Some practical issues with online application servers:
- A dedicated server may be too much for an online application — maximum resource utilization never reached and causing it wasting too much resources. This is especially a problem for a new/developing application.
- A dedicated server is not easily or quickly expandable when more resource is required.
- For small web site owners, like me, server crash means downtime and the time required to take it back online is unpredictable.
A popular solution for efficient resource usage is by using server virtualization (virtual servers/machines). Initially this solved the efficiency problem by putting many virtual servers into a single physical server. This way, we can add up virtual servers until near full resources of the physical server are used. Every individual virtual server can also be scaled easily.
Recently, a virtual server technology called Xen is developed to have a self-healing feature. This is possible by having a software controller to monitor the virtual servers running on several physical servers. If a virtual server is down, the controller will immediately move the virtual server to another running physical server. So, your server is in ‘always-on’ state and this solves the problem #3.
In practice, Virtual Private Servers (VPS) play a significant role in growing the Software-as-a-Service business. The provider can start by using a small VPS and upscale later when the application needs more resources as the business grows.