In situations where a single system isn’t sufficient for hosting a website due to high traffic or reliability concerns, replicating the site across multiple systems is a viable solution. This can be done using a front-end load balancer or multiple DNS entries to distribute traffic.

Virtualmin supports domain replication using backup and restore processes. Replication in Virtualmin is one-way, with a master system acting as the source of truth, and its state periodically copied to one or more replica systems.

Requirements for domain replication

For effective replication, all Virtualmin systems should share domain home directories from a central server, typically using NFS. A databases should also be stored remotely, accessible by all Virtualmin systems. This setup is crucial for dynamic websites to keep application states synchronized. For static content sites, file sharing setup might be optional.

Setting up remote database and NFS

This involves configuring remote database access and exporting/mounting directories via NFS. Detailed guides can be found here:

When databases and home directories are shared, only domain metadata needs replication.

Replicating your first domain

Once all systems are set up with shared storage, follow these steps to replicate a domain:

  1. Create and test domain on master
    Set up your domain, for example domain.com on the master system named master.hostname.com

  2. Transfer domain to replica

    • SSH into the master system as root.
    • Run the command:
      virtualmin transfer-domain --domain domain.com --host replica.hostname.com --replication --output --overwrite

      Note: If SSH keys are not set up, use the --pass flag to specify the password for the root user on the replica system (not recommended).

  3. Update DNS or load balancer
    Update any DNS records or load balancer configuration for the domain.com domain to use the IPs of both master.hostname.com and replica.hostname.com systems.

  4. Test the site
    Test that the site is accessible and functions correctly when traffic is being sent to both systems.

Scheduled replication

After successful manual replication, set up regular scheduled replication to keep settings in sync:

  • Create a cron job on the master system to run:
    virtualmin transfer-domain --domain domain.com --host replica.hostname.com --replication --overwrite
  • If shared storage is used for home directories and databases, a frequent schedule (e.g., every 10 minutes) is feasible, as the amount of data to transfer will be small.

Deleting domains

When deleting a domain from a replica:

  • Use the --preserve-remote flag in the delete-domain API command or the corresponding UI checkbox to avoid removing shared contents.
  • For complete removal, delete the domain from all replicas first, then from the master system.

A built-in domain synchronization feature in Cloudmin will automatically use replication mode when the destination system is running Virtualmin.