Active-active disaster recovery between OceanBase databases

OceanBase Migration Service (OMS) allows you to migrate data between tenants of the same type in OceanBase Database across regions and create active-active disaster recovery projects.

Background

As more users apply OMS in data migration, OMS must adapt to increasingly diverse scenarios. In addition to single-region data migration and data synchronization, OMS supports data migration across regions and active-active data synchronization between remote regions.

OMS supports the following active-active disaster recovery scenarios:

• Local data migration and synchronization

• Local primary/standby disaster recovery

• Local active-active disaster recovery

• Remote data migration and synchronization

• Remote primary/standby disaster recovery

• Remote active-active disaster recovery

Primary/Standby disaster recovery is common in scenarios with data disaster recovery requirements. You can create a real-time data synchronization project between the primary and standby IDCs based on OMS. When the primary IDC encounters disasters or downtime, the business can be switched to the standby IDC to avoid business interruption.

However, primary/standby disaster recovery causes a waste of resources in the standby IDC. Therefore, OMS provides an active-active solution to allow two IDCs to share business traffic.

When you migrate data between tenants of OceanBase Database in an active-active disaster recovery scenario, no forward switchover is performed, and you can enable DDL parameters only for one link.

Prerequisites

• You have created a corresponding schema in the target tenant of OceanBase Database.

• You have created dedicated database users for data migration in the source and destination OceanBase databases, and granted corresponding privileges to the users. For more information, see 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 data migration. Otherwise, the data migration project may be interrupted.

• You can migrate data only between tenants of the same type in OceanBase Database.

  Specifically, you can migrate data from a MySQL tenant to another MySQL tenant, or from an Oracle tenant to another Oracle tenant.

• In an active-active disaster recovery project between MySQL tenants of OceanBase Database, only tables with unique keys can be migrated.

• When you create an active-active disaster recovery project, you cannot select reverse incremental migration.

• OMS does not support data migration from OceanBase Database V1.4.x to V4.x.

• OMS does not support primary keys that contain data of the FLOAT or DOUBLE type.

• When you migrate data from OceanBase Database V1.4.79, OMS supports only OceanBase Database V2.2.77 or later as the destination. When you migrate data from OceanBase Database V2.1.0 or later, OMS supports OceanBase Database of the same version or a different version as the destination.

• You must create a forward link and a reverse link to form an active-active disaster recovery link, which does not support triggers.

• When useTargetIndex is set to false in incremental synchronization, if the destination uses a field of the BINARY type as the primary key or unique key, and the length of the data at the source is different from the length of the field of the BINARY type at the destination, the UPDATE or DELETE operation cannot match data. This causes data quality risks.

• Data source identifiers and user accounts must be globally unique in OMS.

• OMS supports the migration of only objects whose database name, table name, and column name are ASCII-encoded and do not contain special characters. The special characters are spaces, line breaks, and the following characters: . | " ' ` ( ) = ; / & \.

• If temporary tables exist in the MySQL tenant of OceanBase Database of a version earlier than V4.0.0, full migration fails.

• OMS does not support a standby OceanBase database as the source.

Considerations

• If you use OceanBase Database V4.x, we recommend that you enable log archiving. If you enable log archiving, OMS implements incremental synchronization by consuming archive logs after clogs are recycled. For more information about how to enable log archiving, see the Log archive chapter.

• 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 destination 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.

  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.

• In an active-active disaster recovery project in OceanBase Database of a version earlier than V3.2.x, if the source table is a multi-partition table with a global unique index and you update the values of the partitioning key of the table, data may be lost during migration.

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

• When DDL synchronization is disabled, if you change the unique index of the destination, you must restart the Incr-Sync component. Otherwise, the data may be inconsistent.

• Take note of the following points if you want to perform data merge migration:

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

  • We recommend that you manually create schemas at the destination. If you use OMS to create schemas, skip failed objects in the schema migration step.

• In a data migration project where the source is an OceanBase database and DDL synchronization is enabled, if a RENAME operation is performed on a table in the source, we recommend that you restart the project to avoid data loss during incremental synchronization.

• If you select only Incremental Synchronization when you create a data migration project, OMS requires that the local incremental logs in the source database be retained for more than 48 hours.

