migration, incremental synchronization, full verification, and reverse incremental synchronization.

   

   Migration type	Description
   Schema migration	After a schema migration task is started, the data transmission service migrates the definitions of database objects (such as tables, indexes, constraints, comments, and views) from the source database to the target database and automatically filters out temporary tables. Schema migration from a PoarDB-X 2.0 database to a MySQL tenant of OceanBase Database supports the DRDS and AUTO modes. Important In AUTO mode, a table without a primary key is converted into a non-partitioned table without a primary key after it is migrated to OceanBase Database. If you want to create partitions, manually specify the primary key and partitioning key.
   Full migration	Before data migration, assess the performance of the source and target databases. We recommend that you perform data migration during off-peak hours. During full migration, the data transmission service consumes some read and write resources in the source and target databases. This may increase the loads of the databases. For information about performance assessment, see Performance assessment of migration assessment. After a full migration task is started, the data transmission service migrates existing data of tables in the source database to corresponding tables in the target database.
   Incremental synchronization	After an incremental synchronization task is started, the data transmission service synchronizes changed data (data that is added, modified, or removed) from the source database to corresponding tables in the target database. DML Synchronization is supported for Incremental Synchronization. You can select operations as needed. For more information, see Configure DDL/DML synchronization.
   Full verification	After the full migration and incremental synchronization tasks are completed, the data transmission service automatically initiates a full verification task to verify the tables in the source and target databases. If you selected Incremental Synchronization but did not select all DML operations in the DML Synchronization section, you cannot select Full Verification. Before data migration, assess the performance of the source and target databases. We recommend that you perform data migration during off-peak hours. During full verification, the data transmission service consumes some read resources in the source and target databases. This may increase the loads of the databases.
   Reverse incremental synchronization	Data changes made in the target database after the business database switchover are synchronized to the source database in real time through reverse incremental synchronization. Generally, incremental synchronization configurations are reused for reverse incremental synchronization. You can also customize the configurations for reverse incremental synchronization as needed.

5. Click Next. On the Select Migration Objects page, specify migration objects for the data migration task.

   You can select Specify Objects or Match Rules to specify the migration objects. This topic describes how to specify the migration objects by using Specify Objects. For information about matching rules, see Configure and modify matching rules.

   Important

   • The names of tables to be migrated, as well as the names of columns in the tables, must not contain Chinese characters.

   • If a database or table name contains double dollar signs ($$), you cannot create the migration task.

   

   1. In the Select Migration Objects section, select Specify Objects.

   2. In the Source Object(s) list of the Specify Migration Scope section, select the objects to migrate. You can select tables and views of one or more databases.

   3. Click > to add them to the Target Object(s) list.

   The data transmission service allows you to import objects from text files, rename target objects, set row filters, view column information, and remove a single or all migration objects.

   Note

   When you select Match Rules to specify migration objects, object renaming is implemented based on the syntax of the specified matching rules. In the operation area, you can only set filter conditions. For more information, see Configure and modify matching rules.

   Operation	Description
   Import objects	In the list on the right, click Import Objects in the upper-right corner. In the dialog box that appears, click OK. Important This operation will overwrite previous selections. Proceed with caution. In the Import Objects dialog box, import the objects to be migrated. You can import CSV files to rename databases or tables and set row filtering conditions. For more information, see Download and import the settings of migration objects. Click Validate. After you import the migration objects, check their validity. Column field mapping is not supported at present. After the validation succeeds, click OK.
   Rename objects	The data transmission service allows you to rename migration objects. For more information, see Rename a database table.
   Configure settings	The data transmission service allows you to filter rows by using WHERE conditions. For more information, see Use SQL conditions to filter data. You can also view column information of the migration objects in the View Columns section.
   Remove one or all objects	The data transmission service allows you to remove a single object or all migration objects that are added to the right-side list during data mapping. Remove a single migration object In the list on the right, move the pointer over the object that you want to remove, and click Remove to remove the migration object. Remove all migration objects In the list on the right, click Remove All in the upper-right corner. In the dialog box that appears, click OK to remove all migration objects.

6. Click Next. On the Migration Options page, configure the parameters.

   • Full migration

     The following table describes the parameters for full migration, which are displayed only if you have selected Full Migration on the Select Migration Type page.

     

     Parameter	Description
     Read Concurrency	The concurrency for reading data from the source during full migration. The maximum value is 512. A high read concurrency may incur excessive stress on the source, affecting the business.
     Write Concurrency	The concurrency for writing data to the target during full migration. The maximum value is 512. A high write concurrency may incur excessive stress on the target, affecting the business.
     Full Migration Rate Limit	You can choose whether to limit the full migration rate as needed. If you choose to limit the full migration rate, you must specify the records per second (RPS) and bytes per second (BPS). The RPS specifies the maximum number of data rows migrated to the target per second during full migration, and the BPS specifies the maximum amount of data in bytes migrated to the target per second during full migration. Note The RPS and BPS values specified here are only for throttling. The actual full migration performance is subject to factors such as the settings of the source and target and the instance specifications.
     Handle Non-empty Tables in Target Database	Valid values are Ignore and Stop Migration. If you select Ignore, when the data to be inserted conflicts with existing data of a target table, the data transmission service logs the conflicting data while retaining the existing data. Important If you select Ignore, data is pulled in IN mode during full verification. In this case, verification is inapplicable if the target contains data that does not exist in the source, and the verification performance is downgraded. If you select Stop Migration and a target table contains records, an error prompting migration unsupported is reported during full migration. In this case, you must process the data in the target table before continuing with the migration. Important If you click Restore in the dialog box prompting the error, the data transmission service ignores this error and continues to migrate data. Proceed with caution.
     Post-indexing	Specifies whether to create indexes after the full migration is completed. Post-indexing can shorten the time required for full migration. For more information about the considerations on post-indexing, see the description below. Important This parameter is displayed only if you have selected both Schema Migration and Full Migration on the Select Migration Type page. Only non-unique key indexes can be created after the migration is completed. If the target OceanBase database returns the following error during index creation, the data transmission service ignores the error and determines that the index is successfully created, without creating it again. Error message in a MySQL tenant of OceanBase Database: Duplicate key name. Error message in an Oracle tenant of OceanBase Database: name is already used by an existing object. If the target is an OceanBase database and you select Allow for this parameter, the following parameters need to be set: Single Index DDL Concurrency Configuration: the maximum number of concurrent DDL operations allowed for a single index. A larger value indicates higher resource consumption and faster data migration. Maximum concurrent index DDL quantity configuration: the maximum number of post-indexing DDL operations that the system can call at a time.

