This figure illustrates system availability, or the lack of downtime, during an upgrade of multiple middle tiers and an Infrastructure. There are nine stages of the upgrade; the figure shows client connectivity and system state (running or stopped) at each stage. Complete downtime (where clients cannot connect to the middle tier) only occurs at stage 2, when the Infrastructure is being upgraded, and at stage 4, when the Information Storage database is being upgraded. The callouts in the graphic correspond to the items in the numbered list in the following section.