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The Challenges and Benefits of Hierarchical Design for Computer Systems with Critical Requirements


On Hierarchical Design of Computer Systems for Critical Applications




Computer systems are ubiquitous in modern society, and many of them are involved in applications that have critical requirements such as human safety, fault tolerance, high availability, security, privacy, integrity, and timely responsiveness. Examples of such applications include air traffic control, nuclear power plants, medical devices, autonomous vehicles, and e-commerce. Designing computer systems that can satisfy these requirements and continue to do so throughout maintenance and long-term evolution is a challenging task that requires rigorous methods and techniques.




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In this article, we will explore the concept of hierarchical design as a way to cope with the complexity and diversity of critical applications. We will explain what hierarchical design is, why it is important, and what are the challenges and benefits of applying it to computer systems for critical applications. We will also introduce the notions of hierarchical abstraction and layers of trust as key elements of hierarchical design. Finally, we will provide some examples of hierarchical design in practice and discuss some future directions and open problems.


Introduction




What are critical applications?




A critical application is an application that has one or more requirements that must be met at all times and under all circumstances, otherwise serious consequences may occur. These requirements may involve human safety (e.g., preventing harm or death), fault tolerance (e.g., recovering from errors or failures), high availability (e.g., ensuring continuous operation or service), security (e.g., protecting confidentiality or integrity), privacy (e.g., respecting personal data or preferences), integrity (e.g., ensuring correctness or consistency), and timely responsiveness (e.g., meeting deadlines or performance goals).


Critical applications are often characterized by high levels of complexity, uncertainty, dynamism, and interdependence. They may involve multiple components, subsystems, users, stakeholders, environments, and scenarios. They may also face various sources of variability, unpredictability, change, and evolution. Moreover, they may interact with other systems or applications that have different or conflicting requirements or assumptions.


Why is hierarchical design important?




Hierarchical design is a general approach to deal with complex problems by decomposing them into simpler subproblems that can be solved separately and then integrated into a coherent solution. Hierarchical design can help in designing computer systems for critical applications by reducing the cognitive load on the designers, increasing the modularity and reusability of the components, facilitating the verification and validation of the properties, and enabling the adaptation and evolution of the system.


Hierarchical design can also help in addressing the diversity and heterogeneity of critical applications by allowing different levels of abstraction, granularity, representation, specification, implementation, testing, and evaluation. Hierarchical design can enable the use of different methods and techniques that are suitable for different aspects or phases of the system development lifecycle.


What are the challenges and benefits of hierarchical design?




Hierarchical design is not a panacea for designing computer systems for critical applications. It also poses some challenges and limitations that need to be considered and addressed. Some of these challenges include:



  • How to define the appropriate levels of hierarchy and the criteria for decomposing and composing the system?



  • How to ensure the consistency and compatibility among different levels and components of the system?



  • How to cope with the trade-offs and conflicts that may arise among different requirements or properties of the system?



  • How to deal with the emergent or unexpected behaviors or properties that may result from the interactions or integration of the system?



  • How to manage the complexity and overhead of the hierarchical design process and the resulting system?



On the other hand, hierarchical design can also bring some benefits and advantages that can outweigh the challenges and limitations. Some of these benefits include:



  • How to improve the understandability and maintainability of the system by providing a clear structure and organization?



  • How to enhance the reliability and robustness of the system by isolating and containing faults and failures?



  • How to increase the efficiency and scalability of the system by exploiting parallelism and concurrency?



  • How to improve the security and privacy of the system by enforcing boundaries and policies?



  • How to enable the flexibility and extensibility of the system by supporting modifiability and evolution?



Hierarchical Abstraction




What is hierarchical abstraction?




Hierarchical abstraction is a technique for creating and manipulating different levels of detail or generality of a system or a problem. Hierarchical abstraction can help in reducing the complexity and increasing the clarity of a system or a problem by hiding or revealing relevant or irrelevant information. Hierarchical abstraction can also help in bridging the gap between different domains or disciplines by providing a common language or framework.


Hierarchical abstraction can be applied to different aspects or dimensions of a system or a problem, such as data, functions, behaviors, interfaces, architectures, models, etc. Hierarchical abstraction can also involve different types or forms of abstraction, such as generalization, specialization, aggregation, decomposition, refinement, etc.


How does hierarchical abstraction help in designing critical systems?




Hierarchical abstraction can help in designing computer systems for critical applications by providing a systematic way to capture and express the essential features and properties of the system and its environment, as well as the relationships and dependencies among them. Hierarchical abstraction can also help in identifying and separating the concerns and responsibilities of different components or subsystems of the system, as well as the roles and expectations of different users or stakeholders of the system.


Hierarchical abstraction can also help in verifying and validating the correctness and completeness of the system with respect to its requirements and specifications, as well as its assumptions and constraints. Hierarchical abstraction can also help in analyzing and evaluating the performance and quality of the system with respect to its goals and metrics, as well as its risks and uncertainties.


What are some examples of hierarchical abstraction in practice?




Hierarchical abstraction is widely used in practice for designing computer systems for critical applications. Some examples of hierarchical abstraction in practice are:



  • The OSI model is a hierarchical abstraction that defines seven layers of communication protocols for computer networks, from physical to application.



