Fault Tolerance Chapter Notes | AP Computer Science Principles - Grade 9 PDF Download

Introduction

Fault tolerance is a vital concept in AP Computer Science Principles, ensuring that systems like the Internet Network continue to work even when parts fail. This chapter explains fault tolerance, focusing on its role in the Internet’s design and how redundancy makes it possible. It provides examples from both digital and digital non-digital systems, discusses benefits like reliability and scalability, and scalability, highlights drawbacks like increased costs. Understanding fault tolerance helps us understand how networks maintain functionality despite hardware issues or cyberattacks.

What Is Fault Tolerance?

• Fault tolerance means a system can keep working even if one part fails.
• Example from the Internet: Data packets can take alternate paths if a network device (node) or connection fails.
• Non-digital example: A city’s road system is fault-tolerant because you can reach your destination via another route if one road is blocked due to a crash or construction.
• Other non-digital examples:
  • A backup generator keeps a house powered if the main power fails.
  • Multiple heaters in an apartment ensure heating continues if one heater breaks.
• The Internet is designed to be fault-tolerant, mainly through redundancy.

Redundancy

• Redundancy involves adding extra components to a system to prevent failure if some parts stop working.
• In digital systems, redundancy is beneficial, unlike in writing where it’s avoided.
• Examples of redundancy in the Internet:
  • Multiple paths for packets, using extra cables or routers, allow data to reroute if one path fails (e.g., due to a damaged cable or broken router).
  • A company can use multiple Internet providers or wire channels to ensure connectivity if one fails.
• Website redundancy methods (not tested on AP CSP):
  • Load balancing: Uses load balancers to distribute traffic across multiple servers, so the website runs even if one server fails.
  • Failover solutions: Automatically switch to a backup machine if the main system fails.

Advantages of Fault Tolerance

• Increases system reliability by preventing complete shutdowns when parts fail.
  • Parts can fail unexpectedly, often in groups (e.g., natural disasters damaging multiple devices in one area).
  • Example: Fault tolerance in transatlantic communication systems prevents harmful shutdowns affecting users and society.
• Reduces damage from cyberattacks, like Distributed Denial of Service (DDoS) attacks, where servers are flooded with traffic.
  • Redundant servers or connections allow systems to bypass overwhelmed areas and keep operating.
• Enhances scalability by supporting more devices and traffic.
  • Multiple paths in a fault-tolerant routing system handle more packets than a non-redundant system.

Disadvantages of Fault Tolerance

• Requires more resources, like extra cables and routers, increasing costs for materials, construction, and maintenance.
• Areas with fewer resources may have more vulnerable networks due to inability to afford redundancy.
• Not all components can be made fault-tolerant due to cost or design limits.
  • Example: A car cannot have two steering wheels for redundancy.
• Developers must prioritize which parts need fault tolerance based on cost and design constraints.

Key Terms