17.7.2 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.
Group Replication is built on GTID based replication, therefore you should also be aware of Section 22.214.171.124, “Restrictions on Replication with GTIDs”.
Gap Locks. Group Replication's certification process for concurrent transactions 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
For a group in multi-primary mode, unless you rely on
REPEATABLE READsemantics in your applications, we recommend using the
READ COMMITTEDisolation 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. For a group in single-primary mode, only the primary accepts writes, so the
READ COMMITTEDisolation level is not important to 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 Statements”) or named locks (see
Replication Event Checksums. Due to a design limitation of replication event checksums, Group Replication cannot currently make use of them. Therefore set
SERIALIZABLE Isolation Level.
SERIALIZABLEisolation level is not supported in multi-primary groups by default. Setting a transaction isolation level to
SERIALIZABLEconfigures 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
CASCADINGforeign 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=ONon 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.
MySQL Enterprise Audit and MySQL Enterprise Firewall. Prior to version 5.7.21 MySQL Enterprise Audit and MySQL Enterprise Firewall use
MyISAMtables in the
mysqlsystem database. Group Replication does not support
Multi-primary Mode Deadlock. When a group is operating in multi-primary mode,
SELECT .. FOR UPDATEstatements 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. 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.
The maximum number of MySQL servers that can be members of a single replication group is 9. If further members attempt to join the group, their request is refused. This limit has been identified from testing and benchmarking as a safe boundary where the group performs reliably on a stable local area network.
If an individual transaction results in message contents which are large enough that the message cannot be copied between group members over the network within a 5-second window, members can be suspected of having failed, and then expelled, just because they are busy processing the transaction. Large transactions can also cause the system to slow due to problems with memory allocation. To avoid these issues use the following mitigations:
Where possible, try and limit the size of your transactions. For example, split up files used with
LOAD DATAinto smaller chunks.
Use the system variable
group_replication_transaction_size_limitto specify a maximum transaction size that the group will accept. In MySQL 5.7, this system variable defaults to zero, but in MySQL 8.0, it defaults to a maximum transaction size of 150000000 bytes (approximately 143 MB). Transactions above this limit are rolled back and are not sent to Group Replication's Group Communication System (GCS) for distribution to the group. Adjust the value of this variable depending on the maximum message size that you need the group to tolerate, bearing in mind that the time taken to process a transaction is proportional to its size.
Use the system variable
group_replication_compression_thresholdto specify a message size above which compression is applied. This system variable defaults to 1000000 bytes (1 MB), so large messages are automatically compressed. Compression is carried out by Group Replication's Group Communication System (GCS) when it receives a message that was permitted by the
group_replication_transaction_size_limitsetting but exceeds the
group_replication_compression_thresholdsetting. If you set the system variable value to zero, compression is deactivated. For more information, see Section 126.96.36.199, “Message Compression”.
If you have deactivated message compression and do not specify a maximum transaction size, the upper size limit for a message that can be handled by the applier thread on a member of a replication group is the value of the member's
slave_max_allowed_packet system variable, which has a default and maximum value of 1073741824 bytes (1 GB). A message that exceeds this limit fails when the receiving member attempts to handle it. The upper size limit for a message that a group member can originate and attempt to transmit to the group is 4294967295 bytes (approximately 4 GB). This is a hard limit on the packet size that is accepted by the group communication engine for Group Replication (XCom, a Paxos variant), which receives messages after GCS has handled them. A message that exceeds this limit fails when the originating member attempts to broadcast it.