Get MySQL Replication up and running in 5 minutes

MySQL Replication is incredibly simple to get up and running and this (short) post steps you through it.

Simple Master -> Slave Replication

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” (192.168.0.31) is to be our master and “blue” (192.168.0.34) 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

master.cnf:

[mysqld]
server-id=1
log-bin=black-bin.log
datadir=/home/billy/mysql/master/data
innodb_flush_log_at_trx_commit=1
sync_binlog=1

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.

slave.cnf:

[mysqld]
server-id=2
relay-log-index=slave-relay-bin.index
relay-log=slave-relay-bin
datadir=/home/billy/mysql/slave/data

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@192.168.0.34;
master> GRANT REPLICATION SLAVE ON *.* TO repl_user@192.168.0.34 IDENTIFIED BY 'billy';

Step 3: Initialize Replication

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

slave> CHANGE MASTER TO MASTER_HOST='192.168.0.31',
 -> MASTER_USER='repl_user',
 -> MASTER_PASSWORD='billy',
 -> MASTER_LOG_FILE='',
 -> MASTER_LOG_POS=4;

Where:

  • MASTER_HOST: the IP or hostname of the master server, in this example blue or 192.168.0.31
  • 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)
  • 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 |
 +-----+




10 comments

  1. Shlomi Noach says:

    Hi,
    Good write. Typo on last example though, should remove “slave> create database clusterdb;”

    Regards

    • admin says:

      Thanks Shlomi,

      I’ve found that I’ve needed to create the database on the slave as well as the master (after that, table creations get replicated).

      Regards, Andrew.

  2. Shlomi Noach says:

    That’s very strange. Were you applying any replicate-do-db and co. parameters?

    • admin says:

      Hi Schlomi,

      you were quite right – there’s no need to manually create the database on the slave as replication does it for you (I was getting confused with restoring a backup when introducing replication to an established database).

      Thanks again!

  3. magix says:

    First, I got “Error connecting to master: Access denied for user ‘repl_user’@'blue’ (using password: YES)”.

    Solution : GRANT REPLICATION SLAVE ON *.* TO repl_user@’%’ IDENTIFIED BY ‘billy’;

    (Or @’blue’ maybe more secure)

    Then I tried LOAD DATA FROM MASTER;
    But I had to add SUPER priviledge.

    • admin says:

      magix – the “GRANT REPLICATION…” statement is included in Step 2 of the procedure.

      Regards, Andrew.

  4. Tash Pemhiwa says:

    Between step 2 and step 3, I would add this:

    master> show master status;
    ++++–+
    | File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
    ++++–+
    | master-bin.000004 | 4 | | |
    ++++–+
    1 row in set (0.00 sec)

    This gives you the position to use for step 3′s
    MASTER_LOG_POS. This is especially important if your setup requires you to first import data from the master to the slave.

  5. MarkS says:

    Awesome. Yes just correct innodb for DB. I even did this on same box with two instances using localhost address

    Works great, thank you very much.

  6. sajid says:

    it very helpfull..but let me confirm one thing that i need to create master.cnf & slve.cnf new files are can use alrdy existng file like my.cnf ??

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