Fault Tolerance
Help Questions
AP Computer Science Principles › Fault Tolerance
How does this setup differ from a truly fault-tolerant system?
A fault-tolerant system would use a single, more reliable switch instead of two.
In a fault-tolerant system, the backup switch would be from a different manufacturer to avoid common bugs.
In a fault-tolerant system, the backup switch would be automatically and instantly activated upon failure, without manual intervention.
A fault-tolerant system would not require switches and would connect all computers directly to each other.
Explanation
Having a spare part is a form of redundancy, but fault tolerance implies the system can handle the failure automatically with minimal or no downtime. This setup, known as a cold spare, requires manual intervention and results in system downtime, whereas a hot spare in a truly fault-tolerant system would take over automatically.
To best improve the fault tolerance of the business's Internet access, which action should be taken?
Set up a local server to cache frequently visited websites to reduce Internet usage.
Purchase a second, separate Internet connection from a different service provider.
Upgrade the existing Internet connection to a higher bandwidth plan.
Install a new, more powerful router to replace the one provided by the current provider.
Explanation
This action introduces redundancy for the entire Internet connection. If the primary provider has an outage, the business can switch to the secondary provider, thus continuing operations. This directly addresses the single point of failure.
The original design motivation for the network that became the Internet included the need for a communication system that could withstand a large-scale disaster. This motivation directly led to which core principle?
High-speed data transfer
Data encryption
Open protocols
Fault tolerance
Explanation
The need for a network that could survive the loss of some of its nodes is the very definition of fault tolerance. This foundational requirement shaped the Internet's decentralized, packet-switched architecture with redundant paths.
What is a primary trade-off associated with building a fault-tolerant system using redundancy?
An increase in costs and complexity due to the need for additional hardware, software, and maintenance.
A reduction in system security, as redundant components can provide more points of entry for attackers.
A decrease in the overall speed of the system because of the complexity of managing duplicate components.
A limit on the system's ability to scale, since redundancy works best for small, contained networks.
Explanation
Implementing redundancy involves duplicating components (like servers, network paths, power supplies), which adds to the initial cost and the ongoing complexity of managing the system. This is a key trade-off against the benefit of increased reliability.
Which of the following best explains how packet switching supports a fault-tolerant network?
It allows each data packet to be routed independently, enabling the network to bypass failed or congested links.
It establishes a dedicated, fixed circuit between two communicating devices, guaranteeing the connection quality.
It ensures all packets in a message arrive in the correct sequence by numbering them before transmission.
It bundles data from multiple users into a single packet to maximize the use of the network's bandwidth.
Explanation
The independent routing of packets is key. Unlike circuit switching, which uses a fixed path, packet switching allows the network to be flexible and adaptive. If one path fails, only the packets currently on that path are lost; subsequent packets can be sent on working routes.
Which of the following scenarios best illustrates the concept of redundancy in a computing system?
A file is compressed to a smaller size before being sent over a network.
A program is broken into smaller tasks that can be run simultaneously on multiple processors.
An airline's booking system runs on identical servers in two geographically separate locations.
A user protects their account with a password and a fingerprint scan.
Explanation
This is a clear example of redundancy for fault tolerance. The identical servers are duplicate components. If one location fails, the other can take over, ensuring the booking system remains available.
A home security company monitored sensors over a wireless network. Sometimes interference flipped bits in a message, so the system added a small check value to each packet. If the check did not match, the receiver requested the packet again, and the correct alert was delivered. This kept door and window events accurate even on noisy nights. In the example given, how does the described system maintain functionality during a failure?
It boosted volume so alerts sounded louder
It deleted packets to prevent delays
It detected errors and requested resend
It encrypted data to hide sensor names
Explanation
This question tests AP Computer Science Principles skills, specifically understanding fault tolerance in computing systems and networks. Fault tolerance is the ability of a system to continue operating properly in the event of a failure of some of its components. It involves mechanisms like redundancy, error detection and correction, and failover. In the passage, the system uses error detection and correction by adding check values to packets and requesting retransmission when errors are detected. Choice B is correct because it accurately describes how the system detects errors through check values and requests resending of corrupted packets. Choice A is incorrect because encryption is about security and privacy, not error detection - the passage specifically mentions detecting bit flips from interference. To help students, emphasize the difference between error detection (finding mistakes) and other fault tolerance mechanisms. Practice identifying checksum/parity concepts and how they enable reliable communication over unreliable channels.
