5.2 Fault Tolerance and Redundancy

What is fault tolerance?

Fault tolerance is the ability of a system to keep operating properly even when one or more of its components fail. Computing systems are designed with fault tolerance so that a single hardware failure, software bug, or lost connection doesn't cause the entire system to crash or lose data.

Redundancy makes it possible

Redundancy means duplicating critical components, data, or systems so a backup is available if the original fails. Examples include storing multiple copies of data on different servers, backup power supplies, and mirrored hard drives (RAID). If one part fails, the redundant part takes over seamlessly.

Why it matters for the Internet

The Internet itself is fault tolerant by design: it uses redundant routing paths so data packets can be rerouted if a connection or router fails. This distributed, decentralized structure means no single point of failure can bring down the whole network, keeping global communication reliable.

Key terms

Fault tolerance

The ability of a system to continue operating properly even if one or more components fail.

Redundancy

Duplicating critical components, data, or systems to serve as backups if the original fails.

Single point of failure

A component whose failure would cause the entire system to stop working; fault-tolerant design avoids this.

RAID (example of redundancy)

A storage technology that mirrors data across multiple hard drives so data isn't lost if one drive fails.

Internet's fault tolerance

The Internet uses multiple redundant routing paths, so data can be rerouted if a connection or router fails.

Try an AP-style question

A company stores copies of its data in several data centers. Why?

1. So a failure in one doesn't lose the data
2. To make the data harder to access
3. To reduce the number of servers
4. To slow down the website