  • The TCP/IP model is a hierarchical abstraction that defines four layers of communication protocols for internet networks, from network access to application.



  • The UML is a hierarchical abstraction that defines various diagrams for modeling different aspects of software systems, from use cases to classes.



  • The MVC is a hierarchical abstraction that defines three components for designing user interfaces for software systems, from model to view to controller.



  • The NIST framework is a hierarchical abstraction that defines five functions for managing cybersecurity risks for computer systems, from identify to protect to detect to respond to recover.



Layers of Trust




What are layers of trust?




Layers of trust are a concept that describes the degree of confidence or assurance that a system or a component satisfies its requirements or properties under certain conditions or assumptions. Layers of trust can help in expressing and measuring the trustworthiness or dependability of a system or a component with respect to its critical requirements or properties.


Layers of trust can be defined along different dimensions or criteria, such as functionality, performance, availability, reliability, safety, security, privacy, integrity, etc. Layers of trust can also be defined at different levels or scopes, such as component, subsystem, system, network, etc.


How do layers of trust relate to critical requirements?




Layers of trust relate to critical requirements by providing a way to specify and verify the expected behavior or outcome of a system or a component under normal or abnormal situations. Layers of trust can also provide a way to compare and contrast different alternatives or solutions for satisfying critical requirements.


What are some techniques for achieving and verifying layers of trust?




There are various techniques for achieving and verifying layers of trust for computer systems for critical applications. Some of these techniques include:



  • Formal methods are techniques for specifying and verifying the properties of a system or a component using rigorous mathematical logic and reasoning.



  • Testing is a technique for checking the behavior or outcome of a system or a component under specific inputs or scenarios.



  • Simulation is a technique for modeling and analyzing the behavior or outcome of a system or a component under different conditions or assumptions.



  • Redundancy is a technique for increasing the availability or reliability of a system or a component by providing multiple copies or backups.



  • Diversity is a technique for increasing the safety or security of a system or a component by providing different implementations or versions.



Conclusion




Summary of main points




In this article, we have discussed the concept of hierarchical design as a way to cope with the complexity and diversity of computer systems for critical applications. We have explained what hierarchical design is, why it is important, and what are the challenges and benefits of applying it to computer systems for critical applications. We have also introduced the notions of hierarchical abstraction and layers of trust as key elements of hierarchical design. Finally, we have provided some examples of hierarchical design in practice and discussed some future directions and open problems.


Future directions and open problems




Hierarchical design is an active and evolving research area that has many open problems and challenges that need to be addressed. Some of these problems and challenges include:



  • How to define and measure the optimal level of hierarchy and abstraction for a given system or problem?



  • How to ensure the coherence and completeness of the hierarchical design process and the resulting system?



  • How to cope with the trade-offs and conflicts that may arise among different layers or components of the system?



  • How to deal with the emergent or unexpected behaviors or properties that may result from the interactions or integration of the system?



  • How to manage the complexity and overhead of the hierarchical design process and the resulting system?



Hierarchical design is also an interdisciplinary and collaborative research area that requires the involvement and contribution of different domains and disciplines, such as computer science, engineering, mathematics, logic, psychology, sociology, etc. Hierarchical design can also benefit from the integration and combination of different methods and techniques, such as formal methods, testing, simulation, redundancy, diversity, etc.


Hierarchical design is a promising and powerful approach for designing computer systems for critical applications that can satisfy simultaneously a variety of critical requirements such as human safety, fault tolerance, high availability, security, privacy, integrity, and timely responsiveness, and that can continue to do so throughout maintenance and long-term evolution. Hierarchical design can also provide a systematic way to capture and express the essential features and properties of the system and its environment, as well as the relationships and dependencies among them. Hierarchical design can also provide a way to specify and verify the expected behavior or outcome of the system under normal or abnormal situations.


Frequently Asked Questions




Here are some frequently asked questions about hierarchical design of computer systems for critical applications:



  • What is hierarchical design?Hierarchical design is a general approach to deal with complex problems by decomposing them into simpler subproblems that can be solved separately and then integrated into a coherent solution.



  • What are critical applications?Critical applications are applications that have one or more requirements that must be met at all times and under all circumstances, otherwise serious consequences may occur. These requirements may involve human safety, fault tolerance, high availability, security, privacy, integrity, and timely responsiveness.



  • What are hierarchical abstraction and layers of trust?Hierarchical abstraction is a technique for creating and manipulating different levels of detail or generality of a system or a problem. Layers of trust are a concept that describes the degree of confidence or assurance that a system or a component satisfies its requirements or properties under certain conditions or assumptions.



  • What are some examples of hierarchical design in practice?Some examples of hierarchical design in practice are the OSI model, the TCP/IP model, the UML, the MVC, and the NIST framework.



  • What are some future directions and open problems in hierarchical design?Some future directions and open problems in hierarchical design include defining and measuring the optimal level of hierarchy and abstraction, ensuring the coherence and completeness of the hierarchical design process and the resulting system, coping with the trade-offs and conflicts that may arise among different layers or components of the system, dealing with the emergent or unexpected behaviors or properties that may result from the interactions or integration of the system, and managing the complexity and overhead of the hierarchical design process and the resulting system.



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