Having HAProxy check mysql status through a xinetd script

HAProxy is able to load balance MySQL wonderfully. The main issue is how to make sure that the backend MySQL server to forward the request to is up and running (I mean not just to establish a connection to port 3306, I mean something more “complete”, that performs a little operation against the MySQL server).

It is possible to make haproxy check the status of a mysql server using a small shell script managed through the xinetd daemon.

What this script basically does is performs a basic operation against the mysql database then returns http status 200 if the operation was successful or http status 500 if it there was any error (i.e. mysql was not available).

Script

The script looks like this:

#!/bin/bash
#
# This script checks if a mysql server is healthy running on localhost. It will
# return:
#
# "HTTP/1.x 200 OKr" (if mysql is running smoothly)
#
# - OR -
#
# "HTTP/1.x 500 Internal Server Errorr" (else)
#
# The purpose of this script is make haproxy capable of monitoring mysql properly
#
# Author: Unai Rodriguez
#
# It is recommended that a low-privileged-mysql user is created to be used by
# this script. Something like this:
#
# mysql> GRANT SELECT on mysql.* TO 'mysqlchkusr'@'localhost' 
#     -> IDENTIFIED BY '257retfg2uysg218' WITH GRANT OPTION;
# mysql> flush privileges;

MYSQL_HOST="localhost"
MYSQL_PORT="3306"
MYSQL_USERNAME="mysqlchkusr"
MYSQL_PASSWORD="secret"

TMP_FILE="/tmp/mysqlchk.out"
ERR_FILE="/tmp/mysqlchk.err"

#
# We perform a simple query that should return a few results :-p
#
/usr/bin/mysql --host=$MYSQL_HOST --port=$MYSQL_PORT --user=$MYSQL_USERNAME 
	--password=$MYSQL_PASSWORD -e"show databases;" > $TMP_FILE 2> $ERR_FILE

#
# Check the output. If it is not empty then everything is fine and we return
# something. Else, we just do not return anything.
#
if [ "$(/bin/cat $TMP_FILE)" != "" ]
then
	# mysql is fine, return http 200
	/bin/echo -e "HTTP/1.1 200 OKrn"
	/bin/echo -e "Content-Type: Content-Type: text/plainrn"
	/bin/echo -e "rn"
	/bin/echo -e "MySQL is running.rn"
	/bin/echo -e "rn"
else
	# mysql is fine, return http 503
	/bin/echo -e "HTTP/1.1 503 Service Unavailablern"
	/bin/echo -e "Content-Type: Content-Type: text/plainrn"
	/bin/echo -e "rn"
	/bin/echo -e "MySQL is *down*.rn"
	/bin/echo -e "rn"
fi

Steps on the MySQL server

First, you should create the script somewhere, and assign proper permissions:

chown nobody /opt//mysqlchk
chmod   744  /opt//mysqlchk

Then, set permissions into the mysql server:

mysql> GRANT SELECT on mysql.* TO 'mysqlchkusr'@'localhost' 
    -> IDENTIFIED BY 'secret' WITH GRANT OPTION;
mysql> flush privileges;
mysql> exit

Test:

/opt/mysqlchk
HTTP/1.x 200 OK

Now, configure xinetd by adding this line at the bottom of /etc/services:

mysqlchk        9200/tcp                        # mysqlchk

Then add this file /etc/xinetd.d/mysqlchk:

# default: on
# description: mysqlchk
service mysqlchk
{
        flags           = REUSE
        socket_type     = stream
        port            = 9200
        wait            = no
        user            = nobody
        server          = /opt/mysqlchk
        log_on_failure  += USERID
        disable         = no
        only_from       = 0.0.0.0/0 # recommended to put the IPs that need
                                    # to connect exclusively (security purposes)
        per_source      = UNLIMITED # Recently added (May 20, 2010)
                                    # Prevents the system from complaining
                                    # about having too many connections open from
                                    # the same IP. More info:
                                    # http://www.linuxfocus.org/English/November2000/article175.shtml
}

Restart xinetd (you can watch for issues on /var/log/syslog):

/etc/init.d/xinetd stop
/etc/init.d/xinetd start

Test:

telnet localhost 9200
Trying 127.0.0.1...
Connected to localhost.localdomain.
Escape character is '^]'.
HTTP/1.1 200 OK

Content-Type: Content-Type: text/plain

MySQL is running.

Connection closed by foreign host.

Steps on the HAProxy server
Now, in order to make haproxy check the status of the mysql service through the xinetd-managed-script, we should add something similar to this on the haproxy.cfg file:

listen  MySQL 10.135.2.67:3306
        mode    tcp
	option  httpchk
        server  10.135.2.69:3306 10.135.2.69:3306 check port 9200 inter 12000 rise 3 fall 3
        source  10.135.2.67

What is important?

  1. option httpchk.- tells haproxy to check for full http response (i.e. http headers: 2xx OK or 5xx ERROR)
  2. check port XXXX.- tells haproxy to check the status of the service by sending an http request on that port

by

Comments

0 responses to “Having HAProxy check mysql status through a xinetd script”

  1. […] modified the mysqlchk_status.sh script found at SysBible with my […]

  2. Gael Avatar
    Gael

    One thing which is not mentionned here, is that you will need to open your firewall accordingly.

    In this case, protocol TCP on port 9200.

    Otherwise, it works perfectly!

  3. unai Avatar

    That is correct Gael, thank you for pointing it out.

  4. John Avatar
    John

    Thanks for this! The only thing I found I needed to change was:

    server = /opt/mysql-xinetd-monitor/mysqlchk

    changed to

    server = /opt/mysqlchk

  5. […] requests to the non-working slave.  For this I referenced a tutorial written by Unai Rodriquez  – Having HAProxy check mysql status through a xinetd script Essentially, you just create a xinetd service that listens on port 9200 and returns an HTTP status […]

  6. unai Avatar

    Dear John,

    Thank you so much for the feedback! I have just updated the doc.

  7. jose Avatar
    jose

    Thanks, it helps a lot.

  8. David Avatar
    David

    How does the replication works? For instance I make an Insert and the haproxy sends the request for 1 server. How does it propagates to the other mysqlServer?

  9. unai Avatar

    MySQL’s replication will take care of inserting that same data into the other server(s). More info: http://dev.mysql.com/doc/refman/5.0/en/replication.html

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