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:
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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)
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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:
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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’
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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):
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# master my.cnf
[client]
default–character–set=utf8
[mysqld]
character–set–server=utf8
collation–server=utf8_unicode_ci
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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 tocaching_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:
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Last_IO_Errno: 2059
Last_IO_Error: error connecting to master ‘root@127.0.0.1:19025’ – retry–time: 60 retries: 1
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To create a user using mysql_native_password :
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master> CREATE USER ‘replica_user’@‘%’ IDENTIFIED WITH mysql_native_password BY ‘repli$cat’;
master> GRANT REPLICATION SLAVE ON *.* TO ‘replica_user’@‘%’;
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Finally, we can proceed as usual to build the replication:
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master > show master status\G
*************************** 1. row ***************************
File: mysql–bin.000007
Position: 155
Binlog_Do_DB:
Binlog_Ignore_DB:
Executed_Gtid_Set:
1 row in set (0.00 sec)
slave > CHANGE MASTER TO MASTER_HOST=‘127.0.0.1’, MASTER_USER=‘replica_user’, MASTER_PASSWORD=‘repli$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;
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Checking the replication status:
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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)
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Executing a quick test to check if the replication is working:
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master > create database vinnie;
Query OK, 1 row affected (0.06 sec)
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slave > show databases like ‘vinnie’;
+—————————–+
| Database (vinnie) |
+—————————–+
| vinnie |
+—————————–+
1 row in set (0.00 sec)
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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:
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master > alter user replica_user identified with caching_sha2_password by ‘sekret’;
Query OK, 0 rows affected (0.01 sec)
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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”
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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.
Vinicius Grippa
Sponsored by Percona