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 cities.

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 destination tenant of OceanBase Database. OMS allows you to migrate tables and columns. Therefore, you must create a corresponding schema in the destination database before migration.

• 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.

Limits

• You can migrate data only between tenants of the same mode 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 triggers in the destination database for long-term synchronization between these databases.

• You can migrate multiple schemas in a data migration project. The migration granularity ranges from a table to a tenant.

• You can migrate data from OceanBase Database V2.1.0 or later versions. The source and destination tenants can be created in different versions of OceanBase Database.

• If the character set used by the source database is UTF-8, we recommend that you use UTF-8 or a greater character set for the destination database.

• When you migrate data from OceanBase Database V1.4.x, OMS does not support primary keys that contain data of the FLOAT or DOUBLE type.

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

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

• When useTargetIndex = false, if a BINARY field is used as a primary or unique key in the destination and the length of the data to be migrated differs from the length of BINARY fields in the destination, no data is available for UPDATE or DELETE operations.

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

• OMS allows you to migrate only databases and tables whose names are ASCII codes and do not contain special characters such as .|"'`()=;/&\n.

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

• OMS supports full synchronization of tables whose partition fields are not within the primary keys, but does not support DDL operations that create tables.

• You cannot create primary keys and other indexes in the source OceanBase database during data migration on OMS.

Considerations

• If you enable DDL operations for incremental data migration, the drop index command is executed on all indexes, which may cause index loss in the destination database.

• If the clocks are out of synchronization between the nodes or between the client and the server, a negative delay may occur in incremental synchronization or reverse incremental synchronization.

• A table containing a primary key or a not-null unique key but not a function-based unique key is a table with a primary key. If a table in the source Oracle tenant of OceanBase Database contains a function-based unique key in a virtual column, or if the database version is earlier than V2.2.77, OMS cannot accurately identify whether the table contains a function-based unique key and therefore may be inaccurate in determining whether the table has a primary key. This may slow down full migration and full verification and result in data inconsistency in incremental synchronization.

  During incremental synchronization, OMS processes data in parallel based on their UNIQUE constraints. If a table contains function-based unique keys, the related transaction operations must be performed in sequence during incremental synchronization. Otherwise, data inconsistency may occur.

• In an active-active disaster recovery project in OceanBase Database, when the database is of a version earlier than V3.2.x and contains a multi-partition table that has global unique indexes, if you update the value of a partitioning key of the table, data may be lost during migration.

• When you migrate data from one MySQL tenant of OceanBase Database to another, if the length of a field in the destination database is shorter than that in the source database, the data of this field will be truncated in the destination database, causing data inconsistency between the source and destination databases.

• If you change the unique index of the destination, you must restart the incremental synchronization. Otherwise, the data may be inconsistent.

• When you migrate data from one MySQL tenant of OceanBase Database to another, by default, OMS uses the lower_case_table_names = 1 setting in the destination and creates objects named in lower case in the destination.

• If you select migration objects based on matching rules, we recommend that you create standard CTAS statements and set object exclusion rules, such as a.ctas*, to avoid interrupting the data migration project.

• In a multi-table aggregation scenario:

  • We recommend that you map objects in the source and destination databases by importing objects and configuring matching rules.

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

• If you skip the "Check the ROW_MOVEMENT parameter of the database" check item of the source primary database during the migration, data inconsistency may occur when you synchronize tables whose ROW_MOVEMENT 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, specify the following 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	Click the field and select a target tag from the drop-down list. You can 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, select it from the drop-down list. If you have not created an OceanBase data source, click Create Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information, see Create a physical data source of OceanBase Database. Notice The source database cannot be an instance of OceanBase Database V4.0.0.
   Destination	If you have created an OceanBase data source, select it from the drop-down list. If you have not created an OceanBase data source, click Create Data Source in the drop-down list and create one in the dialog box that appears on the right. Notice The destination cannot be an instance of OceanBase Database V4.0.0.
   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.

4. In the dialog box that appears, click OK.

   Note that this project supports only tables with a primary key or a non-null unique index and other tables and views are automatically filtered out.

