Migrate data in active-active disaster recovery scenarios

OceanBase Migration Service (OMS) Community Edition allows you to migrate data between OceanBase Database Community Edition and MySQL databases, and within OceanBase Database Community Edition in active-active disaster recovery scenarios.

Background information

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

At present, OMS Community Edition supports the following data migration projects in active-active disaster recovery scenarios: Migrate data from a MySQL database to OceanBase Database Community Edition, Migrate data from OceanBase Database Community Edition to a MySQL database, and Migrate data between instances of OceanBase Database Community Edition. OMS Community Edition 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 project to synchronize data in real time between the primary and standby IDCs based on OMS Community Edition. When the primary IDC encounters a disaster or is down, 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 Community Edition provides an active-active solution to allow two IDCs to share business traffic. An active-active solution involves two databases. An application writes data to both databases based on the specified rules, and both databases retain all data of the application. In a non-active-active scenario, an application writes data only to one database. In other words, data written to one database will not be written to the other database.

Prerequisites

• You have created a corresponding schema in the destination database. OMS Community Edition allows you to migrate tables and columns. Therefore, you must create a corresponding schema in the destination database before the migration.

• You have created dedicated database users for data migration in the source and destination 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 for database or schema changes during schema migration or full migration. Otherwise, the data migration project may be interrupted.

• In active-active disaster recovery within OceanBase Database Community Edition, only tables with unique keys can be migrated.

• You cannot add or remove migration objects of a project in an active-active disaster recovery scenario when the project is running.

• If the destination is a database, OMS Community Edition does not support triggers in the destination database. If triggers exist in the destination database, the data migration may fail.

• 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, no data is matched for the UPDATE or DELETE operation. This causes data quality risks.

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

• OMS Community Edition supports the migration of only objects whose database name, table name, and column name are ASCII-encoded without special characters. The special characters are line breaks and | " ' ` ( ) = ; / &

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

• The source cannot be a standby OceanBase database.

Considerations

• To ensure the performance of a data migration project, we recommend that you migrate no more than 1,000 tables at a time.

• If you use OceanBase Database Community Edition V4.x, we recommend that you enable log archiving. If you enable log archiving, OMS Community Edition 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 or reverse incremental migration.

  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 within OceanBase Database Community Edition or from OceanBase Database Community Edition to a MySQL database, if OceanBase Database Community Edition is of a version earlier than V3.2.x and a source table is a multi-partition table with a global unique index, data can be lost during the migration after you update the values of the partitioning key of the table.

• Check whether the migration precision of OMS Community Edition for columns 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 considerations if you want to aggregate multiple tables:

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

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

• In a data migration project where the source is OceanBase Database Community Edition 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.

• By default, lower_case_table_names is set to 1 at the destination, 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, if the data type of a column in the source is not supported by OMS Community Edition, implicit data type conversion may occur, 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.

Procedure

1. Create a data migration project.

   1. Log on to the console of OMS Community Edition.

   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	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/Destination	If you have created a 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 parameters of a data source, see corresponding topics in the Create data sources chapter. Active-active disaster recovery scenarios are supported in data migration projects between a MySQL database and OceanBase Database Community Edition.
   Scenarios	Two scenarios are available: Data Migration and Active-Active Disaster Recovery. Select Active-Active Disaster Recovery. If the destination is an OceanBase data source on Alibaba Cloud, you cannot select Active-Active Disaster Recovery.
   Scenario type	This parameter is available only when you migrate data from a MySQL database to OceanBase Database Community Edition. Valid values: Tables with Unique Key and All Tables. If a table has primary key constraints or non-null unique key constraints, this table is a table with a unique key. If you select All Tables, tables with a primary key or a unique key and those without a unique key, except for temporary tables, are included. If you select Active-Active Disaster Recovery for Scenarios, you can migrate only tables with unique keys.

3. Click Next, and then click OK in the dialog box that appears.

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

   Options for Migration Type 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. In a project that migrates schemas from a MySQL database to OceanBase Database Community Edition, the database that does not exist in the destination can be automatically created.
   Full migration	The existing data is migrated from tables in the source database to the corresponding tables in the destination database. If you select Full Migration, we recommend that you collect the statistics of the source database before data migration.
   Incremental synchronization	Changed data in the source database is synchronized to the corresponding tables in the destination database after an incremental synchronization task starts. 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 operations, see topics in the Supported DDL operations for synchronization and limitations chapter.
   Full verification	After the full data migration and incremental data synchronization are completed, OMS Community Edition 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 databases before full verification. For more information about how to collect statistics of OceanBase Database Community Edition, see Manually collect statistics. If you select Incremental Synchronization but do not select all DML operations in the DML Synchronization section, you cannot select Full Verification. OMS Community Edition supports full data verification of only tables with a unique key.

