Single Point of Failure (SPOF)

A single point of failure (SPOF) is one component, such as a server, database, DNS provider, certificate or person, whose failure stops an entire system because there is no backup or alternative path.

Why it matters

Reliability is limited by its weakest dependency. You can run ten web servers behind a load balancer, but if they share one database, one nameserver set or one expiring TLS certificate, a single failure still produces a full outage. Finding SPOFs before they fail is cheaper than discovering them at 3 a.m.

SPOFs are not always hardware. A single engineer who knows the deploy process, one registrar account with no recovery option, or one availability zone are common examples.

Example

A quick audit of a typical site:

Web tier       3 servers behind load balancer      ok
Load balancer  1 instance                           SPOF
Database       1 primary, no replica                SPOF
DNS            2 nameservers, same provider         partial
Certificate    manual renewal, 1 owner              SPOF

Fixes include a second load balancer, a replica with failover, nameservers at two providers and automated renewal.

Common mistakes

Adding redundancy at one layer and ignoring shared dependencies underneath it.
Having a replica that has never been failed over, so nobody knows it works.
Counting people and accounts as non-technical and leaving them off the list.

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