Browsing by Author "Schäfer, David Richard"

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Open Access
Replicated execution of workflows
(2018) Schäfer, David Richard; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h. c.)
Workflows are the de facto standard for managing and optimizing business processes. Workflows allow businesses to automate interactions between business locations and partners residing anywhere on the planet. This, however, requires the workflows to be executed in a distributed and dynamic environment, where device and communication failures occur quite frequently. In case that a workflow execution becomes unavailable through such failures, the business operations that rely on the workflow might be hindered or even stopped, implying the loss of money. Consequently, availability is a key concern when using workflows in dynamic environments. In this thesis, we propose replication schemes for workflow engines to ensure the availability of the workflows that are executed by these engines. Of course, a workflow that is executed by a replicated workflow engine has to yield the same result as a non-replicated execution of that workflow. To this end, we formally define the equivalence of a replicated and a non-replicated execution called Single-Execution-Equivalence. Subsequently, we present replication schemes for both imperative and declarative workflow languages. Imperative workflow languages, such as the Web Service Business Process Execution Language (WS-BPEL), specify the execution order of activities through an ordering relation and are the predominant way of specifying workflow models. We implement a proof-of-concept for demonstrating the compatibility of our replication schemes with current (imperative) workflow technology. Declarative workflow languages provide greater flexibility by allowing the reordering of the activities within a workflow at run-time. We exploit this by executing differently ordered replicas on several nodes in the network for improving availability further.
Open Access
Robust execution of workflows in a distributed environment
(2013) Schäfer, David Richard
In many business applications, workflows are used to describe business processes. Employees and machines get instructions from a plan (the workflow) to be guided or controlled. The workflows make it easier to create and manage business processes. Therefore, using workflows is the standard procedure in the business area today. The distributed execution of workflows plays an important role as almost all nodes are connected to a network today. The importance even increases with the emerging of pervasive environments. Because these systems are prone to failures, it is important to develop reliability methods that ensure that the system works properly even if failures occur. When the robustness of a system in a distributed environment shall be increased, the service that has to be executed is usually replicated and executed by two or more nodes. This means that the exact same behavior is executed by multiple nodes and thereby increases the reliability of the system by being able to cope with node failures. Changing the order of the activities or using alternative activities to increase the robustness is promising because when each node receives a different workflow that achieves the same goal, the possibility of failures should be further reduced by decoupling the replicas in respect of time and hardware dependencies. We developed a robustness metric that evaluates the robustness of a set of workflow replicas. We also developed methods and algorithms that generate workflows with different orders and alternative tasks within reasonable time. Our evaluations show that our proposed methods work significantly better than deploying a brute-force method to achieve the same behavior.
Open Access
Visualisierungstechniken für multiskopische Linsenrasterdisplays
(2012) Schäfer, David Richard
Dreidimensionale Inhalte und Datensätze werden üblicherweise auf 2D-Ausgabegeräten betrachtet. Durch die Reduzierung auf zwei Dimensionen gehen Informationen verloren. Es ist zwar möglich Systeme bereitzustellen, die auf Benutzerinteraktion reagieren, so dass der Inhalt aus unterschiedlichen Ansichten betrachtet werden kann und der Nutzer einen besseren Eindruck der Szene erhält, aber die einzelnen Bilder sind wieder nur 2D-Bilder und der Benutzer muss gewillt und befähigt sein, die richtigen Ansichten zu finden, um einen guten Eindruck von der Szene zu bekommen. Diese Arbeit beschäftigt sich damit, 3D-Inhalte auf multiskopischen Displays auszugeben und zu überprüfen, ob der damit gewonnene Eindruck der Szene, durch einfaches Betrachten, verbessert werden kann. Dazu werden von einem Datensatz ein 2D-Bild, ein Farbanaglyphenbild und ein Linsenrasterdisplay angefertigt. In einer Pilotstudie wird untersucht, auf welchem Bild der Datensatz am besten vermittelt wird. Die Ergebnisse zeigen, dass ein dreidimensionaler Eindruck der Szene, der mit Hilfe des Linsenrasterdisplays oder des Farbanaglyphenbildes erzeugt werden kann, ein besseres Verständnis für den Datensatz mit sich bringt.