5. (Optional) Click Next. On the Select Migration Objects page, specify the migration objects for the migration project.

   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.

     • When you migrate data from a MySQL database to OceanBase Database Community Edition, do not select a table named in the `*_ghc` format.

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

     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 an object	OMS Community Edition allows you to rename migration objects. For more information, see Rename a database table.
     Configure settings	OMS Community Edition 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 object in the View Column section.
     Remove one or all objects	OMS Community Edition allows you to remove a single object or all objects to be migrated to the destination database during data mapping. 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.

     Notice

     When you migrate data from a MySQL database to OceanBase Database Community Edition, set Object Exclusion Rule to {database_name}.*_ghc.

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

   To view or modify parameters of the Full-Import or Incr-Sync component, click Configuration Details in the upper-right corner of the Full Migration or Incremental Synchronization section. For more information about the parameters, see Coordinator.

   • Schema migration

     This section is displayed only if you have selected Schema Migration on the Select Migration Type page. You can specify the Character Set Mapping and Collation Mapping parameters as needed.

   • Full migration

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

     Parameter	Description
     Concurrency Speed	Valid values: Stable, Normal, Fast, and Custom. The amount of resources to be consumed by a full data migration task varies based on the migration performance. If you select Custom, you can set Read Concurrency, Write Concurrency, and JVM Memory as needed.
     Processing Strategy When Records Exist in Target Object	Valid values: Ignore and Stop Migration. If you select Ignore, when the data to be inserted conflicts with the existing data of a destination table, OMS Community Edition retains the existing data and records the conflict data. Notice If you select Ignore, data is pulled in IN mode for verification. In this case, the scenario where the destination table contains more data than the source table 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 Community Edition ignores this error and continues to migrate data. Proceed with caution.
     Writing Method	Valid values: SQL (specifies to write data to tables by using INSERT or REPLACE) and Direct Load (specifies to write data through bypass import). The limitations on the Direct Load are as follows: This write mode is supported for full migration only when the destination is an instance of OceanBase Database Community Edition of a version later than V4.2.1. The size of data in a single row cannot exceed 2 MB. Only data duplication caused by primary key constraints can be handled, and that caused by unique key constraints cannot be handled. Generated columns, triggers, and user-defined types (UDTs) are not supported. Bypass import applies to empty tables and cannot be interrupted. In other words, resumable transmission is not supported. Therefore, if bypass import is used for full migration and is interrupted, the data in the source is read again even after the project resumes normal. If an imported table contains large object (LOB) fields, execute the following statement to disable the enable_rebalance parameter for OceanBase Database Community Edition: SHOW PARAMETERS LIKE 'enable_rebalance'; ALTER SYSTEM SET enable_rebalance = false; Requests in a queue may time out if data is concurrently written in bypass mode. We recommend that you set soure.sliceWorkerNum to 1. The number and order of fields in the source table must be the same as those in the destination table. In a multi-table aggregation scenario, data writing through bypass import is not supported.
     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 Migration on the Select Migration Type page.

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

     • If you use OceanBase Database Community Edition V4.x, adjust the following parameters of the sys tenant and business tenants by using a command-line interface (CLI) client.

       • Modify 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;
         

       • Modify the parameters of a user 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 Community Edition 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 table describes the incremental synchronization parameters, which are displayed only if you have selected Incremental Synchronization on the Select Migration Type page.

     Parameter	Description
     Concurrency Speed	Valid values: Stable, Normal, Fast, and Custom. The amount of resources to be consumed by an incremental synchronization task varies based on the synchronization performance. If you select Custom, you can set Read Concurrency, Write Concurrency, and JVM Memory as needed.
     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.
     Incremental Record Retention Time	The duration that incremental parsed files are cached in OMS Community Edition. A longer retention period results in more disk space occupied by the Store component of OMS Community Edition.

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

   During the precheck, OMS Community Edition checks the read and write privileges of the database users and the network connectivity of the databases. The data migration project 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 error 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. 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.

   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.