Models, Abstractions, and Architectures: The Missing Links in Cyber-Physical Systems Bharathan Balaji†, Mohammad Abdullah Al Faruque‡, Nikil Dutt‡, Rajesh Gupta†, Yuvraj Agarwal# †University of California, San Diego ‡University of California, Irvine #Carnegie Mellon University †{bbalaji,gupta}@cs.ucsd.edu, ‡{alfaruqu,dutt}@uci.edu, #[email protected] ABSTRACT iad set of requirements and create prototypical system archetypes. Bridging disparate realms of physical and cyber system compo- This requires our ability to create useful abstractions of CPS, and nents requires models and methods that enable rapid evaluation of reasoning and optimization tools that use these abstractions. design alternatives in cyber-physical systems (CPS). The diverse Consider an example of a personal home assistant application. intellectual traditions of physical and mathematical sciences makes It will take care of your basic chores such as payment of monthly this task exceptionally hard. This paper seeks to explore poten- bills, switch on lights as you enter your home, or remind you to tial solutions by examining specific examples of CPS applications buy milk. The app would also ensure stable electricity according to in automobiles and smart buildings. Both smart buildings and au- your demand, making use of solar panels, exploiting the batteries tomobiles are complex systems with embedded knowledge across in your electric car when not in use, and bidding for electricity several domains. We present our experiences with development of from the grid when needed. The app would modulate the lighting CPS applications to illustrate the challenges that arise when exper- and thermal environment based on ongoing activities - sleeping, tise across domains is integrated into the system, and show that cre- cooking, watching TV, etc. It would notify you about things that ation of models, abstractions, and architectures that address these may need your attention, such as a broken pipe, malfunctioning challenges are key to next generation CPS applications. refrigerator, or high water usage bills. Each aspect of this app requires interaction with other physical systems such as home appliances, the power grid, and the security Categories and Subject Descriptors system. To develop such applications, we need to create an ecosys- D.2.11 [Software Architectures]: Data Abstraction; tem that makes it easy for programmers to access relevant infor- D.4.7 [Organization and Design]: Real-time systems and embed- mation across these systems. However, each system has domain ded systems specific aspects, and the information sought is a result of complex interactions within the system. We need models that capture this General Terms complexity, so that applications can access contextual information without requiring extensive domain expertise across all these dis- Design parate systems. For instance, the power grid for a modern home will consist of solar panels, energy storage, appliance load, and Keywords connection to the community grid. The grid model would capture Cyber-Physical Systems, Smart Buildings, Automobiles, Abstrac- the interaction between these components, and provide information tions, Models, Architectures such as expected electricity usage, or suggest changes that will re- duce electricity bills. Similarly, occupants within the home will 1. INTRODUCTION have certain behavioral patterns, preferences, and habits that need to be learned and captured in an occupant model. Proliferation of computing and communication technologies has Each of these models often naturally interact with each other made it possible to build societal-scale systems that are at the thresh- and third party applications written on top need to extract and ex- old of transforming societal infrastructure for transportation, en- change information from multiple models/systems. Thus, the mod- ergy, healthcare. This opportunity for improved societal infrastruc- els need to provide abstractions that enable other applications to ture can be exploited only if we are able to build effective Cyber- use it. These abstractions simplify application development, and Physical Systems (CPS) applications that will incorporate a myr- lead to development of complex models that can be reused across Permission to make digital or hard copies of all or part of this work for different applications. Such abstractions are common in modern personal or classroom use is granted without fee provided that copies are not computing systems. For instance, solid state drives hide the com- made or distributed for profit or commercial advantage and that copies bear plexity of flash memory, and expose byte addressable memory to this notice and the full citation on the first page. Copyrights for components the processors. Similar abstractions need to be created for various of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to CPS domains. redistribute to lists, requires prior specific permission and/or a fee. Request We need architectures that support these models and abstrac- permissions from [email protected]. tions, and provide a unified view of the system that has built in DAC ’15 San Francisco, CA, USA privacy and security mechanisms. The architecture needs to sup- Copyright is held by the author(s). Publication rights licensed to ACM. port a system that is scalable and maintainable. Design of these ACM 978-1-4503-3520-1/15/06 ...$15.00 models, abstractions and architectures is the next big challenge for http://dx.doi.org/10.1145/2744769.2747936. creation of societal scale CPS applications. In this paper, we use Third Party two domains – smart buildings and automobiles, as exemplars to illustrate these challenges and outline data exchange mechanisms Applications Visualize Maintain Analyze Control and design alternatives. REST/Native API 2. BACKGROUND AND MOTIVATION RESTful Web Service Building Management Cyber-Physical Systems seek to bring the benefits of internet Metadata Access scale computing and networking to physical systems like trans- Info Control Storage System portation and buildings. Existing methods for application devel- opment create their own models and abstractions as needed, and Data Connectors as the applications become more complex, the common parts are Information modularized into libraries or services that can be reused across ap- Source plications. There are tools available that allows the application to Building A Building B Building C Building D be tested and debugged in a limited environment. Errors in pro- gramming are often benign, and security flaws do not directly im- pact physical systems. Figure 1: Software Architecture of BuildingDepot [7] With CPS applications, many of the tools for development, li- available at this time, based on our experience with the case stud- braries for abstraction of functionalities and simulators or emula- ies, we present the challenges faced using specific examples across tors for testing or debugging are yet to be built. Some modeling smart buildings and automobiles. tools and simulators such as architectural CAD and fluid mechan- ics simulation tools are available, but they are restricted to domain specific applications. As CPS applications targeting emerging areas 3. CASE STUDY I: SMART BUILDINGS like the Smart Grid will directly impact physical systems, develop- Buildings form an essential part of modern infrastructure, and is ers need to capture the complexity of the natural world, need to a prototypical example of a complex CPS. Many different domains ensure correctness across a range of scenarios, and make programs are embedded in a building to meet its wide range of requirements that are robust against physical damage and external attacks. - architecture and construction, electrical system for lights and ap- To reduce developer burden, to decrease the cost of application pliances, and other systems that manage water, safety and security. development and to encourage reuse across applications, an infras- Use of sensors and technology is common in modern buildings, and tructure needs to be created that supports creation of CPS applica- they provide a fertile ground for development of CPS applications. tions. We propose that models be created that capture domain spe- cific complexity, and expose information with relevant abstractions 3.1 Building Management Architectures to the developers. Models can be design centric, user centric, infor- In most buildings today, each of the systems is maintained by mation centric, or centered towards operations or controls. Model separate contractors with little information flow across them. Con- development can be complex [25], and hence, creation of these sider the HVAC (Heating, Ventilation and Air Conditioning) sys- models can be delegated to trusted entities to reduce errors and cer- tem, that use sensors such as temperature and air flow, distributed tified by standardized bodies. Tools for debugging, simulation and across the building for monitoring and control of the thermal envi- emulation can be developed based on these models. ronment. Usually, a single vendor provides the equipment, sensors, Currently, CPS applications that span across domains need de- and management software. Examples of such Building Manage- velopers that are familiar with all the domains relevant to the ap- ment Systems (BMS) include Metasys from Johnson Controls [2] plication. Significant expertise is required to make use of available and Apogee from Siemens