High availability (HA) is a critical aspect of modern IT infrastructure, ensuring that services and applications remain accessible even in the face of hardware failures, network issues, or other disruptions. In Linux environments, achieving high availability involves a combination of clustering, load balancing, and failover mechanisms. This article will guide you through setting up high availability using tools like Pacemaker, Corosync, and HAProxy.

Understanding High Availability

High availability aims to minimize downtime and ensure continuous service availability. This is typically achieved through redundancy and failover mechanisms. In a Linux environment, this often involves:

1. Clustering: Grouping multiple servers to work together as a single system.
2. Load Balancing: Distributing incoming network traffic across multiple servers.
3. Failover: Automatically switching to a standby server if the primary server fails.

Setting Up a High Availability Cluster with Pacemaker and Corosync

Pacemaker and Corosync are widely used tools for setting up high availability clusters in Linux.

Step-by-Step Guide

1. Install Pacemaker and Corosync

   First, install the required packages on all nodes in your cluster.

   sudo apt-get update
   sudo apt-get install pacemaker corosync

2. Configure Corosync

   Edit the Corosync configuration file (/etc/corosync/corosync.conf) to define your cluster nodes and settings.

   totem {
      version: 2
      secauth: on
      cluster_name: my_cluster
      transport: udpu
   }
   
   nodelist {
      node {
          ring0_addr: node1
          nodeid: 1
      }
      node {
          ring0_addr: node2
          nodeid: 2
      }
   }
   
   quorum {
      provider: corosync_votequorum
   }

3. Start and Enable Corosync and Pacemaker

   Start the services and enable them to start on boot.

   sudo systemctl start corosync
   sudo systemctl enable corosync
   sudo systemctl start pacemaker
   sudo systemctl enable pacemaker

4. Verify Cluster Status

   Use the crm command to check the status of your cluster.

   sudo crm status

5. Configure Resources and Constraints

   Define the resources (e.g., IP addresses, services) and constraints (rules for failover) for your cluster.

   sudo crm configure primitive my_service ocf:heartbeat:apache \
      op monitor interval=30s
   sudo crm configure clone my_service-clone my_service
   sudo crm configure colocation col_my_service inf: my_service-clone
   sudo crm configure order ord_my_service inf: my_service-clone

Load Balancing with HAProxy

HAProxy is a powerful tool for load balancing and can be used in conjunction with Pacemaker and Corosync for high availability.

Step-by-Step Guide

1. Install HAProxy

   sudo apt-get install haproxy

2. Configure HAProxy

   Edit the HAProxy configuration file (/etc/haproxy/haproxy.cfg).

   global
      log /dev/log    local0
      log /dev/log    local1 notice
      chroot /var/lib/haproxy
      stats socket /run/haproxy/admin.sock mode 660 level admin
      stats timeout 30s
      user haproxy
      group haproxy
      daemon
   
   defaults
      log     global
      mode    http
      option  httplog
      option  dontlognull
      timeout connect 5000
      timeout client  50000
      timeout server  50000
   
   frontend http_front
      bind *:80
      default_backend http_back
   
   backend http_back
      balance roundrobin
      server server1 192.168.1.101:80 check
      server server2 192.168.1.102:80 check

3. Start and Enable HAProxy

   sudo systemctl start haproxy
   sudo systemctl enable haproxy

Conclusion

By following these steps, you can set up a high availability environment in Linux using Pacemaker, Corosync, and HAProxy. This setup ensures that your services remain accessible and resilient to failures, providing a robust infrastructure for your applications.