14.18.1.1 Function which Configures Group Replication Primary

The following function enables you to set a member of a single-primary replication group to take over as the primary. The current primary becomes a read-only secondary, and the specified group member becomes the read-write primary. The function can be used on any member of a replication group running in single-primary mode. This function replaces the usual primary election process; see Section 20.5.1.1, “Changing the Primary”, for more information.

If a standard source to replica replication channel is running on the existing primary member in addition to the Group Replication channels, you must stop that replication channel before you can change the primary member. You can identify the current primary using the MEMBER_ROLE column in the Performance Schema table replication_group_members, or the group_replication_primary_member status variable.

Any uncommitted transactions that the group is waiting on must be committed, rolled back, or terminated before the operation can complete. Before MySQL 8.0.29, the function waits for all active transactions on the existing primary to end, including incoming transactions that are started after the function is used. From MySQL 8.0.29, you can specify a timeout for transactions that are running when you use the function. For the timeout to work, all members of the group must be at MySQL 8.0.29 or higher.

When the timeout expires, for any transactions that did not yet reach their commit phase, the client session is disconnected so that the transaction does not proceed. Transactions that reached their commit phase are allowed to complete. When you set a timeout, it also prevents new transactions starting on the primary from that point on. Explicitly defined transactions (with a START TRANSACTION or BEGIN statement) are subject to the timeout, disconnection, and incoming transaction blocking even if they do not modify any data. To allow inspection of the primary while the function is operating, single statements that do not modify data, as listed in Permitted Queries Under Consistency Rules, are permitted to proceed.

• group_replication_set_as_primary()

  Appoints a specific member of the group as the new primary, overriding any election process.

  Syntax:

  STRING group_replication_set_as_primary(member_uuid[, timeout])

  Arguments:

  • member_uuid: A string containing the UUID of the member of the group that you want to become the new primary.

  • timeout: An integer specifying a timeout in seconds for transactions that are running on the existing primary when you use the function. You can set a timeout from 0 seconds (immediately) up to 3600 seconds (60 minutes). When you set a timeout, new transactions cannot start on the primary from that point on. There is no default setting for the timeout, so if you do not set it, there is no upper limit to the wait time, and new transactions can start during that time. This option is available from MySQL 8.0.29.

  Return value:

  A string containing the result of the operation, for example whether it was successful or not.

  Example:

  SELECT group_replication_set_as_primary(‘00371d66-3c45-11ea-804b-080027337932’, 300);

  For more information, see Section 20.5.1.1, “Changing the Primary”.