How to fix ERROR 1200: Not configured as slave in MySQL

First, verify that the server has a valid, unique server ID. Run this command:

SHOW VARIABLES LIKE 'server_id';

The result should show a non-zero value. If it shows 0 or is missing, the server cannot participate in replication. Each server in a replication topology must have a unique server_id.

Edit your MySQL configuration file (/etc/my.cnf or /etc/mysql/my.cnf on Linux, or C:\\ProgramData\\MySQL\\MySQL Server\\my.ini on Windows) and ensure it contains the required replication settings:

[mysqld]
server-id=2
log_bin=mysql-bin
relay-log=mysql-relay-bin
read-only=1

Key settings explained:
- server-id=2 - Unique ID for this slave (must be different from master and other slaves)
- log_bin=mysql-bin - Enable binary logging (required for replication)
- relay-log=mysql-relay-bin - Relay log for received changes
- read-only=1 - Optional: makes slave read-only to prevent accidental writes

After editing, save the file and restart MySQL:

# Linux/macOS
sudo systemctl restart mysql

# Or if using a Docker container:
docker restart <container_name>

On the slave server, run the CHANGE MASTER TO command with the correct master connection details. You'll need information from the master server:

First, on the MASTER server, run:

SHOW MASTER STATUS;

This shows:
- File (binary log file name)
- Position (log position)

Then on the SLAVE server, run:

CHANGE MASTER TO
  MASTER_HOST='192.168.1.100',
  MASTER_USER='replication_user',
  MASTER_PASSWORD='your_password',
  MASTER_LOG_FILE='mysql-bin.000001',
  MASTER_LOG_POS=154;

Replace with your actual values:
- MASTER_HOST - IP address or hostname of the master server
- MASTER_USER - Username of the replication user on the master
- MASTER_PASSWORD - Password for that user
- MASTER_LOG_FILE - From SHOW MASTER STATUS output
- MASTER_LOG_POS - From SHOW MASTER STATUS output

After configuring the slave, start replication:

START SLAVE;

Then check the status:

SHOW SLAVE STATUS\G

Look for these critical fields:
- Slave_IO_Running: Yes - Network thread is connected to master
- Slave_SQL_Running: Yes - SQL thread is executing master's changes
- Last_Error: (empty) - No replication errors
- Seconds_Behind_Master: 0 - Slave is caught up with master

If both IO and SQL threads show "Yes" and there are no errors, replication is working correctly.

If CHANGE MASTER TO returns an error:

1. Empty MASTER_HOST error: Do not set MASTER_HOST to an empty string. Either provide a valid host or use CHANGE MASTER TO ... MASTER_HOST='actual_host'.

2. Duplicate server-id: Ensure no other server in your replication topology has the same server_id. Each server must be unique.

3. File does not exist: Verify the MASTER_LOG_FILE exists on the master. Run SHOW BINARY LOGS; on the master to list available logs.

4. Invalid credentials: Test the replication user credentials by connecting from the slave:

mysql -h 192.168.1.100 -u replication_user -p

5. Network connectivity: Ensure the slave can reach the master on port 3306:

telnet 192.168.1.100 3306