Internet servers are normally expected to be available 24 hours a day, 7 days a week. E-commerce sites, global B2B (business-to-business) sites, and any other revenue-producing sites may be critical to the companies that run them, and their unavailability could prove to be very expensive. The approach taken to ensure that servers remain in service even when they are being upgraded depends on the type of server in use. There are two categories to consider: server clusters and single servers.