This Visio diagram was used when building a concept design for our data centre for the eventengine.org applicaion. You can download the VSD file here and use for your own designs data-centre-business-continuity
Our primary concern when building a robust infrastructure is making use of a live-live environment across two data centres. With business continuity in mind our objective was to operate with minimal redundancy in technology, so all servers can provide application services without being in a warm standby. A good choice for multi site load balance is the Zeus ZXTM solution, with a heartbeat configured between the two hosted environments we were assured of continuality of service during an outage of one of the data centres.
You can find out more about the Zeus ZXTM on their website.

