Historically, vendor solutions for disaster recovery have been created for on-site use for individual enterprises. The client company concerned was the sole owner of the user data involved, and disaster recovery could be implemented without having to worry about anybody else. The cloud computing model changes that situation. It’s possible to use cloud services to have your own dedicated servers and instances of applications, or to share physical space but still have your own application (as in multi-instance setups). However, multi-tenancy (perhaps the defining feature of cloud architectures) makes the application of disaster recovery solutions rather more delicate.
The difference between multi-tenancy and multi-instance configurations is that multi-tenancy uses one instance of an application to serve several customers (or ‘tenants’). The software application designed for multi-tenancy partitions its data and configuration and makes a virtual instance of the application available to each customer. By comparison, a multi-instance architecture provides a physically separate software instance to each customer. Disaster recovery software has not always provided suitable access for cloud providers to drive the hypervisor that is typically a part of the DR solution. For multi-tenancy solutions, a number of cloud providers have resorted to doing their own software development in order to provide workable, manageable DR solutions.
But just to spice things up a little more, cloud services may be multi-tenant at the SaaS (Software as a Service) level or the IaaS (Infrastructure as a Service) level. Salesforce.com is an example of SaaS-level multi-tenancy, with “72,500 customers who are supported by 8 to 12 multi-tenant instances… in a 1:5000 ratio” (Computerworld, 2010). An IaaS multi-tenancy provider example is Amazon. The challenge for SaaS customers is to find a solution for increasing availability in a situation where the SaaS provider (like Salesforce.com) manages almost everything in the system for its customers. A growing contingent of RaaS (Recovery as a Service) providers may be able to help, although customers should first ascertain if the RaaS concerned applies to just IaaS or also to SaaS implementations.