Failover in MySQL Chain Replication We have a 4 server setup, 1 master, 3 daisy-chained slaves, in the following setup:
A (master)   -&gt;   B (slave)   -&gt;   C (slave)   -&gt;   D (slave)
(the servers B and C and D are running with log-slave-updates)
In normal operation everything works as expected: if we add new data to A, we see it show up quickly in B and C and D
Now we want to create a failure scenario -- we shutdown A and want to make B the new master:
B (master)   -&gt;   C (slave)   -&gt;   D (slave)
It seems like what we want to do is fairly simple -- switch B from Slave to Master
We are trying to follow the documentation &quot;Switching Sources During Failover&quot;
https://dev.mysql.com/doc/refman/8.0/en/replication-solutions-switch.html
The doc says &quot; On the replica Replica 1 being promoted to become the source, issue STOP REPLICA | SLAVE and RESET MASTER.&quot;
So if we're reading correctly, to switch B from Slave to Master all we have to do is run:
STOP SLAVE
RESET MASTER

Running &quot;STOP SLAVE&quot; causes no issues, but running &quot;RESET MASTER&quot; breaks the replication to downstream staves C and D. This is the error on C:
Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'Client requested master to start replication from position &gt; file size'
or sometimes:
Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'I/O error reading log event; the first event 'mysql-bin.000001' at 14282060, the last event read from './mysql-bin.000001' at 14282683, the last byte read from './mysql-bin.000001' at 14282683.'
So what is the point of &quot;RESET MASTER&quot; and why does it break the chain? Is there any harm in omitting it/how does one properly do a failover in MySQL chain replication?
You're not switching sources so there is nothing required to migrate from an A master to a B master.
To clear up the record of A being a master of B however the following may be desirable:

(B) STOP SLAVE will stop (B) from trying to get more binlog events
(B) RESET SLAVE ALL will make (B) forget about the (A) master.
(B) RESET MASTER is wrong. (B) is the master of (C). It was even before (A) is removed.