In this blog post, we’ll discuss how to set a replication from MySQL 8.0 to MySQL 5.7. There are some situations that having this configuration might help. For example, in the case of a MySQL upgrade, it can be useful to have a master that is using a newer version of MySQL to an older version slave as a rollback plan. Another example is in the case of upgrading a master x master replication topology.
Officially, replication is only supported between consecutive major MySQL versions, and only from a lower version master to a higher version slave. Here is an example of a supported scenario:
5.7 master –> 8.0 slave
while the opposite is not supported:
8.0 master –> 5.7 slave
In this blog post, I’ll walk through how to overcome the initial problems to set a replication working in this scenario. I’ll also show some errors that can halt the replication if a new feature from MySQL 8 is used.
Here is the initial set up that will be used to build the topology:
slave > select @@version; +---------------+ | @@version | +---------------+ | 5.7.17-log | +---------------+ 1 row in set (0.00 sec) master > select @@version; +-----------+ | @@version | +-----------+ | 8.0.12 | +-----------+ 1 row in set (0.00 sec)
First, before executing the CHANGE MASTER command, you need to modify the collation on the master server. Otherwise the replication will run into this error:
slave > show slave status\G
Last_Errno: 22
Last_Error: Error 'Character set '#255' is not a compiled character set and is not specified in the '/opt/percona_server/5.7.17/share/charsets/Index.xml' file' on query. Default database: 'mysql8_1'. Query: 'create database mysql8_1'
This is because the default character_set and the collation has changed on MySQL 8. According to the documentation:
The default value of the character_set_server and character_set_database system variables has changed from latin1 to utf8mb4.
The default value of the collation_server and collation_database system variables has changed from latin1_swedish_ci to utf8mb4_0900_ai_ci.
Let’s change the collation and the character set to utf8 on MySQL 8 (it is possible to use any option that exists in both versions):
# master my.cnf [client] default-character-set=utf8 [mysqld] character-set-server=utf8 collation-server=utf8_unicode_ci
You need to restart MySQL 8 to apply the changes. Next, after the restart, you have to create a replication user using mysql_native_password.This is because MySQL 8 changed the default Authentication Plugin to caching_sha2_password which is not supported by MySQL 5.7. If you try to execute the CHANGE MASTER command with a user using caching_sha2_password plugin, you will receive the error message below:
Last_IO_Errno: 2059 Last_IO_Error: error connecting to master 'root@127.0.0.1:19025' - retry-time: 60 retries: 1
To create a user using mysql_native_password :
master> CREATE USER 'replica_user'@'%' IDENTIFIED WITH mysql_native_password BY 'repli$cat'; master> GRANT REPLICATION SLAVE ON *.* TO 'replica_user'@'%';
Finally, we can proceed as usual to build the replication:
master > show master status\G *************************** 1. row *************************** File: mysql-bin.000007 Position: 155 Binlog_Do_DB: Binlog_Ignore_DB: Executed_Gtid_Set: 1 row in set (0.00 sec) slave > CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_USER='replica_user', MASTER_PASSWORD='repli$cat',MASTER_PORT=19025, MASTER_LOG_FILE='mysql-bin.000007', MASTER_LOG_POS=155; start slave; Query OK, 0 rows affected, 2 warnings (0.01 sec) Query OK, 0 rows affected (0.00 sec) # This procedure works with GTIDs too slave > CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_USER='replica_user', MASTER_PASSWORD='repli$cat',MASTER_PORT=19025,MASTER_AUTO_POSITION = 1 ; start slave;
Checking the replication status:
master > show slave status\G *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 127.0.0.1 Master_User: replica_user Master_Port: 19025 Connect_Retry: 60 Master_Log_File: mysql-bin.000007 Read_Master_Log_Pos: 155 Relay_Log_File: mysql-relay.000002 Relay_Log_Pos: 321 Relay_Master_Log_File: mysql-bin.000007 Slave_IO_Running: Yes Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 155 Relay_Log_Space: 524 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Master_SSL_Allowed: No Master_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: 0 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 100 Master_UUID: 00019025-1111-1111-1111-111111111111 Master_Info_File: /home/vinicius.grippa/sandboxes/rsandbox_5_7_17/master/data/master.info SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates Master_Retry_Count: 86400 Master_Bind: Last_IO_Error_Timestamp: Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: Auto_Position: 0 Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version: 1 row in set (0.01 sec)
Executing a quick test to check if the replication is working:
master > create database vinnie; Query OK, 1 row affected (0.06 sec)
slave > show databases like 'vinnie'; +-------------------+ | Database (vinnie) | +-------------------+ | vinnie | +-------------------+ 1 row in set (0.00 sec)
Caveats
Any tentative attempts to use a new feature from MySQL 8 like roles, invisible indexes or caching_sha2_password will make the replication stop with an error:
master > alter user replica_user identified with caching_sha2_password by 'sekret'; Query OK, 0 rows affected (0.01 sec)
slave > show slave status\G
Last_SQL_Errno: 1396
Last_SQL_Error: Error 'Operation ALTER USER failed for 'replica_user'@'%'' on query. Default database: ''. Query: 'ALTER USER 'replica_user'@'%' IDENTIFIED WITH 'caching_sha2_password' AS '$A$005$H MEDi\"gQ
wR{/I/VjlgBIUB08h1jIk4fBzV8kU1J2RTqeqMq8Q2aox0''
Summary
Replicating from MySQL 8 to MySQL 5.7 is possible. In some scenarios (especially upgrades), this might be helpful, but it is not advisable to have a heterogeneous topology because it will be prone to errors and incompatibilities under some cases.
You might also like:
- Migrating database charsets to utf8mb4: a story from the trenches
- This webinar might also have some useful pointers Troubleshooting issues with MySQL character sets
The post Replicating from MySQL 8.0 to MySQL 5.7 appeared first on Percona Database Performance Blog.