Applies To: CLOUD VPS DEDICATED

A Cross Site Cluster, when used in conjunction with other tools, such as Cross Site Private Network, allows users to create clusters of servers or load balancers that span across multiple network zones. This provides redundancy between network zones which can be used to provide an automatic failover in the event of an outage.

Cross Site Cluster Configurations

There are two main configurations available for the Cross Site Cluster, this is "Live / Standby" and "Live / Live", these are looked at in more detail below.

Live/Standby Configuration


In this configuration, all servers are allocated the same additional IP addresses. Only the servers in Reading will receive all of the traffic under normal conditions. If a server begins to fail its health checks, traffic to this will be directed to the other server in Reading.

If both servers in Reading fail their health checks, or there is a data centre wide outage, traffic will be redirected automatically to the standby server in Dunsfold.

Live/Live Configuration


Ideally, in the above scenario, 4 IP addresses would be allocated to the servers, and only one of them would be “preferenced” for each server. This would mean that under normal operation, traffic for a particular IP address would only be routed to one server. However, if the health checks of this server fails, the traffic for this IP address will be re-routed over to one or more of the other members of the cluster. Round-robin DNS can then be used to distribute traffic to all of these IP addresses, and hence between all members of the cluster across both sites.

Configuring a new Cross Site Cluster

To setup a new Cross Site Cluster, is quite simple. First you need to setup some Health Checks, these are essentially rules to tell the Cross Site Cluster when a member of the cluster is considered "Up" or "Down", these can be done on the "Manage Health Checks" page. Once the Health Check have been added, you can then add the servers that you require to be part of the Cluster into the Cross Site Cluster, this can be done on the "Manage Servers" page, once all the Servers have been added you then need to ensure they are "Enabled". After that the Cross Site Cluster should then be active.

Further Documentation on the Manage Health Check and Manage Server features can be found here;

Manage Health Checks for a Cross Site Cluster
Manage Servers in a Cross Site Cluster

Caveats

There are some constraints that must be considered for the Cross Site Cluster service:

  • Servers and Load Balancers must be on separate firewalls. This may mean that in order to add some members to the cluster, they may need to be moved to a different host/firewall. Our support team can advise on this and it may result in some downtime.
  • Servers should be on a different primary VLAN. This is because we are unable to ensure that traffic is directed to the correct host, as any host on the same vLAN with the IP configured can respond.
  • Servers shouldn't have a matching preference for an IP address as any other server in the same network zone. This is because traffic will only ever be routed to one of these servers, even if the health checks are passing for both. In this configuration, a health check failure on one server will still cause the traffic to be re-routed to a different server with the same preference.
  • A Cross Site Cluster can't be used with Open Stack Instances, Load Balancers or similar.