Gera Weiss

Behavioral programming

A novel paradigm for programming reactive systems centered on naturally specified modular behavior.

Automata based interfaces for control and scheduling

We propose the use of formal languages of infinite words over the alphabet of task identifiers as an interface between control designs and software implementations. We argue that this approach is more flexible than the classical real-time scheduling framework based on periodic tasks, and allows composition of interfaces by language-theoretic operations. We show that finite automata over infinite words offer analyzable representation and can capture many interesting interface specifications such as exponential stability of switched linear systems.

Regular Specifications of Resource Requirements for Embedded Control Software

For embedded control systems, a schedule for the allocation of resources to a software component can be described by an infinite word whose ith symbol models the resources used at the ith sampling interval. Dependency of performance on schedules can be formally modeled by an automaton (omega-regular language) which captures all the schedules that keep the system within performance requirements. We show how such an automaton is constructed for linear control designs and exponential stability or settling time performance requirements. Then, we explore the use of the automaton for online scheduling and for schedulability analysis. As a case study, we examine how this approach can be applied for the LQG control design. We demonstrate, by examples, that online schedulers can be used to guarantee performance in worst-case condition together with good performance in normal conditions. We also provide examples of schedulability analysis.

Modeling and Analysis of Multi-hop Control Networks

We propose a mathematical framework, inspired by the Wireless HART specification, for modeling and analyzing multi-hop communication networks. The framework is designed for systems consisting of multiple control loops closed over a multi-hop communication network. We separate control, topology, routing, and scheduling and propose formal syntax and semantics for the dynamics of the composed system. The main technical contribution of the paper is an explicit translation of multi-hop control networks to switched systems. We describe a Mathematica notebook that automates the translation of multihop control networks to switched systems, and use this tool to show how techniques for analysis of switched systems can be used to address control and networking co-design challenges.

RTComposer: a framework for real-time components with scheduling interfaces

We present a framework for component-based design and scheduling of real-time embedded software. Each component has a clearly specified interface that includes the methods used for sensing, computation, and actuation, along with a requirement given as a regular set of macro-schedules. Each macro-schedule is an infinite sequence that specifies, for every time slot, the set of component methods invoked in that slot. The macro-scheduler composes the specifications of all the components, along with the platform specification that constrains which methods can be executed within a single slot, to generate a feasible macro-schedule. Within a slot, we use logical execution time semantics, and this micro-scheduling is implemented on top of a native priority-based scheduler. With this approach, each component can be specified and analyzed in a platform-independent way, and at the same time, the performance can vary with changing load and changing processing speed. We describe an implementation using Real-Time Java. Scheduling specifications can be given as periodic tasks, or using temporal logic, or as omega-automata. Components can be added dynamically, and non-real-time components are allowed. We demonstrate the benefits of the approach using case studies.

Model-checking behavioral programs

System specifications are often structured as collections of scenarios and use-cases that describe desired and forbidden sequences of events. A recently proposed behavioral programming approach, which evolved from the visual language of live sequence charts (LSCs), calls for coding software modules in alignment with such scenarios. We present a methodology and a supporting model-checking tool for verifying behavioral Java programs, without having to first translate them into a specific input language for the model checker. Our method facilitates early discovery of conflicting or under-specified scenarios, which can often be resolved by adding new scenarios rather than by changing existing code. Also, counterexamples provided by the tool are themselves event sequences that can serve directly for refinements and corrections. Our tool reduces the size of the execution state-space using an abstraction that focuses on behaviorally interesting states and treats transitions between them as atomic.

Programming coordinated behavior in java

Following the scenario-based approach to programming which centered around live sequence charts (LSCs), we propose a general approach to software development in Java. A program will consist of modules called behavior threads (b-threads), each of which independently describes a scenario that may cross object boundaries. We identify a protocol and a coordination mechanism that allow such behavioral programming. Essentially, runs of programs are sequences of events that result from three kinds of b-thread actions: requesting that events be considered for triggering, waiting for triggered events, and blocking events requested by other b-threads. The coordination mechanism synchronizes and interlaces b-threads execution yielding composite, integrated system behavior. The protocol idioms and the coordination mechanism of b-threads are implemented as a Java library called BPJ. Throughout the exposition we illustrate benefits of the approach and discuss the merits of behavioral programming as a broad, implementation-independent paradigm.

Scalable scheduling algorithms for wireless networked control systems

In this paper, we address the problem of designing scalable scheduling and routing policies over a time-triggered multi-hop control network, when closing a considerable number of control loops on the same network. The key idea is to formally define by means of regular languages the set of schedules for each control loop that satisfy a given control specification, and to exploit operators on regular languages to compute the set of schedules for the whole system. In order to test our methodology, we address a mineral floatation control problem derived from the Boliden (a swedish mining company) mine in Garpenberg, and propose a scheduling solution that can be implemented on systems compliant with communication protocols for wireless networks (e.g. the WirelessHART specification).

A decentralized approach for programming interactive applications with JavaScript and blockly

We present a decentralized-control methodology and a tool-set for developing interactive user interfaces. We focus on the common case of developing the client side of Web applications. Our approach is to combine visual programming using Google Blockly with a single-threaded implementation of behavioral programming in JavaScript. We show how the behavioral programming principles can be implemented with minimal programming resources, i.e., with a single-threaded environment using coroutines. We give initial, yet full, examples of how behavioral programming is instrumental in addressing common issues in this application domain, e.g., that it facilitates separation of graphical representation from logic and handling of complex inter-object scenarios. The implementation in JavaScript and Blockly (separately and together) expands the availability of behavioral programming capabilities, previously implemented in LSC, Java, Erlang and C++, to audiences with different skill-sets and design approaches.

Non-intrusive Repair of Reactive Programs

We show how, under certain conditions, programs written in the behavioral programming approach can be modified (e.g., as result of new requirements or discovered bugs) using automatically-generated code modules. Given a trace of undesired behavior, one can generate a relatively small piece of code, whose execution is interwoven at run time with the rest of the system and brings about the desired changes without modifying existing code, and without introducing new bugs. At the core of our approach is the ability of a thread of behavior to prevent the triggering of events from other threads. Our repair algorithms apply model checking to the program and transform the counterexamples produced by the model-checker into corrective modules. Our work is supported by a proof-of-concept tool, which creates understandable modules that can be further manually managed as part of ongoing incremental system development.