When an abnormally large message is sent between Group Replication group members, it can result in some group members being reported as failed and expelled from the group. This is because the single thread used by Group Replication's group communication engine (XCom, a Paxos variant) is occupied processing the message for too long, so some of the group members might report the receiver as failed. By default, large messages are automatically split into fragments that are sent separately and reassembled by the recipients.
The system variable
specifies a maximum message size for Group Replication
communications, above which messages are fragmented. The default
maximum message size is 10485760 bytes (10 MiB). The greatest
permitted value is the same as the maximum value of the
variable (1 GB).
must be less than
because the applier thread cannot handle message fragments larger
than the maximum permitted packet size. To switch off
fragmentation, specify a zero value for
As with most other Group Replication system variables, you must restart the Group Replication plugin for the change to take effect. For example:
STOP GROUP_REPLICATION; SET GLOBAL group_replication_communication_max_message_size= 5242880; START GROUP_REPLICATION;
Message delivery for a fragmented message is considered complete when all the fragments of the message have been received and reassembled by all the group members. Fragmented messages include information in their headers that enables a member joining during message transmission to recover the earlier fragments that were sent before it joined. If the joining member fails to recover the fragments, it expels itself from the group.
The Group Replication communication protocol version in use by the
group must allow fragmentation. You can obtain the communication
protocol in use by a group using
which returns the oldest MySQL Server version that the group
supports. If necessary, use
to set the communication protocol version high enough (8.0.16 or
above) to allow fragmentation. For more information, see
Section 126.96.36.199, “Setting a Group's Communication Protocol Version”.
If a replication group cannot use fragmentation because some
members do not support it, the system variable
can be used to limit the maximum size of transactions the group
accepts. Transactions above this size are rolled back. You can
to allow additional time (up to an hour) before a member under
suspicion of having failed is expelled from the group.