  If you select Full Data Migration and Incremental Synchronization when you create a data migration project, OMS requires that the local incremental logs in the source database be retained for at least 7 days. Otherwise, the data migration project will fail or the data in the source and destination databases will be inconsistent because OMS cannot obtain incremental logs.

• Take note of the following limitations on MySQL tenants of OceanBase Database:

  • For data migration between MySQL tenants of OceanBase Database, the default value of the lower_case_table_names parameter for the destination is 1, and the destination database objects are created with lowercase names.

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

    • When you manually create a table schema at the destination, if the data types of any columns are not supported by OMS, implicit data type conversion may occur at the destination, which causes inconsistent column types between the source and destination.

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

• Take note of the following limitations on Oracle tenants of OceanBase Database:

  • A table containing a primary key or a non-null unique key, which must not be a function-based unique key, is considered a table with a primary key.

    If the source OceanBase database is of a version earlier than V2.2.77 or a source table contains a function-based unique key on a virtual column, OMS may fail to accurately identify whether the table contains a function-based unique key, which may lead to misjudgment of the table with a primary key. This may slow down full migration and full verification and cause data inconsistency in incremental synchronization.

  • In an Oracle tenant of OceanBase Database V2.2.76 or earlier, the ALL_TABLES view may mistake partitioned tables for non-partitioned tables. As a result, OMS may read incorrect data from the view. In this case, manually create this table at the destination. For more information, see Create a table.

  • If you skip the "source-primary-database ROW_MOVEMENT check" precheck item during the migration, data inconsistency is expected during the synchronization of a table whose value of the ROW_MOVEMENT field is enable.

Create a forward link

1. Create a data migration project.

   

   1. Log on to the OMS console.

   2. In the left-side navigation pane, click Data Migration.

   3. On the Data Migration page, click Create Migration Project in the upper-right corner.

2. On the Select Source and Destination page, configure the parameters.

   Parameter	Description
   Migration Project 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.
   Tag (Optional)	Click the field and 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 projects.
   Source	If you have created an OceanBase data source, which can be a physical data source or an ApsaraDB for OceanBase data source, select it from the drop-down list. If not, 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 a physical OceanBase data source or Create a public cloud OceanBase data source.
   Destination	If you have created an OceanBase data source, which can be a physical data source or an ApsaraDB for OceanBase data source, select it from the drop-down list. If not, click New Data Source in the drop-down list and create one in the dialog box that appears on the right.
   Scenarios	Two scenarios are available: Data Migration and Active-Active Disaster Recovery. Select Active-Active Disaster Recovery. Notice The source and destination data sources must be tenants of the same type in OceanBase Database. The source and destination nodes must belong to different regions.

3. Click Next. In the dialog box that appears, click OK.

   Note that this project supports only tables and views with a primary key or a non-null unique index and those without are automatically filtered out.

   

4. On the Select Migration Type page, configure the parameters.

   Migration types available for the forward link include Schema Migration, Full Migration, Incremental Synchronization, and Full Verification.

   Migration type	Description
   Schema migration	When a schema migration task starts, OMS migrates the definitions of data objects, such as tables, indexes, constraints, comments, and views, from the source database to the destination database. Temporary tables are automatically filtered out.
   Full migration	When a full migration task starts, OMS migrates the existing data from tables in the source database to the corresponding tables in the destination database. If you select Full Data Migration, we recommend that you collect the statistics of the source OceanBase database before the data migration. For more information, see Manually collect statistics.
   Incremental synchronization	When an incremental synchronization task starts, OMS synchronizes changed data in the source database to the corresponding tables in the destination database. Data changes are data addition, modification, and deletion. Options for Incremental Synchronization are DML Synchronization and DDL Synchronization. You can select the options as needed. For more information about DDL synchronization, see DDL synchronization between MySQL tenants of OceanBase Database or DDL synchronization between Oracle tenants of OceanBase Database. Incremental Synchronization has the following limitations: You can enable DDL synchronization in either the forward or reverse link. If you select DDL Synchronization, when you perform a DDL operation that cannot be synchronized by OMS in the source database, data migration may be interrupted. If the DDL operation creates a new column, we recommend that you set the attribute of the column to NULL. Otherwise, data migration may be interrupted.
   Full verification	After the full data migration and incremental data synchronization are completed, OMS automatically initiates a full data verification task to verify the data tables in the source and destination databases. If you select Full Verification, we recommend that you collect the statistics of both the source and destination OceanBase databases before full verification. If you have selected Incremental Synchronization but did not select all DML statements in the DML Synchronization section, OMS does not support full verification.

