How to Use xinetd to Build a Quick MySQL Health-Check Service for HAProxy

Scenario

Using HAProxy to load‑balance MySQL is common; to improve availability, HAProxy should stop forwarding requests as soon as a MySQL instance fails or replication breaks.

How HAProxy Detects MySQL Issues

HAProxy can query an HTTP service that runs a health‑check script; based on the HTTP status (200 for healthy, 503 for unhealthy) HAProxy decides whether the MySQL node is usable.

Solution Overview

Write a shell script that checks MySQL status and exits with 200 (healthy) or 503 (unhealthy) together with a message.

Expose the script via a simple HTTP service.

Configure HAProxy to call this HTTP service and interpret the response.

Quick Way to Expose a Shell Script as HTTP – xinetd

xinetd is an extended Internet daemon that can listen on a port, launch a script when a connection arrives, and return the script’s output directly.

HAProxy → xinetd → mysql‑check script, allowing HAProxy to obtain MySQL status.

xinetd Configuration Example

Open port 9200 and return the output of a test script.

Install xinetd if not present (e.g., yum install xinetd on CentOS 7).

1. Test script (test.sh)

#!/bin/bash
echo `uptime | egrep -o 'up ([0-9]+) days' | awk '{print $2}'`

This script prints the number of days the server has been up. Make it executable with chmod +x test.sh.

2. xinetd service definition (/etc/xinetd.d/helloworld)

service helloworld
{
    disable     = no
    port        = 9200
    socket_type = stream
    protocol    = tcp
    wait        = no
    user        = root
    server      = /root/test.sh
    server_args = test
}

Key fields: port specifies the listening port, server points to the script to execute.

3. Add service to /etc/services

# Comment out the original 9200 entry and add:
helloworld      9200/tcp
#wap-wsp        9200/tcp
#wap-wsp        9200/udp

4. Restart xinetd

systemctl restart xinetd.service

5. Test the service

Use nc localhost 9200 or telnet localhost 9200 to verify the script output is returned.

The service correctly outputs the script result, completing the example.

For a complete HAProxy‑aware MySQL health‑check implementation, see the GitHub article linked in the original post.