Introduction

In today’s digital world, ensuring high availability (HA) for services is paramount. Downtime can lead to lost revenue and damage to reputation. Many businesses turn to load balancers and clustering solutions to maintain service continuity. This article explores how to implement high availability in Linux servers using HAProxy, a robust and high-performance TCP/HTTP load balancer.

What is HAProxy?

HAProxy (High Availability Proxy) is an open-source software renowned for its efficiency in distributing workload across multiple servers, making it a reliable solution for increasing application availability and scalability. Its advanced features, including SSL termination, sticky sessions, and extensive logging capabilities, make it a popular choice for various applications.

Key Components of a High Availability Setup

To set up a highly available environment, you need to consider two primary components:

  1. Load Balancer: Distributes incoming traffic across multiple backend servers.

  2. Server Cluster: A group of backend servers that will respond to incoming requests.

Typically, you’d deploy a pair of HAProxy servers to operate in an active-passive or active-active mode, along with a monitoring tool to handle failover.

Prerequisites

Before getting started with the implementation, ensure you have:

  • At least two Linux servers (Debian/Ubuntu or CentOS/RHEL).

  • Access to root or superuser privileges.

  • Installation of HAProxy on both servers.

  • A shared storage or a mechanism to synchronize configuration files.

Installation of HAProxy

On Ubuntu/Debian

sudo apt update

sudo apt install haproxy

On CentOS/RHEL

sudo yum install epel-release

sudo yum install haproxy

Configuration

To configure HAProxy for high availability, follow these steps:

Step 1: Basic HAProxy Configuration

Edit the HAProxy configuration file located at /etc/haproxy/haproxy.cfg. Below is a sample configuration for load balancing HTTP traffic:

global

    log /dev/log    local0

    log /dev/log    local1 notice

    chroot /var/lib/haproxy

    stats socket /var/run/haproxy.sock mode 660 level admin

    stats timeout 30s

    user haproxy

    group haproxy

    daemon



defaults

    log global

    mode http

    option httplog

    option dontlognull

    timeout client 30s

    timeout server 30s

    timeout connect 5s



frontend http_front

    bind *:80

    default_backend http_back



backend http_back

    balance roundrobin

    server web1 192.168.1.101:80 check

    server web2 192.168.1.102:80 check

Step 2: Enable HAProxy

After saving the configuration, enable and start HAProxy:

sudo systemctl enable haproxy

sudo systemctl start haproxy

Step 3: Check HAProxy Status

To ensure HAProxy is running, check its status:

sudo systemctl status haproxy

Step 4: Setup Health Checks

It’s crucial to configure health checks for your backend servers to automatically remove any unhealthy servers from the load balancing rotation. The check parameter in the configuration above enables this feature.

Implementing High Availability with Keepalived

For high availability, you can use Keepalived alongside HAProxy. Keepalived allows for failover between multiple HAProxy instances.

Step 1: Install Keepalived

# On Ubuntu/Debian

sudo apt install keepalived



# On CentOS/RHEL

sudo yum install keepalived

Step 2: Configure Keepalived

Edit the Keepalived configuration file, usually located at /etc/keepalived/keepalived.conf, to set up virtual IP management. Below is a simple configuration example:

vrrp_instance VI_1 {

    state MASTER

    interface eth0

    virtual_router_id 51

    priority 100

    advert_int 1

    authentication {

        auth_type PASS

        auth_pass yourpassword

    }

    virtual_ipaddress {

        192.168.1.200

    }

}

Step 3: Enable Keepalived

Then, enable and start Keepalived:

sudo systemctl enable keepalived

sudo systemctl start keepalived

Testing Your Setup

  1. Access the Load Balancer: Open a web browser and access the virtual IP http://192.168.1.200. You should see the response from one of the backend servers.

  2. Simulate a Failure: Simulate a failure by stopping HAProxy on the active node and checking if traffic automatically reroutes to the standby node.

sudo systemctl stop haproxy

Check if the virtual IP remains accessible.

Monitoring and Logging

Monitoring your HAProxy and Keepalived setup is vital for maintaining high availability. HAProxy provides a built-in web interface for monitoring, which you can enable in the configuration. Add the following to your frontend section:

frontend stats 

    bind *:8080

    stats enable

    stats uri /stats

    stats auth YourUsername:YourPassword

You can access this by navigating to http://YourVirtualIP:8080/stats.

Conclusion

Implementing high availability with HAProxy and Keepalived is essential for maintaining service continuity in production environments. This approach not only helps in load balancing but also provides redundancy through failover mechanisms. With the right monitoring and configuration, your Linux servers can achieve robust high availability tailored to your needs.

By following the steps in this guide, you can ensure your services remain accessible, reliable, and resilient against unexpected failures.

About the Author: [Author Name] is a tech enthusiast with a background in systems administration and networking. With a passion for Linux and open-source technologies, they enjoy sharing knowledge through articles and blogs.