MySQL 8.4 Release Notes
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
replication_group_members table.
Any uncommitted transactions that the group is waiting on must be committed, rolled back, or terminated before the operation can complete. 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 MySQL version 8.0.29 or newer.
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.
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);This function waits for all ongoing transactions and DML operations to finish before electing the new primary. In MySQL 8.4, it also waits for the completion of any ongoing DDL statements such as
ALTER TABLE. Operations that are considered DDL statements for this purpose are listed here:ALTER TABLE
This also includes any open cursors (see Section 15.6.6, “Cursors”).
For more information, see Section 20.5.1.1, “Changing the Primary”.