17.7.2 Group Replication Limitations

The following known limitations exist for Group Replication.

• 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 the READ COMMITTED isolation level with Group Replication. InnoDB does not use gap locks in READ 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 13.3.5, “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 to SERIALIZABLE 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 defined CASCADING 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 setting group_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.

• Creating a Cluster from a Group.  Using the adoptFromGR option with the dba.createCluster() function to create a cluster based on an existing deployment of Group Replication fails with an error that the instance is already part of a replication group. This happens in MySQL Shell's default wizard mode only. A workaround is to disable wizard mode by launching mysqlsh with the --no-wizard command option.