Depending on the nature of the computation, it may be important to be able to quickly recover from a crisis and resume computation. A bank, for example, might be able to tolerate a four-hour loss of computing facilities during a fire, but it could not tolerate a ten-month period to rebuild a destroyed facility, acquire new equipment, and resume operation.
Most computer manufacturers have several spare machines of most models that can be delivered to any location within 24 hours in the event of a real crisis. Sometimes the machine will come straight from assembly; other times the system will have been in use at a local office. Machinery is seldom the hard part of the problem. Rather, the hard part is deciding where to put the equipment in order to begin a temporary operation.
A cold site or shell is a facility with power and cooling available, in which a computing system can be installed to begin immediate operation. Some companies maintain their own cold sites, and other cold sites can be leased from disaster recovery companies. These sites usually come with cabling, fire prevention equipment, separate office space, telephone access, and other features. Typically, a computing center can have equipment installed and resume operation from a cold site within a week of a disaster.