MAN06 Failover for MySQL grid based services

!!MySQL DB Replication

The DB replication process is one of the ways to get scability and higher availability. This process will consist in keeping DB copies of a main host (MASTER) in a secondary host (SLAVE). By architecture, the slave host will only have read access to the Database entries.

In this document we use 'stunnel' for security reasons but we are still searching information on mysql ssl connections which we can apply in the future.

As a case example, I'm following the method used to replicate a voms server DB.

!!!GENERIC SETUP info

MASTER hostname: server1.lip.pt 
MASTER username: root 
MASTER mysql superuser: root 
MASTER mysql replication user: db_rep 
SLAVE hostname: server2.lip.pt
SLAVE username: root
SLAVE mysql superuser: root
DB to replicate: DB_*

!!!MASTER setup

__Step 1__: Configure stunnel to get secure connection between MASTER and SLAVE. Create /etc/stunnel.conf with the following contents:

# Authentication stuff
verify = 2
CApath = /etc/grid-security/certificates
cert = /etc/grid-security/hostcert.pem
key = /etc/grid-security/hostkey.pem

# Auth fails in chroot because CApath is not accessible
#chroot = /var/run/stunnel
#debug = 7
output = /var/log/stunnel.log
pid = /var/run/stunnel/master.pid

setuid = nobody
setgid = nobody

# Service-level configuration
\[mysql\]
accept  = 3307
connect = 127.0.0.1:3306

__Step 2__: Start the service 'stunnel' and add it to rc.local

mkdir /var/run/stunnel
chown nobody:nobody /var/run/stunnel
stunnel /etc/stunnel.conf
echo 'stunnel /etc/stunnel.conf' >> /etc/rc.local

__Step 3__: Setup the firewall to allow connections from SLAVE (ex.:server2.lip.pt) on port 3307 TCP

# MySQL replication
-A INPUT -s 200.143.244.108 -m state --state NEW -m tcp -p tcp --dport 3307 -j ACCEPT

__Step 4__: Edit /etc/my.cnf

Setup de ID of the Master (usually 1), setup the binary log for the DB to be replicated.

[mysqld]
server-id=1
# create replication log
log-bin = /path_to_log_file/log_file.index
# but only for this database
binlog-do-db=DB_2
binlog-do-db=DB_1

OBS. Please ensure that the path declared under log-bin has the mysql:mysql ownerships, and that the binary log exists.

__Step 5__: Restart MySQL

/etc/init.d/mysqld restart

__Step 6__: Add a specific Replication User

GRANT REPLICATION SLAVE ON *.* TO 'db_rep'@'127.0.0.1' IDENTIFIED BY '16_char_password';

__Step 7__: Make a backup of the Databases

mysqldump -u root -p --default-character-set=latin1 --master-data=2 --databases DB_1 DB_2 > dump.sql

The option "--master-data=2" writes a comment on dump.sql that show the log file and the ID to be used on the Slave Setup. This option also lock all the tables while they are beeing copied, avoiding problems.

!!!SLAVE Setup

OBS.It must be set the same timezone on master and slave. This prevent problems with the script that runs to verify consistency between servers.

__Step 1__: Setup stunnel. Create /etc/stunnel.conf with the following contents:

# Authentication stuff
verify = 2
CApath = /etc/grid-security/certificates
cert = /etc/grid-security/hostcert.pem
key = /etc/grid-security/hostkey.pem

# Auth fails in chroot because CApath is not accessible
#chroot = /var/run/stunnel
#debug = 7
output = /var/log/stunnel.log
pid = /var/run/stunnel/master.pid

setuid = nobody
setgid = nobody

# Use it for client mode
client = yes

# Service-level configuration
[mysql]
accept  = 127.0.0.1:3307
connect = server1.lip.pt:3307

__Step 2__: Start stunnel and add to rc.local

mkdir /var/run/stunnel
chown nobody:nobody /var/run/stunnel
stunnel /etc/stunnel.conf
[root@voms root]# echo 'stunnel /etc/stunnel.conf' >> /etc/rc.local

__Step 3__: Edit /etc/my.cnf

Setup the ID of SLAVE (ID 2).

\[mysqld\]
server-id=2
replicate-do-db=DB_1
replicate-do-db=DB_2

__Step 4__: Restart MySQL and insert dump.sql created on Master

/etc/init.d/mysql restart
mysql -u root -p < dump.sql

__Step 5__: Start slave

login in mysql of slave

mysql -u root -p

Run the 2 queries above changing the values xxxxx and yyyyy by the values on top of dump.sql file.

CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=3307,
                MASTER_USER='db_rep', MASTER_PASSWORD='16_char_password',
                MASTER_LOG_FILE='xxxxx', MASTER_LOG_POS=yyyyy;
SLAVE START;

__Step 6__: Check if everything is OK:

On mysql prompt run "show slave status\G", check the replication status.

Slave_IO_State: Waiting for master to send event

If status are diferent check if you can access to port 3307 of MASTER running

telnet server1.lip.pt 3307

If you can't reach it, you should also verify what is wrong on master, for example the firewall. Also you can see mysql errors on /var/lib/mysql

!!!TROUBLESHOOTING ===

* /var/log/stunnel.log
* SHOW MASTER STATUS;
* SHOW PROCESSLIST;
* SHOW SLAVE STATUS;

http://dev.mysql.com/doc/refman/5.1/en/replication-options-slave.html#option_mysqld_read-only