5. (Optional) Click Next.

   If you have selected Schema Migration or Incremental Synchronization but no related parameters are configured for the source OceanBase database, the More About Data Sources dialog box appears, prompting you to configure related parameters. For more information about the parameters, see Create a physical OceanBase data source or Create a public cloud OceanBase data source.

   After you configure the parameters, click Test Connection. After the test succeeds, click OK.

6. Click Next. On the Select Migration Objects page, specify the migration objects and migration scope.

   You can select Specify Objects or Match Rules to specify the migration objects.

   • Select Specify Objects. Then select the objects to be migrated on the left and click > to add them to the list on the right. You can select tables and views of one or more databases as the migration objects.

     Notice

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

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

     • After you select migration objects by using the Specify Objects option, the DDL operations take effect only for selected objects, and table creation is not supported.

     • OMS automatically filters out unsupported tables.

     OMS also allows you to import objects from text, rename objects, set row filters, view column information, and remove a single object or all objects to be migrated.

     

     Operation	Description
     Import objects	In the list on the right of the Specify Migration Scope section, click Import Objects in the upper-right corner. In the dialog box that appears, click OK. Notice 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/tables and set row filtering conditions. For more information, see Download and import the settings of migration objects. Click Validate. After the validation succeeds, click OK.
     Rename objects	OMS allows you to rename the migration objects. For more information, see Rename a migration or synchronization object.
     Configure settings	OMS allows you to use the WHERE clause to filter rows. For more information, see Use SQL conditions to filter data. You can also view column information of the migration object in the View Column section.
     Remove one or all objects	During data mapping, OMS allows you to remove one or more selected objects to be migrated or synchronized to the destination. To remove a single migration object: In the list on the right of the Specify Migration Scope section, move the pointer over the target object, and click Remove. To remove all migration objects: In the list on the right of the Specify Migration Scope section, click Remove All in the upper-right corner. In the dialog box that appears, click OK.

   • Select Match Rules. For more information, see Configure matching rules for migration objects.

7. Click Next. On the Migration Options page, configure the following parameters.

   • Full migration

     The following parameters are displayed only if you have selected Full Data Migration on the Select Migration Type page.

     

     Parameter	Description
     Full Migration Resource Configuration	You can select Small, Medium, or Large to use the corresponding default values of Read Concurrency, Write Concurrency, and Memory. You can also customize the resource configurations for full migration. Through resource configuration for the Full-Import component, you can limit the resource consumption of a project in the full migration phase. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.
     Processing Strategy When Destination Table Has Records	Valid values: Ignore and Stop Migration. If you select Ignore, when the data to be written conflicts with the existing data of a destination table, OMS logs the conflicting data while retaining the existing data. Notice If you select Ignore, data is pulled in IN mode for full verification. In this case, the scenario where the destination contains more data than the source cannot be verified, and the verification efficiency will be decreased. If you select Stop Migration and a destination table contains records, an error is returned during full migration, indicating that the migration is not allowed. In this case, you must clear the data in the destination table before you can continue with the migration. Notice After an error is returned, if you click Resume in the dialog box, OMS ignores this error and continues to migrate data. Proceed with caution.
     Whether to Allow 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. Notice This feature is supported only if you have selected both Schema Migration and Full Data Migration on the Select Migration Type page. Only non-unique key indexes can be created after the migration is completed. OceanBase Database V1.x does not support the post-indexing feature.

     If post-indexing is allowed, we recommend that you adjust the parameters based on the hardware conditions of your OceanBase database and the current business traffic.

     • If you use OceanBase Database V4.x, adjust the following parameters of the sys tenant and business tenants by using a CLI client.

       • Adjust the parameters of the sys tenant

         // parallel_servers_target specifies the queuing conditions for parallel queries on each server. 
         // To maximize performance, we recommend that you set this parameter to a value greater than, for example, 1.5 times, the number of physical CPU cores. In addition, make sure that the value does not exceed 64, to prevent database kernels from contending for locks. 
         set global parallel_servers_target = 64; 
         

       • Adjust the parameters of a business tenant

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

     • If you use OceanBase Database V2.x or V3.x, adjust the following parameters of the sys tenant by using a CLI client.

