It is interesting that most of the posts so far have focused on ensuring that our original poster has sufficient business acumen to make the decision to build a clustered hosting environment. There are reasons other than straight margins why downtime for a website is an absolute no-go. For instance, I work for a medium sized data center. Although we do few direct conversions through the website, the embarrassment of that site going down more than justifies a clustered solution. I will assume that OP has done the math.
That being said, I have had excellent experience with ultramonkey / ldirectord. Ldirector has a single primary conf file that provides for pseudo custom service requests to check availability. I have found this to be much more intuitive than Windows clustering services, however if you are planning to have IIS boxes using SSL you may run into trouble loadbalancing HTTPS traffic.
The problem with 2 boxes and heartbeat only is that often times a box will stop serving websites but will not drop ping. You need a service that is smart enough to realize that a 404 page is not what you are looking for. That being said, custom validation queries can include SQL queries, SMTP, IMAP and POP sessions, HTTP requests that look for specific responses, etc.
This would need to sit on a dedicated firewall in front of at least two identical hosts. Note this introduces a single point of failure - a philosophically sound cluster will have two identical firewalls running heartbeat.
Another point of failure will be the switch providing link to these hosts. I would recommend redundant uplinks configured using VRRP to avoid lost availability due to a dead switchport.
I can go on, and the scale of a cluster topology is limited only by one's imagination, but I think this is a good start.
Josh Wieder
Atlantic.Net
