Preparations

Before you create standby tenants, you must select the source and creation method, specify the number of standby tenants to be created, and configure resources for the standby tenants.

Select the source for standby tenants

A standby tenant synchronizes logs from the primary tenant or another standby tenant. A deployment architecture where a standby tenant synchronizes logs from another standby tenant is called a cascaded standby database or a cascaded standby tenant.

You can select the source for the standby tenants based on your business needs.

Select the number of standby tenants to be created

OceanBase Database does not limit the number of standby tenants that synchronize logs from a primary or standby tenant.

However, if the Physical Standby Database solution is based on log archiving, log synchronization depends on the storage medium. In this case, you must determine the number of standby tenants to be created based on your business needs.

If your Physical Standby Database solution is based on network, a standby tenant must read logs from the source tenant during synchronization. This consumes not only the CPU, memory, and I/O resources of the source tenant, but also the network bandwidth between the standby tenant and the source tenant. In this case, you must determine the number of standby tenants to be created based on the resource usage of the source tenant and your business needs.

Select the method for creating standby tenants

OceanBase Database allows you to create standby tenants by using one of the following three methods based on your business scenario:

• Create an empty standby tenant

  If the primary tenant is a new tenant, or if the cluster to which the primary tenant belongs or the log archiving medium stores complete logs of the primary tenant since its creation, you can create standby tenants without relying on baseline or minor compaction data other than the logs. In this case, you can use the CREATE STANDBY TENANT statement to create a standby tenant.

  To check whether the primary tenant has complete logs, perform the following steps:

  1. Log on as the administrator to the primary tenant or the sys tenant of the cluster to which the primary tenant belongs.

  2. Execute the following statement to query comparison information about log streams in the primary tenant.

     • Query the information from the sys tenant of the cluster to which the primary tenant belongs

       (SELECT LS_ID FROM oceanbase.CDB_OB_LS_HISTORY WHERE TENANT_ID = xxxx) EXCEPT (SELECT LS_ID FROM oceanbase.CDB_OB_LS WHERE TENANT_ID = xxxx);
       

       or

       (SELECT LS_ID FROM oceanbase.CDB_OB_LS_HISTORY WHERE TENANT_ID = xxxx) MINUS (SELECT LS_ID FROM oceanbase.CDB_OB_LS WHERE TENANT_ID = xxxx);
       

       You can query the DBA_OB_TENANTS view for the ID of the primary tenant.

     • Query the information from the primary tenant

       MySQL mode

       Oracle mode

       (SELECT LS_ID FROM oceanbase.DBA_OB_LS_HISTORY) EXCEPT (SELECT LS_ID FROM oceanbase.DBA_OB_LS);
       

       or

       (SELECT LS_ID FROM oceanbase.DBA_OB_LS_HISTORY) MINUS (SELECT LS_ID FROM oceanbase.DBA_OB_LS);
       

       (SELECT LS_ID FROM SYS.DBA_OB_LS_HISTORY) MINUS (SELECT LS_ID FROM SYS.DBA_OB_LS);
       

     You can query the DBA_OB_LS_HISTORY or CDB_OB_LS_HISTORY view for information about all log streams created in the tenant, and query the DBA_OB_LS or CDB_OB_LS view for the log stream that provides services in the tenant. If the query result is not empty, the query result shows the log streams that were once created in the primary tenant but were deleted later due to reasons such as load balancing or scaling.

     In this case, the logs of the primary tenant are incomplete, and you cannot use the CREATE STANDBY TENANT statement to create a standby tenant. If the query result is empty, proceed to the next step.

  3. Execute the following statement to query log streams of the primary tenant.

     • Query log streams of the primary tenant from the sys tenant of the cluster to which the primary tenant belongs

       SELECT LS_ID, BEGIN_LSN FROM oceanbase.GV$OB_LOG_STAT WHERE TENANT_ID = xxxx AND ROLE = 'LEADER' ;
       

       You can query the DBA_OB_TENANTS view for the ID of the primary tenant.

