Implementing High Availability for Nginx with Keepalived on CentOS

Preparation

Two virtual machines with IPs 192.168.16.128 and 192.168.16.129 are required, each with Nginx already installed.

Install Nginx

Update the yum repository and install Nginx:

rpm -ivh http://nginx.org/packages/centos/7/noarch/RPMS/nginx-release-centos-7-0.el7.ngx.noarch.rpm
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

Install Nginx:

yum -y install nginx

Control commands:

systemctl start nginx   # start Nginx
systemctl stop nginx    # stop Nginx

What is High Availability?

High Availability (HA) aims to minimise service downtime. In production, Nginx often acts as a reverse proxy; if it fails, all external services become unavailable. Keepalived can provide HA for Nginx.

Problems Addressed

When Nginx crashes, the service is inaccessible. Using Keepalived with VRRP allows automatic failover.

Dual‑Machine Hot‑Standby Scheme

One server provides the service (MASTER) while the other stays on standby (BACKUP). If the MASTER fails, the BACKUP takes over.

What is Keepalived?

Keepalived was originally designed to manage LVS clusters and implements VRRP for HA. It can also provide HA for services such as Nginx, HAProxy, MySQL, etc.

Failover Mechanism

Keepalived uses VRRP heartbeats. The MASTER continuously sends multicast heartbeats to the BACKUP. If the BACKUP stops receiving heartbeats, it assumes the virtual IP (VIP) and services.

Implementation Process

Install Keepalived

yum -y install keepalived

Configure Master (192.168.16.128)

Edit /etc/keepalived/keepalived.conf :

# detection script
vrrp_script chk_http_port {
    script "/usr/local/src/check_nginx_pid.sh"   # check Nginx status
    interval 2
    weight 2
}

# VRRP instance
vrrp_instance VI_1 {
    state MASTER
    interface ens33
    virtual_router_id 66
    priority 100
    advert_int 1
    authentication {
        auth_type PASS
        auth_pass 1111
    }
    track_script { chk_http_port }
    virtual_ipaddress { 192.168.16.130 }
}

Configure Backup (192.168.16.129)

# detection script
vrrp_script chk_http_port {
    script "/usr/local/src/check_nginx_pid.sh"
    interval 2
    weight 2
}

vrrp_instance VI_1 {
    state BACKUP
    interface ens33
    virtual_router_id 66
    priority 99
    advert_int 1
    authentication {
        auth_type PASS
        auth_pass 1111
    }
    track_script { chk_http_port }
    virtual_ipaddress { 192.168.16.130 }
}

Health‑Check Script (check_nginx_pid.sh)

#!/bin/bash
# Check if Nginx is running
A=`ps -C nginx --no-header | wc -l`
if [ $A -eq 0 ]; then
    systemctl start nginx   # try to start Nginx
    if [ `ps -C nginx --no-header | wc -l` -eq 0 ]; then
        # if start fails, stop Keepalived to trigger VIP takeover
        killall keepalived
    fi
fi

Make the script executable:

chmod 775 check_nginx_pid.sh

Failover Demonstration

Access the service via the virtual IP 192.168.16.130 . Initially the page shows 192.168.16.128 , indicating the MASTER is serving.

Stop Nginx on the MASTER:

systemctl stop nginx   # stop Nginx on 192.168.16.128

The VIP still points to the MASTER because the script restarts Nginx. After shutting down the MASTER machine, the VIP automatically moves to the BACKUP ( 192.168.16.129 ), confirming successful failover.

Keepalived also supports additional features such as email alerts, which are not covered here.