A system has two redundant load balancers, three web servers behind them, and a single database server used by all the web servers. Among the devices listed here, which is a single point of failure (a point whose stopping stops the whole system)?

Answer: C. The database server

A single point of failure (SPOF) is a point where a stop in just that one place stops the whole system. The load balancers are made redundant with two units, and there are three web servers and the load balancers can remove a failed server from distribution, so processing continues even if any one of them stops. There is only one database server and every web server uses it, so if it stops, the whole system stops. Making the database server redundant too, for example with a cluster configuration, removes this single point of failure.

• A. They are made redundant with two units, so if one stops the other continues processing.
• B. There are three, and the load balancers can remove a failed server and distribute requests to the rest.
• C. Correct. There is only one, and if it stops none of the web servers can process requests.