Daniel Graff

Design and implementation of the Rebeca publish/subscribe middleware

Publish/subscribe is used increasingly often as a communication mechanism in loosely-coupled distributed applications. Research and product development have focused mostly on efficiency issues and neglected methodological support to build concrete middleware implementations so far. In this paper, we present the novel design of the REBECA publish/subscribe middleware that is based on the experience gained with previous versions. As basic design concept, we focus on a modular pipeline architecture that is built around a minimal, but extendable publish/ subscribe core. With respect to modularity, we employ the concept of features that are well-defined aspects of a software systems functionality, encapsulated in pluggable modules, and, thereby, facilitate a separation of concerns. We address the composition of features and show how this is realized in REBECAs pipeline architecture with independently working plugins that can influence passing messages in three dedicated stages.

Modeling Group Scheduling Problems in Space and Time by Timed Petri Nets

Dealing with cyber-physical systems CPS puts a strong emphasis on the interaction between computing and non-computing elements. Since the physical world is characterized by being strongly distributed and concurrent, this is also reflected in the computational world making the design of such systems a challenging task. If a number of tasks shall be executed on a CPS which are bound to time and space, may have dependencies to other tasks and requires a specific amount of computing devices, a solution requires a four-dimensional space-time schedule which includes positioning of the devices resulting in an NP-hard problem. In this paper, we address the problem of spatial-temporal group scheduling using Timed Petri nets. We use Timed Petri nets in order to model the spatial, temporal, ordered and concurrent character of our mobile, distributed system. Our model is based on a discrete topology in which devices can change their location by moving from cell to cell. Using the time property of Petri nets, we model movement in a heterogeneous terrain as well as task execution or access to other resources of the devices. Given the modeling, we show how to find an optimal schedule by translating the problem into a shortest path problem, which is solvable with the known method of dynamic programming.

On the Need of Systemic Support for Spatio-Temporal Programming of Mobile Robot Swarms

In this paper, we present SwarmOS -- a distributed operating system for mobile robot swarms. SwarmOS features transaction-based spatio-temporal programming of mobile robot swarms on a systemic level. We show the programming model and resource management in SwarmOS. Swarm applications consist of concurrent, distributed and context-aware actions. We provide distributed transactions in order to guarantee atomic execution of a set of dependent actions. We distinguish between schedulability and executability of a set of actions: the first one is checked by the space-time scheduler which is a core service of the execution environment. The scheduler plans actions in space and time and computes spatio-temporal trajectories if robot movement is necessary. In order to guarantee executability of a distributed transaction of spatio-temporal actions, we present the concept of path alternatives and a time-based two-phase commit protocol in order to assure consistency. We show the feasibility of our approach by performing experiments on our testbed.

Systemic support for transaction-based spatial-temporal programming of mobile robot swarms

In this paper, we present an approach to support transaction-based spatial-temporal programming of mobile robot swarms on a systemic level. We introduce a programming model for swarms of mobile robots. Swarm applications consist of concurrent, distributed and context-aware actions. We provide distributed transactions in order to guarantee atomic execution of a set of dependent actions. We distinguish between schedulability and executability of a set of actions. In order to guarantee executability of a distributed transaction of spatial-temporal actions, we present the concept of path alternatives and a time-based two-phase commit protocol in order to assure consistency. We show the feasibility of our approach by a proof-of-concept.

Programming and Managing the Swarm -- An Operating System for an Emerging System of Mobile Devices

Todays situation is characterized by an increasing pervasiveness of a plethora of mobile devices featuring different capabilities and exhibiting different system interfaces making the handling of these devices and especially the cooperation between different devices a complex task. In this paper, we consider the sum of all these devices as one emerging system (the swarm) and present an approach of a swarm operating system that on a systemic level manages these devices (local devices give up their autonomy) while providing a common interface to user applications. We provide a programming model for distributed mobile applications that abstracts from error-prone aspects such as distribution and concurrency by giving the programmer a systemic view to system resources. The model allows the programmer to define actions that can be restricted in space and time. Together with a high level goal, an entire application emerges implicitly based on those defined actions. In order to execute such applications, we present an architecture for a runtime system that uses virtualization techniques in order to execute multiple independently developed applications in parallel. The system follows a service-oriented architecture: one of the core services is the space-time scheduler that plans applications (a set of actions) in time and space.

Spatio-Temporal Coordination of Mobile Robot Swarms

Context-aware applications that require access to physical space and time are a necessity in cyber-physical systems. We focus on the design of a cyber-physical operating system in which a space-time scheduler is the core-component responsible for resource management. Given a set of space-time tasks and a set of mobile robots that move through physical space, a main objective remains in finding a mapping of tasks to robots. In this paper, we address the problem of scheduling a set of tasks with spatio-temporal constraints in space and time. We present an online-scheduler that computes collision-free spatio-temporal trajectories for the robots in order to execute the space-time tasks. As side condition, collisions with static as well as dynamic obstacles must be avoided at all times. The scheduler consists of two components: a job scheduler that uses a heuristic and performs a coarse-grained scheduling and a trajectory planner that takes the output of the job scheduler and computes spatio-temporal trajectories.

The Swarm as a Service: Virtualization of Motion

A defining aspect of Cyber-physical systems are the sensors and actuators of a computing system, which interact with the physical world. While sensors and actuators can take on many different forms, one type of actuator, namely motors, usually has the highest influence on the lifetime due to the high energy consumption. Therefore, it is necessary to reduce physical movement of robots as much as possible without reducing the quality of the application. In this paper we introduce virtual movement of applications, meaning that parts of an application can be executed on arbitrary robots that can reach the locations defined by the application programmer with minimal effort. As we assume that robots will be shared by multiple applications, programming virtual movement within the application is not only cumbersome, but impossible, as the location and status of a robot may be changed by a different application. Therefore, we provide a middleware abstraction which takes care of virtual movement and hides it from the application programmer.