How to fix ERROR 1201: Could not initialize master info in MySQL

Connect to the slave MySQL server and stop replication before making any changes:

STOP SLAVE;

If you are using MySQL 8.0.22 or later, use STOP REPLICA instead:

STOP REPLICA;

Wait for any running SQL statements to complete.

Clear the existing master info and relay log files to remove corrupted data:

RESET SLAVE;

For MySQL 8.0.22 or later:

RESET REPLICA;

This command deletes the master.info file, relay-log.info file, and all relay log files. If you need to preserve relay logs for debugging, skip this step and manually delete the files from the MySQL data directory.

Ensure the MySQL user has proper read and write permissions on the data directory:

sudo chown -R mysql:mysql /var/lib/mysql
sudo chmod 750 /var/lib/mysql

Replace /var/lib/mysql with your actual MySQL data directory if different. This is critical for MySQL to create and access the new master.info file.

Establish a new replication connection with correct master parameters:

CHANGE MASTER TO
  MASTER_HOST='your.master.host',
  MASTER_USER='replication_user',
  MASTER_PASSWORD='password',
  MASTER_LOG_FILE='mysql-bin.000001',
  MASTER_LOG_POS=0;

For MySQL 8.0.23 or later, use CHANGE REPLICATION SOURCE instead:

CHANGE REPLICATION SOURCE TO
  SOURCE_HOST='your.master.host',
  SOURCE_USER='replication_user',
  SOURCE_PASSWORD='password',
  SOURCE_LOG_FILE='mysql-bin.000001',
  SOURCE_LOG_POS=0;

Replace the master host, credentials, and log file name with your actual values. Get the correct MASTER_LOG_FILE and MASTER_LOG_POS from the master using SHOW MASTER STATUS.

Start the slave with the new configuration:

START SLAVE;

For MySQL 8.0.22 or later:

START REPLICA;

Check that replication is running correctly:

SHOW SLAVE STATUS\G

For MySQL 8.0.22 or later:

SHOW REPLICA STATUS\G

Verify that:
- Slave_IO_Running: Yes (or Replica_IO_Running: Yes)
- Slave_SQL_Running: Yes (or Replica_SQL_Running: Yes)
- Seconds_Behind_Master: 0 or a small number (not NULL)

If either running status is "No", check the error output and consult the MySQL error log.