MySQL Replication

Set up MySQL master-slave replication

Master-slave data replication allows for replicated data to be copied to multiple computers for backup and analysis by multiple parties. Needed changes identified by a group member must to be submitted to the designated “master” of the node. This differs from Master-Master replication, in which data can be updated by any authorized contributor of the group.

MySQL allows you to build up complex replication hierarchies, such as multi-master, chains of read slaves, backup databases at a remote site or any combination of these. This post focuses on a simple single master to single slave topology – the more complex solutions are built from this basic building block.

This post also makes the assumption that the 2 MySQL Servers have been installed but that there is no existing data in the master that needs to be copied to the slave – it’s not complex to add that extra requirement and it will be covered in a future post.

Server “black” ( is to be our master and “blue” ( the slave.

Step 1: Edit the configuration files & start the MySQL Servers

The first step in setting up replication involves editing the “my.cnf” file on the servers that will serve as the master and slave. A default is provided with the MySQL installation but in case there is already a production MySQL database running on these servers, we provide local configuration files “master.cnf” and “slave.cnf” that will be used when starting up the MySQL servers.

At a minimum we’ll want to add two options to the [mysqld] section of the master.cnf file:

  • log-bin: in this example we choose black-bin.log
  • server-id: in this example we choose 1. The server cannot act as a replication master unless binary logging is enabled. The server_id variable must be a positive integer value between 1 to 2^32








Note: For the greatest possible durability and consistency in a replication setup using InnoDB with transactions, you should also specify the innodb_flush_log_at_trx_commit=1, sync_binlog=1 options.

Next, you’ll need to add the server-id option to the [mysqld] section of the slave’s slave.cnf file. The server-id value, like the master_id value, must be a positive integer between 1 to 2^32, It is also necessary that the ID of the slave be different from the ID of the master. If you are setting up multiple slaves, each one must have a unique server-id value that differs from that of the master and from each of the other slaves.







Now, start the MySQL servers using the service manager or directly from the command line if not being run as a service:

[billy@black ~]$ mysqld --defaults-file=/home/billy/mysql/master/master.cnf &
[billy@blue ~]$ mysqld --defaults-file=/home/billy/mysql/slave/slave.cnf&

Step 2: Create Replication User

Create an account on the master server that the slave server can use to connect. This account must be given the REPLICATION SLAVE privilege:

[billy@black ~]$ mysql -u root --prompt='master> 'master> CREATE USER repl_user@;master> GRANT REPLICATION SLAVE ON *.* TO repl_user@ IDENTIFIED BY 'billy';



| File             | Position  | Binlog_Do_DB | Binlog_Ignore_DB |


| master-bin.000004 |        4 |              | mysql            |


1 row in set (0.00 sec)

Step 3: Initialize Replication

We are now ready to initialize replication on the slave; issue a CHANGE MASTER command:


-> MASTER_USER=’repl_user’,





  • MASTER_HOST: the IP or hostname of the master server, in this example blue or
  • MASTER_USER: this is the user we granted the REPLICATION SLAVE privilege to in Step 2, in this example, “repl_user”
  • MASTER_PASSWORD: this is the password we assigned to ”rep_user” in Step 2
  • MASTER_LOG_FILE: is an empty string (wouldn’t be empty if there were existing writes to be picked up from the master)  Ex:-master-bin.000004
  • MASTER_LOG_POS: is 4 (would likely be different if there were existing writes to be picked up from the master)

Finally, start replication on the slave:

slave> start slave;

Step 4: Basic Checks

Now we are ready to perform a basic check to ensure that replication is indeed working. In this example we insert a row of data into the “simples” table on the master server and then verify that these new rows materialize on the slave server:

master> create database clusterdb

;master> create table clusterdb.simples (id int not null primary key) engine=ndb;

master> insert into clusterdb.simples values (999),(1),(2),(3);

slave> select * from clusterdb.simples;


| id  |


|   1 |

|   2 |

|   3 |

| 999 |