A cloud storage service ran on a primary server and a backup server in different data centers. When the primary server crashed during peak use, the system automatically switched users to the backup within seconds, and most people kept uploading files without noticing. The service logged the failure and later repaired the primary server, then restored it as the main system. Based on the scenario described, in the scenario, what role does Failover play?
It switched service to a backup server
It reduced file sizes before sending
It permanently shut down the service
It split traffic evenly to speed uploads
Explanation
This question tests AP Computer Science Principles skills, specifically understanding fault tolerance in computing systems and networks. Fault tolerance is the ability of a system to continue operating properly in the event of a failure of some of its components. It involves mechanisms like redundancy, error detection and correction, and failover. In the passage, the system uses automatic failover to maintain functionality when the primary server crashes, switching users to a backup server within seconds. Choice B is correct because it accurately reflects how the system automatically switched service to the backup server when the primary failed, which is the definition of failover. Choice A is incorrect because it describes load balancing, not failover - the system didn't split traffic evenly but rather redirected all traffic to the backup. To help students, emphasize that failover specifically involves switching from a failed component to a working backup, not distributing load. Practice identifying the difference between failover (switching to backup) and load balancing (distributing work).
A large data center hosted websites for local hospitals. To keep services available, the building used 2 power feeds from different utility lines. If both feeds dropped, battery units kept the servers running long enough for diesel generators to start. During a storm, one utility line failed, but the servers stayed online because the other feed continued providing electricity. Later that night, the second line flickered and the batteries bridged the gap until the generator took over. Staff reported that patients could still access appointment pages the entire time. In the example given, what is the primary benefit of using Redundancy as illustrated in the passage?
It keeps services available during outages
It speeds up website loading every day
It prevents any component from failing
It reduces the price of electricity
Explanation
This question tests AP Computer Science Principles skills, specifically understanding fault tolerance in computing systems and networks. Fault tolerance is the ability of a system to continue operating properly in the event of a failure of some of its components. It involves mechanisms like redundancy, error detection and correction, and failover. In the passage, the system uses multiple layers of redundancy including two power feeds, battery units, and diesel generators to maintain continuous power supply. Choice B is correct because it accurately identifies that redundancy keeps services available during outages, as demonstrated when the hospital websites remained accessible throughout the storm. Choice D is incorrect because redundancy doesn't prevent failures but rather ensures continued operation despite failures. To help students, emphasize that redundancy provides backup options rather than preventing failures entirely. Watch for: confusion between prevention and mitigation strategies, and overgeneralization about what redundancy can achieve.
A small business hosted its website in a cloud service with 2 regions. The website ran in Region 1, but Region 1 had an outage. The cloud provider redirected requests to Region 2 automatically, and customers still browsed products. Later, the provider restored Region 1 and returned traffic gradually. Based on the scenario described, which mechanism ensures system operation when a component fails?
Manual typing of each customer request
Automatic Failover to another region
Extra animations to keep users engaged
A larger logo to improve brand recall
Explanation
This question tests AP Computer Science Principles skills, specifically understanding fault tolerance in computing systems and networks. Fault tolerance is the ability of a system to continue operating properly in the event of a failure of some of its components. It involves mechanisms like redundancy, error detection and correction, and failover. In the passage, the cloud service uses automatic failover between regions - when Region 1 has an outage, the provider automatically redirects requests to Region 2. Choice A is correct because it accurately identifies automatic failover as the mechanism that maintains service by switching from the failed region to a working region. Choice B is incorrect because manual typing of requests would be inefficient and doesn't relate to automated fault tolerance mechanisms. To help students, emphasize that cloud providers often use geographic redundancy with automatic failover between regions. Practice identifying how DNS redirection and load balancers enable seamless regional failover.