5. On the Select Migration Type page, specify related parameters.

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

   Migration type	Description
   Schema Migration	The definitions of data objects, such as tables, indexes, constraints, comments, and views, are migrated from the source database to the destination database. Temporary tables are automatically filtered out.
   Full Migration	If you select Full 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	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 limits: You can enable DDL synchronization in either the forward or reverse link. If you select DDL Synchronization, when you perform a DDL operation that is not supported by OMS in the source database, data migration may fail. 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	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.

6. (Optional) Click Next.

   Specify the related information for the source OceanBase database.

   • For incremental synchronization, specify the ConfigUrl, username, and password.

   • For schema migration, specify the username and password.

   • For the migration of a table without a unique key, specify the password of _OCEANBASE_INNER_DRC_USER.

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

   After you configure the parameters, click Test Connectivity. After the test succeeds, click Save.

7. Click Next. On the Select Migration Objects page, select the migration objects and migration scope.

   You can select one of the following two modes to migrate objects: Specify Objects or Match Rules. If you have selected DDL Synchronization, only the Match Rules option is available.

   • 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.

     When you migrate data between OceanBase databases, OMS allows you to import objects through text, rename object names, set row filters, view column information, and remove a single object or all objects to be migrated.

     Operation	Steps
     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	OMS allows you to rename the migration objects. For more information, see Rename a database table.
     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/Remove All	During data mapping, OMS allows you to remove one or more selected objects to be migrated to the destination. To remove a single migration object: In the list on the right of the Specify Migration Scope section, hover 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.

8. Click Next. On the Migration Options page, specify the following parameters.

   Parameter	Description
   Incremental Synchronization Start Timestamp	If you have set the migration type to Full Migration, this parameter is not displayed. If you have selected a migration type other than Full Migration, specify a point in time after which the data is to be synchronized. The default value is the current system time. You can select a point in time or enter a timestamp. Notice You can select the current time or a point in time earlier than the current time. This parameter is closely related to the retention period of archived logs. Generally, you can start data synchronization from the current timestamp.
   Concurrency for Full Migration	The value can be Smooth, Normal, or Fast. The quantity of resources to be consumed by a full data migration task varies based on the migration performance. You can also modify the configurations of the Full-Import component to customize the concurrency. Notice: To enable this feature, select Full Migration on the Select Migration Type page.
   Full Verification Concurrency	The value can be Smooth, Normal, or Fast. Different quantities of resources of the source and destination databases are consumed at different concurrencies. You can also modify the configurations of the Full-Verification component to customize the concurrency.
   Incremental Record Retention Time	The duration that incremental parsed files are cached in OMS. A longer retention period indicates more disk space occupied by the Store component of OMS.
   Whether to Allow Destination Table to Be Not Empty During Full Migration	If destination tables are allowed to be not empty during full migration, full verification is performed in IN mode. Notice: To enable this feature, select Full Migration on the Select Migration Type page.
   Whether to Allow Post-indexing	You can specify whether to allow post-indexing after full migration is completed. Post-indexing can shorten the time of full migration. Notice: To enable this feature, select 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.

9. Click Precheck to start a pre-check on the data migration project.

   During the pre-check, 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. If an error is returned during the pre-check:

   • You can identify and troubleshoot the problem and then perform the pre-check again.

   • You can click Skip in the Actions column of the pre-check item with the error. A dialog box will be displayed, prompting the impact caused if you skip this error. If you want to continue, click OK in the dialog box.

10. 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 objects will be executed based on the selected migration type. 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, specify the following parameters.

3. Click Next.

4. In the dialog box that appears, click OK.

   Note that this project supports only tables with a primary key or a non-null unique index and other tables are automatically filtered out.

5. On the Select Migration Type page, specify related parameters.

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

6. (Optional) Click Next.

   For incremental synchronization, specify the ConfigUrl, username, and password of the source database. For the migration of a table without a unique key, you need to specify the password of _OCEANBASE_INNER_DRC_USER.

   If no related parameters are configured for the source database, the More about Data Sources dialog box appears, prompting you to configure related parameters. For more information, see Create a physical data source of OceanBase Database.

   After you configure the parameters, click Test Connectivity. After the test succeeds, click Save.

7. Click Next. On the Select Migration Objects page, select the migration objects and migration scope.

   When you create a reverse link, OMS also allows you to import objects through text, rename object names, 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.

8. Click Next. On the Migration Options page, specify the following parameters.

9. Click Precheck to start a pre-check on the data migration project.

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