By default, members that find themselves in a minority due to a
network partition do not automatically leave the group. You can
use the system variable
to set a number of seconds for a member to wait after losing
contact with the majority of group members, and then exit the
group. Setting a timeout means you do not need to pro-actively
monitor for servers that are in a minority group after a network
partition, and you can avoid the possibility of creating a
split-brain situation (with two versions of the group
membership) due to inappropriate intervention.
When the timeout specified by
elapses, all pending transactions that have been processed by
the member and the others in the minority group are rolled back,
and the servers in that group move to the
ERROR state. You can use the
system variable to force the member to try to rejoin the group
automatically at this point. This feature is active by default;
the member makes three auto-rejoin attempts. If the auto-rejoin
procedure does not succeed or is not attempted, the minority
member then follows the exit action specified by
Consider the following points when deciding whether or not to set an unreachable majority timeout:
In a symmetric group, for example a group with two or four servers, if both partitions contain an equal number of servers, both groups consider themselves to be in a minority and enter the
ERRORstate. In this situation, the group has no functional partition.
While a minority group exists, any transactions processed by the minority group are accepted, but blocked because the minority servers cannot reach quorum, until either
STOP GROUP_REPLICATIONis issued on those servers or the unreachable majority timeout is reached.
If you do not set an unreachable majority timeout, the servers in the minority group never enter the
ERRORstate automatically, and you must stop them manually.
Setting an unreachable majority timeout has no effect if it is set on the servers in the minority group after the loss of majority has been detected.
If you do not use the
variable, the process for operator invention in the event of a
network partition is described in
Section 20.7.8, “Handling a Network Partition and Loss of Quorum”. The
process involves checking which servers are functioning and
forcing a new group membership if necessary.