The Truth Is Out There: Preservation and the Cloud

With the recent introductions of DuraCloud, Preservica, Glacier, and others, preservation has joined most other applications in being offered as a cloud service, Preservation as a Service (PaaS). Does PaaS make technical, economic or business sense? What characteristics make applications cloud-friendly? If outsourcing to third-party cloud services is such a great idea, why do companies that get big enough all build their own clouds?