The article discusses the importance of fault-tolerant distributed systems in ensuring continuous service availability, especially in critical applications such as online transaction processing, process control, and computer-based communications. It highlights the challenges in designing fault-tolerant systems, including the complexity of managing component failures and the lack of clear terminology. The article introduces basic architectural concepts and proposes a structured approach to understanding fault-tolerance issues. Key points include: 1. **Basic Architectural Concepts**: Services, servers, and the "depends" relation are fundamental to understanding system architecture. 2. **Failure Classification**: Failures are categorized into omission, timing, response, and crash failures, each with specific behaviors. 3. **Failure Semantics**: The failure semantics of a server specify the likely failure behaviors it can exhibit. 4. **Hierarchical Failure Masking**: Failures at lower levels can propagate to higher levels, requiring hierarchical masking techniques. 5. **Group Failure Masking**: Redundant servers can mask individual failures, ensuring service availability. 6. **Choosing a Failure Semantics**: The choice of failure semantics depends on the stochastic requirements of the system. 7. **Hardware Architectural Issues**: architectures are classified into coarse and fine granularity, with examples like Tandem, VAX Cluster, IBM XRF, Stratus, and Sequoia. The article emphasizes the importance of balancing redundancy and cost in fault-tolerant systems to achieve optimal performance and reliability.The article discusses the importance of fault-tolerant distributed systems in ensuring continuous service availability, especially in critical applications such as online transaction processing, process control, and computer-based communications. It highlights the challenges in designing fault-tolerant systems, including the complexity of managing component failures and the lack of clear terminology. The article introduces basic architectural concepts and proposes a structured approach to understanding fault-tolerance issues. Key points include: 1. **Basic Architectural Concepts**: Services, servers, and the "depends" relation are fundamental to understanding system architecture. 2. **Failure Classification**: Failures are categorized into omission, timing, response, and crash failures, each with specific behaviors. 3. **Failure Semantics**: The failure semantics of a server specify the likely failure behaviors it can exhibit. 4. **Hierarchical Failure Masking**: Failures at lower levels can propagate to higher levels, requiring hierarchical masking techniques. 5. **Group Failure Masking**: Redundant servers can mask individual failures, ensuring service availability. 6. **Choosing a Failure Semantics**: The choice of failure semantics depends on the stochastic requirements of the system. 7. **Hardware Architectural Issues**: architectures are classified into coarse and fine granularity, with examples like Tandem, VAX Cluster, IBM XRF, Stratus, and Sequoia. The article emphasizes the importance of balancing redundancy and cost in fault-tolerant systems to achieve optimal performance and reliability.