Fault Tolerance (College Board AP® Computer Science Principles): Revision Note
Fault tolerance concepts
What is fault tolerance?
Fault tolerance is the ability of a system to continue functioning even when one or more of its components fail
The Internet has been engineered to be fault-tolerant, with abstractions for routing and transmitting data
When a system can support failures and still continue to function, it is called fault-tolerant
Fault tolerance matters because elements of complex systems fail at unexpected times, often in groups
Fault tolerance allows users to continue to use the network even when these failures occur
How the Internet mitigates failure
The Internet is built from many interconnected components (routers, switches, cables, servers)
If a single component fails, the system is designed so that data can still reach its destination through alternative routes
Because the Internet has multiple paths between devices, the failure of a single device or connection does not prevent the system from continuing to function
Redundancy & trade-offs
What is redundancy?
Redundancy is the inclusion of extra components that can be used to mitigate failure of a system if other components fail
On the Internet, redundancy is often accomplished by having more than one path between any two connected devices
If a particular device or connection on the Internet fails, subsequent data will be sent via a different route, if possible
How redundancy supports reliability
The redundancy of routing options between two points increases the reliability of the Internet
It also helps the Internet scale to more devices and more people, because multiple paths can share the load
Redundancy provides fault tolerance by ensuring that the failure of a single connection does not prevent communication
Concept | What it means | How it applies to the Internet |
|---|---|---|
Fault tolerance | A system continues functioning despite component failure | Data still reaches its destination when a router or link goes down |
Redundancy | Extra components included to mitigate failure | Multiple paths exist between any two devices |
Redundancy trade-offs
Redundancy within a system often requires additional resources, such as more hardware or more connections
However, redundancy provides the benefit of fault tolerance, making the system more reliable
Examiner Tips and Tricks
Exam questions may present a network diagram and ask what happens when a connection fails. Look for alternate paths between the sender and receiver to determine whether communication can continue.
You may also be asked to identify vulnerabilities to failure in a network. These are typically connections or devices where there is only one path or no backup — a part of the network with only a single connection between two devices is more vulnerable than a part with multiple paths.
For the AP Create Performance Task, fault tolerance is unlikely to appear directly in your program, but understanding how the Internet handles failure helps you explain how your program relies on network reliability if it uses web-based data or APIs.
Worked Example
A network has three paths connecting Device A to Device B. One of the three paths fails due to a hardware malfunction.
Which of the following best describes what happens?
(A) Device A can no longer communicate with Device B
(B) All data is lost because the failed path cannot be recovered
(C) Data is rerouted through one of the remaining paths, and communication continues
(D) The network must be restarted before communication can resume
[1]
Answer:
(C) Data is rerouted through one of the remaining paths, and communication continues [1 mark]
Redundancy on the Internet means multiple paths exist between devices. When a particular connection fails, subsequent data is sent via a different route, if possible, so communication continues without restarting the network.
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