When you upgrade servers that participate in a replication setup, the procedure for upgrading depends on the current server versions and the version to which you are upgrading. This section provides information about how upgrading affects replication. For general information about upgrading MySQL, see Section 2.11, “Upgrading MySQL”

When you upgrade a master to 8.0 from an earlier MySQL release series, you should first ensure that all the slaves of this master are using the same 8.0.x release. If this is not the case, you should first upgrade the slaves. To upgrade each slave, shut it down, upgrade it to the appropriate 8.0.x version, restart it, and restart replication. Relay logs created by the slave after the upgrade are in 8.0 format.

Changes affecting operations in strict SQL mode (STRICT_TRANS_TABLES or STRICT_ALL_TABLES) may result in replication failure on an upgraded slave. If you use statement-based logging (binlog_format=STATEMENT), if a slave is upgraded before the master, the nonupgraded master will execute statements without error that may fail on the slave and replication will stop. To deal with this, stop all new statements on the master and wait until the slaves catch up. Then upgrade the slaves. Alternatively, if you cannot stop new statements, temporarily change to row-based logging on the master (binlog_format=ROW) and wait until all slaves have processed all binary logs produced up to the point of this change. Then upgrade the slaves.

The default character set has changed from latin1 to utf8mb4 in MySQL 8.0. In a replicated setting, when upgrading from MySQL 5.7 to 8.0, it is advisable to change the default character set back to the character set used in MySQL 5.7 before upgrading. After the upgrade is completed, the default character set can be changed to utf8mb4. Assuming that the previous defaults were used, one way to preserve them is to start the server with these lines in the my.cnf file:

[mysqld]
character_set_server=latin1
collation_server=latin1_swedish_ci

After the slaves have been upgraded, shut down the master, upgrade it to the same 8.0.x release as the slaves, and restart it. If you had temporarily changed the master to row-based logging, change it back to statement-based logging. The 8.0 master is able to read the old binary logs written prior to the upgrade and to send them to the 8.0 slaves. The slaves recognize the old format and handle it properly. Binary logs created by the master subsequent to the upgrade are in 8.0 format. These too are recognized by the 8.0 slaves.

In other words, when upgrading to MySQL 8.0, the slaves must be MySQL 8.0 before you can upgrade the master to 8.0. Note that downgrading from 8.0 to older versions does not work so simply: You must ensure that any 8.0 binary log or relay log has been fully processed, so that you can remove it before proceeding with the downgrade.

Some upgrades may require that you drop and re-create database objects when you move from one MySQL series to the next. For example, collation changes might require that table indexes be rebuilt. Such operations, if necessary, are detailed at Section 2.11.4, “Changes in MySQL 8.0”. It is safest to perform these operations separately on the slaves and the master, and to disable replication of these operations from the master to the slave. To achieve this, use the following procedure:

If you are upgrading an existing replication setup from a version of MySQL that does not support global transaction identifiers to a version that does, you should not enable GTIDs on either the master or the slave before making sure that the setup meets all the requirements for GTID-based replication. See Section 17.1.3.4, “Setting Up Replication Using GTIDs”, which contains information about converting existing replication setups to use GTID-based replication.

Prior to MySQL 8.0.16, when the server is running with global transaction identifiers (GTIDs) enabled (gtid_mode=ON), do not enable binary logging by mysql_upgrade (the --write-binlog option). As of MySQL 8.0.16, the server performs the entire MySQL upgrade procedure, but disables binary logging during the upgrade, so there is no issue.

It is not recommended to load a dump file when GTIDs are enabled on the server (gtid_mode=ON), if your dump file includes system tables. mysqldump issues DML instructions for the system tables which use the non-transactional MyISAM storage engine, and this combination is not permitted when GTIDs are enabled. Also be aware that loading a dump file from a server with GTIDs enabled, into another server with GTIDs enabled, causes different transaction identifiers to be generated.