     • Query the information from the primary tenant

       MySQL mode

       Oracle mode

       SELECT LS_ID, BEGIN_LSN FROM oceanbase.GV$OB_LOG_STAT WHERE ROLE = 'LEADER' ;
       

       SELECT LS_ID, BEGIN_LSN FROM SYS.GV$OB_LOG_STAT WHERE ROLE = 'LEADER' ;
       

     A sample query result is as follows:

     +-------+-----------+
     | LS_ID | BEGIN_LSN |
     +-------+-----------+
     |     1 |         0 |
     |  1001 |         0 |
     +-------+-----------+
     2 rows in set
     

     BEGIN_LSN indicates the earliest log sequence number (LSN) stored in the current log stream replica. If the value of BEGIN_LSN is 0, the current log stream replica contains complete logs generated since the tenant was created.

     In the query result, the value of BEGIN_LSN for the log stream replica is 0, which indicates that the log stream replica contains complete logs generated since the tenant was created. If the value of BEGIN_LSN for all log stream replicas of the tenant is 0, all log stream replicas of the tenant contain complete logs, and you can use the CREATE STANDBY TENANT statement to create an empty standby tenant.

• Create a standby tenant by using the physical backup and restore (with complete logs) feature

  You can use this method to create standby tenants in any scenarios. For more information about the physical backup and restore feature, see Overview.

• Create a standby tenant by using the BACKUP DATABASE PLUS ARCHIVELOG feature

  When you create a standby tenant by using the regular physical backup and restore (with complete logs) feature, you must use shared storage such as Alibaba Cloud Object Storage Service (OSS) or Network File System (NFS) to store data backups and archive logs, and the shared storage must be accessible to the clusters to which the primary and standby tenants belong. These requirements are hard to meet for users who use OceanBase Database Community Edition or have deployed OceanBase Database in standalone mode on a single server.

  Therefore, you can use the BACKUP DATABASE PLUS ARCHIVELOG feature to generate a database snapshot for all data and archive logs of the primary tenant on the local server if the primary tenant is in standalone mode, or on the shared storage in OceanBase Database Community Edition when the primary tenant belongs to a cluster. The snapshot contains baseline data and archive logs required for running the database. Then you can upload the snapshot to a medium accessible to the cluster where you want to create a standby tenant, and finally restore the standby tenant from the database snapshot.

Configure resources for standby tenants

Before you create standby tenants, you must allocate a resource pool to standby tenants, configure CPU, memory, log disk space, and IOPS resources for standby tenants, and specify the number of replicas, primary zone, and locality for standby tenants.

The resource specifications of a standby tenant can be different from those of the primary tenant. We recommend that you select appropriate resource specifications for standby tenants based on your business needs and the business load of the primary and standby tenants. You can calculate the amount of physical resources required by each resource unit of the standby tenant based on the primary tenant and the number of resource units of the standby tenant to be created.

Assume that the tenant ID of the primary tenant is 1002 and two resource units are to be created for the standby tenant. After you log on to the sys tenant of the cluster to which the primary tenant belongs, you can execute the following statement to calculate the amount of physical resources required by each resource unit of the standby tenant:

obclient [oceanbase]> CALL DBMS_OB_LIMIT_CALCULATOR.CALCULATE_MIN_PHY_RES_NEEDED_BY_STANDBY_TENANT(PRIMARY_TENANT_ID => 1002, STANDBY_TENANT_UNIT_NUM => 2);

The result is as follows:

+------------------------+------------+
| PHYSICAL_RESOURCE_NAME | MIN_VALUE  |
+------------------------+------------+
| memstore               |          0 |
| memory                 | 4294967296 |
| data_disk              |          0 |
| clog_disk              | 2147483648 |
| cpu                    |          0 |
+------------------------+------------+
5 rows in set