       // parallel_servers_target specifies the queuing conditions for parallel queries on each server. 
       // To maximize performance, we recommend that you set this parameter to a value greater than, for example, 1.5 times, the number of physical CPU cores. In addition, make sure that the value does not exceed 64, to prevent database kernels from contending for locks. 
       set global parallel_servers_target = 64; 
       // data_copy_concurrency specifies the maximum number of concurrent data migration and replication tasks allowed in the system. 
       alter system set data_copy_concurrency = 200; 
       

   • Incremental synchronization

     The following parameters are displayed only if you have selected Incremental Synchronization on the Select Migration Type page.

     

     Parameter	Description
     Incremental Log Pull Resource Configuration	You can select Small, Medium, or Large to use the corresponding default value of Memory. You can also customize the resource configurations for incremental log pull. Through resource configuration for the Store component, you can limit the resource consumption of a project in log pull in the incremental synchronization phase. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.
     Incremental Data Write Resource Configuration	You can select Small, Medium, or Large to use the corresponding default values of Write Concurrency and Memory. You can also customize the resource configurations for incremental data write. Through resource configuration for the Incr-Sync component, you can limit the resource consumption of a project in data writes in the incremental synchronization phase. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.
     Incremental Record Retention Time	The duration that incremental parsed files are cached in OMS. A longer retention period results in more disk space occupied by the Store component.
     Incremental Synchronization Start Timestamp	If you have set the migration type to Full Data Migration, this parameter is not displayed. If you have selected Incremental Synchronization but not Full Data 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.

   • Full verification

     The following parameters are displayed only if you have selected Full Verification on the Select Migration Type page.

     

     Parameter

     Description

     Full Verification Resource Configuration

     You can select Small, Medium, or Large to use the corresponding default values of Read Concurrency and Memory. You can also customize the resource configurations for full verification. Through resource configuration for the Full-Verification component, you can limit the resource consumption of a project in the full verification phase. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.

   If the parameter settings on the page cannot meet your requirements, you can click Parameter Configuration in the lower part of the page to configure more specific settings. You can also reference an existing project or component template.

   

8. Click Precheck to start a precheck on the data migration project.

   During the precheck, OMS checks the read and write privileges of the database users and the network connections of the databases. The data migration project can be started only after it passes all check items. You can perform one of the following operations if an error is returned during the precheck:

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

   • Click Skip in the Actions column of a failed precheck item. In the dialog box that appears, you can view the prompt for the consequences of the operation and click OK.

9. Click Start Project. If you do not need to start the project now, click Save to go to the details page of the data migration project. You can start the project later as needed.

   OMS allows you to modify the migration objects when the data migration project is running. For more information, see View and modify migration objects. After a data migration project is started, the migration subtasks will be executed based on the selected migration types. For more information, see the "View migration details" section in the View details of a data migration project topic.

Create a reverse link

You can create a reverse link only after the schema migration task on the forward link is completed.

1. On the Data Migration page, click Create Migration Project in the upper-right corner.

2. On the Select Source and Destination page, configure the parameters.

3. Click Next. In the dialog box that appears, click OK.

   Note that this project supports only tables and views with a primary key or a non-null unique index and those without are automatically filtered out.

4. On the Select Migration Type page, configure the parameters.

   For the reverse link, set Migration Type to Incremental Synchronization.

   Notice

   You can enable DDL synchronization in either the forward or reverse link.

5. (Optional) Click Next.

   If you have selected Incremental Synchronization without configuring the required parameters for the source OceanBase database, the More About Data Sources dialog box appears, prompting you to configure the parameters. For more information about the parameters, see Create a physical OceanBase data source or Create a public cloud OceanBase data source.

   After you configure the parameters, click Test Connection. After the test succeeds, click OK.

6. Click Next. On the Select Migration Objects page, specify the migration objects and migration scope.

   When you create a reverse link, OMS also allows you to import objects through text, rename objects, set row filters, view column information, and remove one or all objects to be migrated. For more information, see the section that describes how to create a forward link.

7. Click Next. On the Migration Options page, configure the incremental synchronization parameters.

8. Click Precheck to start a precheck on the data migration project.

9. Click Start Project to start the incremental synchronization task for the project.