     If post-indexing is allowed, we recommend that you use a CLI client to modify the following parameters for business tenants based on the hardware conditions of OceanBase Database and your current business traffic:

     // Specify the limit on the file memory buffer size.
     alter system set _temporary_file_io_area_size = '10' tenant = 'xxx'; 
     // Disable throttling in OceanBase Database V4.x.
     alter system set sys_bkgd_net_percentage = 100;  

   • Incremental synchronization

     The following table describes the parameters for incremental synchronization, which are displayed only if you have selected Incremental Synchronization on the Select Migration Type page.

     

     Parameter	Description
     Write Concurrency	The concurrency for writing data to the target during incremental synchronization. The maximum value is 512. A high write concurrency may incur excessive stress on the target, affecting the business.
     Incremental Synchronization Rate Limit	You can choose whether to limit the incremental synchronization rate as needed. If you choose to limit the incremental synchronization rate, you must specify the RPS and BPS. The RPS specifies the maximum number of data rows synchronized to the target per second during incremental synchronization, and the BPS specifies the maximum amount of data in bytes synchronized to the target per second during incremental synchronization. Note The RPS and BPS values specified here are only for throttling. The actual incremental synchronization performance is subject to factors such as the settings of the source and target and the instance specifications.
     Incremental Synchronization Start Timestamp	This parameter is not displayed if you have selected Full Migration on the Select Migration Type page. If you have selected Incremental Synchronization but not Full Migration, specify a point in time after which the data is to be synchronized. The default value is the current system time. For more information, see Set an incremental synchronization timestamp.

   • Reverse incremental synchronization

     The following table describes the parameters for reverse incremental synchronization, which are displayed only if you have selected Reverse Increment on the Select Migration Type page. By default, incremental synchronization configurations are reused for reverse incremental synchronization.

     

     You can choose not to reuse the incremental synchronization configurations and configure reverse incremental synchronization as needed.

     Parameter	Description
     Write Concurrency	The concurrency for writing data to the source during reverse incremental synchronization. The maximum value is 512. A high concurrency may incur excessive stress on the source, affecting the business.
     Reverse Increment Rate Limit	You can choose whether to limit the reverse incremental synchronization rate as needed. If you choose to limit the reverse incremental synchronization rate, you must specify the requests per second (RPS) and bytes per second (BPS). The RPS specifies the maximum number of data rows synchronized to the source per second during reverse incremental synchronization, and the BPS specifies the maximum amount of data in bytes synchronized to the source per second during reverse incremental synchronization. Note The RPS and BPS values specified here are only for throttling. The actual reverse incremental synchronization performance is subject to factors such as the settings of the source and target and the instance specifications.
     Incremental Synchronization Start Timestamp	This parameter is not displayed if you have selected Full Migration on the Select Migration Type page. If you have selected Incremental Synchronization but not Full Migration, the forward switchover start timestamp (if any) is used by default. This parameter cannot be modified.

   • Advanced parameters

     This section is displayed only if the target is a MySQL tenant of OceanBase Database V4.3.0 or later and you have selected Schema Migration on the Select Migration Type page.

     

     This parameter specifies the storage type for target table objects during schema migration. The storage types supported for target table objects are Default, Row storage, Column storage, and Hybrid columnar storage. For more information, see default_table_store_format.

     Note

     The value Default means that other parameters are automatically set based on the parameter configurations of the target. Table objects in schema migration are written to the corresponding schemas based on the specified storage type.

7. Click Precheck to start a precheck on the data migration task.

   During the precheck, the data transmission service checks the read and write privileges of the database users and the network connections of the databases. A data migration task can be started only after it passes all check items. If an error is returned during the precheck, you can perform the following operations:

   • Identify and troubleshoot the problem and then perform the precheck again.

   • Click Skip in the Actions column of the failed precheck item. In the dialog box that prompts the consequences of the operation, click OK.

8. After the precheck succeeds, click Start Task.

   If you do not need to start the task now, click Save. You can start the task later on the Migration Tasks page or by performing batch operations. For more information about batch operations, see Perform batch operations on data migration tasks.

   The data transmission service allows you to modify the migration objects and their row filtering conditions when a migration task is running. For more information, see View and modify migration objects and their filter conditions. After the data migration task is started, it is executed based on the selected migration types. For more information, see View migration details.

References

• Create a PolarDB-X 2.0 data source

• Create an OceanBase data source

• Configure and modify matching rules

• View details of a data migration task