High availability, high reliability - goals each growing service seeks to meet. Because there is nothing more deadly for the traffic than tens of seconds of connecting to your service or even no connectivity at all because just one of the parts has failed.
Simple diagram shows how it is possible to design reliable services load balancing in case of only one access line (which is the most common case nowadays, more access lines come usually with more complex cases). Virtis company is using this type of design when creating smaller server farms based on software components and also when creating more demanding solutions that require specialized hardware.
This kind of design is also possible to realize in steps which can help smaller companies launching their first complex products. We recommend to combine this design with Virtualization at the backend level - that actually provides customers with the solution that many current providers call (although not entirely correctly) cloud.