Protection mode and protection level

The protection mode defines whether the primary cluster synchronously or asynchronously transfer logs to different standby clusters, how the standby clusters receive the logs, and whether the data is lossless.

OceanBase Database provides two fields to specify the protection mode and the status of the primary and standby clusters: PROTECTION_MODE and PROTECTION_LEVEL. This topic describes the values of the fields in two scenarios.

Switch from the maximum performance mode to the maximum protection mode

By default, the primary and standby clusters are in maximum performance mode. In this mode, the primary cluster asynchronously transfers logs to all standby clusters. The data of the standby clusters is out-of-sync with that of the primary cluster and is lossy. Both the protection mode and protection level of the primary and standby clusters are displayed as MAXIMUM PERFORMANCE.

In maximum protection mode, the primary cluster synchronously transfers logs to the standby cluster that is in SYNC mode so that the data of the standby clusters is lossless. The entire process of this mode is not atomic, and intermediate states exist. The following figure shows the state transition process of the <PROTECTION_MODE, PROTECTION_LEVEL> tuple when the protection mode switches from maximum performance to the maximum protection.

The state transitions are described as follows:

• Initial status of the primary and standby clusters

  The primary and standby clusters are in maximum performance mode. Both PROTECTION_MODE and PROTECTION_LEVEL are MAXIMUM PERFORMANCE.

• Status of the primary cluster after it receives the protection mode switchover command:

  1. PROTECTION_MODE of the primary cluster changes to MAXIMUM PROTECTION, and PROTECTION_LEVEL of the primary cluster is still MAXIMUM PERFORMANCE. Then, the primary cluster instructs all servers to synchronously transfer logs to the standby cluster that is in SYNC mode.

  2. After all servers enter the SYNC mode, PROTECTION_LEVEL of the primary cluster changes to MAXIMUM PROTECTION, indicating that the primary cluster enters the SYNC mode.

     Notice

     Data writes are not interrupted in this process. Only the logs written after the primary cluster enters the SYNC mode are synchronously transferred, and the previously committed logs are not affected.

• Status of a standby cluster in SYNC mode after it perceives the protection mode switchover:

  1. PROTECTION_MODE of the standby cluster changes toMAXIMUM PROTECTION, and PROTECTION_LEVEL of the standby cluster is still MAXIMUM PERFORMANCE.

  2. After the primary cluster enters the SYNC mode, PROTECTION_LEVEL of the standby cluster changes to RESYNCHRONIZATION, indicating that the logs are being synchronized to the standby cluster. In this state, the standby cluster synchronizes logs from the primary cluster.

  3. After logs are synchronized, PROTECTION_LEVEL of the standby cluster changes to MAXIMUM PROTECTION, indicating that the data of the standby cluster is consistent with that of the primary cluster and is losses.

• Status of a standby cluster in ASYNC mode after it perceives the protection mode switchover:

  PROTECTION_MODE of the standby cluster changes to MAXIMUM PROTECTION. The standby cluster is in ASYNC mode, so PROTECTION_LEVEL of the standby cluster is always MAXIMUM PERFORMANCE.

Switch from the maximum performance mode to the maximum availability mode

The maximum availability mode combines the maximum performance and maximum protection modes. In maximum availability mode, the primary cluster has two states: synchronously transferring logs to the standby cluster that is in SYNC mode, and asynchronously transferring logs to the standby cluster that is in SYNC mode. The primary cluster dynamically switches between the two states, affecting the status of the standby clusters. The following figure shows the state transition process of the <PROTECTION_MODE, PROTECTION_LEVEL> tuple when the protection mode switches from maximum performance to maximum availability. The state transitions are described as follows:

• Initial status of the primary and standby clusters

  The primary and standby clusters are in maximum performance mode. Both PROTECTION_MODE and PROTECTION_LEVEL are MAXIMUM PERFORMANCE.

• Status of the primary cluster after it receives the protection mode switchover command:

  1. PROTECTION_MODE of the primary cluster changes to MAXIMUM AVAILABILITY, and PROTECTION_LEVEL of the primary cluster is still MAXIMUM PERFORMANCE, indicating that the primary cluster is still in ASYNC mode.

