Replicating from MySQL 8.0 to MySQL 5.7

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 '[email protected]: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 .