18.8.2 Group Replication Limitations
The following known limitations exist for Group Replication. Note that the limitations and issues described for multi-primary mode groups can also apply in single-primary mode clusters during a failover event, while the newly elected primary flushes out its applier queue from the old primary.
Tip
Group Replication is built on GTID based replication, therefore you should also be aware of Section 17.1.3.6, “Restrictions on Replication with GTIDs”.
-
Replication Event Checksums. Due to a design limitation of replication event checksums, Group Replication cannot currently make use of them. Therefore set
--binlog-checksum=NONE
. -
Gap Locks. The certification process does not take into account gap locks, as information about gap locks is not available outside of
InnoDB
. See Gap Locks for more information.Note
Unless you rely on
REPEATABLE READ
semantics in your applications, we recommend using theREAD COMMITTED
isolation level with Group Replication. InnoDB does not use gap locks inREAD COMMITTED
, which aligns the local conflict detection within InnoDB with the distributed conflict detection performed by Group Replication. -
Table Locks and Named Locks. The certification process does not take into account table locks (see Section 13.3.6, “LOCK TABLES and UNLOCK TABLES Syntax”) or named locks (see
GET_LOCK()
). -
SERIALIZABLE Isolation Level.
SERIALIZABLE
isolation level is not supported in multi-primary groups by default. Setting a transaction isolation level toSERIALIZABLE
configures Group Replication to refuse to commit the transaction. -
Concurrent DDL versus DML Operations. Concurrent data definition statements and data manipulation statements executing against the same object but on different servers is not supported when using multi-primary mode. During execution of Data Definition Language (DDL) statements on an object, executing concurrent Data Manipulation Language (DML) on the same object but on a different server instance has the risk of conflicting DDL executing on different instances not being detected.
-
Foreign Keys with Cascading Constraints. Multi-primary mode groups (members all configured with
group_replication_single_primary_mode=OFF
) do not support tables with multi-level foreign key dependencies, specifically tables that have definedCASCADING
foreign key constraints. This is because foreign key constraints that result in cascading operations executed by a multi-primary mode group can result in undetected conflicts and lead to inconsistent data across the members of the group. Therefore we recommend settinggroup_replication_enforce_update_everywhere_checks=ON
on server instances used in multi-primary mode groups to avoid undetected conflicts.In single-primary mode this is not a problem as it does not allow concurrent writes to multiple members of the group and thus there is no risk of undetected conflicts.
-
Very Large Transactions. Individual transactions that result in GTID contents which are large enough that it cannot be copied between group members over the network within a 5 second window can cause failures in the group communication. To avoid this issue try and limit the size of your transactions as much as possible. For example, split up files used with
LOAD DATA INFILE
into smaller chunks. -
Multi-primary Mode Deadlock. When a group is operating in multi-primary mode,
SELECT .. FOR UPDATE
statements can result in a deadlock. This is because the lock is not shared across the members of the group, therefore the expectation for such a statement might not be reached. -
Replication Filters. Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the
group_replication_applier
orgroup_replication_recovery
channels.
https://dev.mysql.com/doc/refman/8.0/en/group-replication-limitations.html