  2. Then, the primary cluster constantly checks whether log synchronization to the standby clusters. If yes, the primary cluster instructs all servers to synchronously transfer logs to the standby cluster that is in SYNC mode.

  3. After all servers enter the SYNC mode, PROTECTION_LEVEL of the primary cluster changes to MAXIMUM AVAILABILITY, indicating that the primary cluster enters the SYNC mode.

  4. The primary cluster remains in SYNC mode. If the primary cluster detects a log synchronization exception in the standby cluster in SYNC mode, the primary cluster is automatically downgraded to the ASYNC mode, and PROTECTION_LEVEL of the primary cluster changes to MAXIMUM PERFORMANCE.

• Status of a standby cluster in SYNC mode after it perceives the protection mode switchover:

  1. PROTECTION_MODE of the standby cluster changes to MAXIMUM AVAILABILITY.

  2. Same the case in the maximum protection mode, after the primary cluster enters the SYNC mode, PROTECTION_LEVEL of the standby cluster changes to RESYNCHRONIZATION, indicating that logs are being synchronized to the standby cluster.

  3. After logs are synchronized, PROTECTION_LEVEL of the standby cluster changes to MAXIMUM AVAILABILITY, indicating that the standby cluster is in SYNC mode, and the data of the standby cluster is consistent with that of the primary cluster and is losses.

• Status of a standby cluster in ASYNC mode after it perceives the protection mode switchover:

  PROTECTION_MODE of the standby cluster changes to MAXIMUM AVAILABILITY. The standby cluster is in ASYNC mode, so PROTECTION_LEVEL of the standby cluster is always MAXIMUM PERFORMANCE.

As can be learned from the foregoing two scenarios, PROTECTION_MODE is shared across the primary and standby clusters, and indicates the data protection capabilities of the primary and standby clusters in the current configurations.

The values of PROTECTION_MODE are described as follows:

• MAXIMUM PERFORMANCE: The maximum performance mode. In this mode, the primary cluster asynchronously transfers logs to all standby clusters.

• MAXIMUM PROTECTION: The maximum protection mode. In this mode, the primary cluster synchronously transfers logs to the standby cluster that is in SYNC mode.

• MAXIMUM AVAILABILITY: The maximum availability mode. In this mode, the primary cluster synchronously transfers logs to the standby cluster that is in SYNC mode and can be automatically downgraded to the ASYNC mode as required.

PROTECTION_LEVEL is specific to clusters and has different meanings for the primary cluster and the standby clusters. PROTECTION_LEVEL of the primary cluster specifies whether the primary cluster synchronously transfers logs to standby clusters. PROTECTION_LEVEL of a standby cluster indicates whether the standby cluster is in SYNC mode and whether data is lossless.

The values of PROTECTION_LEVEL are described as follows:

• MAXIMUM PERFORMANCE: indicates that the primary and standby clusters are in ASYNC mode.

• MAXIMUM PROTECTION: indicates that the primary cluster synchronously transfers logs to the standby cluster that is in SYNC mode with maximum protection, or indicates that a standby cluster is in SYNC mode and data is lossless.

• MAXIMUM AVAILABILITY: indicates that the primary cluster synchronously transfers logs to the standby cluster that is in SYNC mode with maximum availability, or indicates that the standby cluster is in SYNC mode and data is lossless.

• RESYNCHRONIZATION: indicates that logs are being synchronized to the standby cluster. This state is unavailable for the primary cluster.

The following table describes the protection levels of the primary and standby clusters in different protection modes.

View the protection mode and protection level of a cluster

Log on to the cluster and check the PROTECTION_MODE and PROTECTION_LEVEL columns in the V$OB_CLUSTER view.

obclient> SELECT PROTECTION_MODE, PROTECTION_LEVEL FROM V$OB_CLUSTER;
+--------------------+--------------------+
| PROTECTION_MODE    | PROTECTION_LEVEL   |
+--------------------+--------------------+
| MAXIMUM PROTECTION | MAXIMUM PROTECTION |
+--------------------+--------------------+
1 row in set