The Ultimate Cheapskate Cluster

This is a variant of the Redundant Load Balancers Using VRRP document, this time with the web servers and load balancers running on just two servers. It's difficult to imagine a smaller cluster.

Pen offers fault tolerance by automatically rerouting traffic from servers that are offline. But what if the load balancer goes down?

Using VRRP, it is possible to run two load balancers in an active-passive failover configuration. This requires a special vrrp daemon which can be found here. See the instructions that come with vrrpd how to install it.

Example network

Install apache, pen and vrrpd on the two load balancer hosts. Apache is configured to start before pen and listen on only one address, 10.1.1.5 on server 1 and 10.1.1.6 on server 2. This is done with the Listen directive in httpd.conf:

Listen 10.1.1.5:80

Pen will listen on all remaining addresses, i.e. 10.1.1.2/3 and 10.1.1.4.

Start pen on both load balancers and check that both work by surfing to http://10.1.1.2/ and http://10.1.1.3/.

Now start vrrpd like this on both load balancers:

vrrpd -i eth0 -v 1 10.1.1.4

Now try surfing to http://10.1.1.4/. One of the load balancers will be active and respond at that address. Disconnect that load balancer from the network to simulate a failure. Now the other load balancer will take over the address, restoring functionality.

In the example network, the firewall uses NAT, although that is in no way necessary. A Cisco PIX would be configured something like this:

static (inside,outside) 193.12.6.25 10.1.1.4 netmask 255.255.255.255 0 0
conduit permit tcp host 193.12.6.25 eq 80 any

Corrections and addition on The Ultimate Cheapskate Cluster from the mailing list.