Fault Tolerance
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AP Computer Science Principles › Fault Tolerance
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 permanently shut down the service
It split traffic evenly to speed uploads
It switched service to a backup server
It reduced file sizes before sending
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 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?
Automatic Failover to another region
A larger logo to improve brand recall
Manual typing of each customer request
Extra animations to keep users engaged
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.
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 encrypted data to hide sensor names
It deleted packets to prevent delays
It detected errors and requested resend
It boosted volume so alerts sounded louder
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.
An online store ran its checkout service on 2 servers: one active and one standby. When the active server’s disk failed, the standby server took over the same network address and continued processing carts. Customers mostly saw no interruption, and orders still completed. The failed disk was replaced later and the standby returned to waiting mode. In the example given, in the scenario, what role does Failover play?
It switched checkout to the standby server
It caused checkout to stop until repairs
It compressed images to load pages faster
It prevented any server from ever failing
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 failover when the active server's disk fails - the standby server takes over the same network address and continues processing. Choice A is correct because it accurately describes the failover process where checkout switched from the failed active server to the standby server. Choice C is incorrect because failover doesn't prevent failures from occurring; it provides a mechanism to continue operating when failures do happen. To help students, emphasize that failover involves automatic switching to backup systems when primary systems fail. Practice identifying active-standby configurations and how they maintain service continuity.
A delivery company sent route updates from trucks to headquarters over a cellular network. Some messages arrived with mistakes due to weak signal. Each message included extra bits that helped the receiver spot an error and rebuild the original data, so dispatch still saw correct locations. This kept tracking reliable without calling drivers repeatedly. Based on the scenario described, which mechanism ensures system operation when a component fails?
Load balancing across multiple web pages
Turning off updates to avoid wrong data
Error detection and correction in messages
Adding more drivers to reduce traffic
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 through extra bits that help spot errors and rebuild original data when messages are corrupted by weak signals. Choice A is correct because it accurately identifies error detection and correction as the mechanism that ensures reliable communication despite transmission errors. Choice B is incorrect because load balancing distributes work across resources, which is different from correcting transmission errors in messages. To help students, emphasize that error correction codes allow systems to recover from data corruption without retransmission. Practice identifying different error handling strategies: detection only vs. detection and correction.
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 city’s emergency alert system sent short messages from a central office to many radio towers. Because messages traveled through noisy links, a few packets occasionally arrived with altered bits. Each packet included extra information that let the receiver notice a mismatch. When a mismatch appeared, the receiver corrected small errors when possible; otherwise it requested the packet again. This helped ensure that evacuation notices were not changed into confusing text. During a thunderstorm, the system still delivered accurate alerts to most towers. In the example given, which mechanism ensures system operation when a component fails?
A faster antenna prevents all interference
Load balancing spreads users across servers
Turning off the network avoids wrong messages
Error detection and correction keeps data accurate
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 emergency alert system uses error detection and correction to identify and fix corrupted bits in messages, ensuring accurate delivery of critical information. Choice A is correct because it accurately identifies error detection and correction as the mechanism that keeps data accurate despite noisy transmission links. Choice C is incorrect because it suggests preventing interference entirely, which is impossible in real-world conditions as shown by the thunderstorm scenario. To help students, emphasize that error detection and correction handles data corruption, not component failures. Watch for: confusion between different fault tolerance mechanisms and unrealistic expectations about preventing all errors.