single point of failure (SPOF)

A single point of failure (SPOF) is a potential risk posed by a flaw in the design, implementation or configuration of a circuit or system in which one fault or malfunction causes an entire system to stop operating.

In a data center or other information technology (IT) environment, a single point of failure can compromise the availability of workloads – or the entire data center – depending on the location and interdependencies involved in the failure.

Consider a data center where a single server runs a single application. The underlying server hardware would present a single point of failure for the application’s availability. If the server failed, the application would become unstable or crash entirely; preventing users from accessing the application, and possibly even resulting in some measure of data loss. In this situation, the use of server clustering technology would allow a duplicate copy of the application to run on a second physical server. If the first server failed, the second would take over to preserve access to the application and avoid the SPOF.

Consider another example where an array of servers is networked through a single network switch. The switch would present a single point of failure. If the switch failed (or simply disconnected from its power source), all of the servers connected to that switch would become inaccessible from the remainder of the network. For a large switch, this could render dozens of servers and their workloads inaccessible. Redundant switches and network connections can provide alternative network paths for interconnected servers if the original switch should fail, avoiding the SPOF.

It is the responsibility of the data center architect to identify and correct single points of failure that appear in the infrastructure’s design. However, it’s important to remember that the resiliency needed to overcome single points of failure carries a cost (e.g. the price of additional servers within a cluster or additional switches, network interfaces and cabling). Architects must weigh the need for each workload against the additional costs incurred to avoid each SPOF. In some cases, designers may determine that the cost to correct a SPOF is costlier than the benefits of the workloads at risk.

This was last updated in February 2012

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