I think web hosting uptime is one of the main criterion people usually look for in a web hosting service. Even though 99.9% uptime is the industry standard, but it rarely happens. It is interesting to know the factors that affect the uptime of a server. I think hardware plays a major role in maintaining the uptime. It can largely affect speed, stability, reliability and overall performance of a server. A high capacity infrastructure can ensure a high uptime and minimize downtime. Another important factor affecting the uptime is the software being used. A well written code ensures better uptime and regular updations rule out any bugs. The number of clients trying to access a particular website also influence uptime. A DoS (denial of service) attack can contribute to downtime of a server. I think these are the main factors that contribute to the downtime/uptime of a server. Any additions are welcome.