Guido Wirtz

Negotiating SLAs - An Approach for a Generic Negotiation Framework for WS-Agreement

The current Web Services Agreement specification draft proposes a simple request-response protocol for agreement creation only addressing bilateral offer exchanges. This paper proposes a framework augmenting this WS-Agreement to enable negotiations according to a variety of bilateral and multilateral negotiation protocols. The framework design is based on a thorough analysis of taxonomies for negotiations from the literature in order to allow for capturing a variety of different negotiation models within a single, WS-Agreement compatible, framework. In order to provide for the intended flexibility, the proposed protocol takes a two-stage approach: a meta-protocol is conducted among interested parties to agree on a common negotiation protocol first before the real negotiation is carried out in the second step due to the protocol established in the first step.

The OCoN Approach to Workflow Modeling in Object-Oriented Systems

Workflow management aims at modeling and executing application processes in complex technical and organizational environments. Modern information systems are often based on object-oriented design techniques, for instance, the Unified Modeling Language (UML). These systems consist of application objects which collaborate to achieve a common goal. Although application objects collaborate in the context of business processes that can be supported by workflow technology, workflow modeling is typically done with proprietary workflow languages. Hence, two separate formalisms are present for modeling application objects and workflows. In this paper we try to remedy this situation by proposing the use of Object Coordination Nets (OCoN) for workflow modeling. OCoN nets provide a seamless integration with UML structure diagrams. The OCoN formalism also helps to deal with all relevant aspects of modeling complex workflow systems in a scalable and consistent manner.

BPEL conformance in open source engines

Service-oriented systems are increasingly implemented in a process-based fashion. Multiple languages for building process-based systems are available today, but the Business Process Model and Notation (BPMN) is becoming ubiquitous. With BPMN 2.0 released in 2011, execution semantics were introduced, supporting the definition of executable processes. Nowadays, more and more process engines directly support the execution of BPMN processes. However, the BPMN specification is lengthy and complex. As there are no official tests and no certification authority, it is very likely that engines a) implement only a subset of the language features and b) implement language features differently. In other words, we suspect that engines do not conform to the standard, despite the fact that they claim support for it. This prohibits the porting of processes between different BPMN vendors, which is an acclaimed goal of the language. In this paper, we investigate the standard conformance of open source BPMN engines to provide a clear picture of the current state of the implementation of BPMN. We develop a testing approach that allows us to build fully BPMN-compliant tests and automatically execute these tests on different engines. The results demonstrate that state of-the-art BPMN engines only support a subset of the language. Moreover, they indicate that porting BPMN processes is only feasible when using basic language constructs.

Modeling Distributed Software Systems with Object Coordination Nets

A dedicated approach for the design and development of distributed software systems is outlined. The method may be used in combination with any standard object-oriented analysis and design method as far as static aspects are concerned. A new formalism based on extended Petri-Nets { so called Object Coordination Nets (OCoNs) { is used to describe the coordination of the behavior of a class on a per service (method) basis, the allocation of object resources as well as behavioral contracts. The paper discusses the rationale behind the key points of the OCoN approach, introduces the method and illustrates it by means of an extended example.

Closing the gap between object-oriented modeling of structure and behavior

The UML as standardized language for visual object-oriented modeling allows to capture the requirements as well as the structure and behavior of complex software systems. With the increasing demands of todays systems, behavior aspects like concurrency, distribution and reactivity become more important. But the language concepts of the UML for describing behavioral aspects are weak compared to its concepts for describing structures. Besides a lack of visual expressiveness, a deeper integration with the structure specification is missing. In order to close this gap, an expressive language for modeling object-oriented behavior is proposed with the OCoN approach. It describes contracts, object scheduling as well as control and data flow of services in a Petri-net-like form. A seamless visual embedding of contract specifications into service and object scheduling specifications is provided by different net types.

Performance Measurements and Statistics of Tor Hidden Services

Tor (the onion routing) provides a secure mechanism for offering TCP-based services while concealing the hidden servers IP address. In general the acceptance of services strongly relies on its QoS properties. For potential Tor users, provided the anonymity is secured, probably the most important QoS parameter is the time until they finally get response by such a hidden service. Internally, overall response times are constituted by several steps invisible for the user. We provide comprehensive measurements of all relevant latencies and a detailed statistical analysis with special focus on the overall response times. Thereby, we gain valuable insights that enable us to give certain statistical assertions and to suggest improvements in the hidden service protocol and its implementation.

Towards Application Portability in Platform as a Service

Cloud Computing has been one of the most vibrant topics in the last years. Especially Platform as a Service (PaaS) is said to be a game changer for future application development. Taking away most of the configuration work, it pledges to foster rapid application development which seems even more important in a world of complex scalable distributed systems. Whereas Infrastructure as a Service (IaaS) is in the process of consolidation and standardization, the PaaS market is largely fragmented offering varying ecosystem capabilities. In this situation, application portability is a major concern for companies utilizing PaaS to avoid vendor lock-in and to retain the ability for future strategical decisions. To categorize portability problems of PaaS, we define a model of current PaaS offerings and identify different portability perspectives. Starting from the model, we derive a standardized profile with a common set of capabilities that can be found among PaaS providers and matched with one another to check application portability based on ecosystem capabilities. We validate our findings with a comprehensive data set of 68 PaaS offerings together with a web-based application for portability matching. We also identify further portability problems by porting the application to different PaaS vendors, validating ecosystem portability and giving hints for future research directions.

Seamless visual object-oriented behavior modeling for distributed software systems

To ease the development of distributed systems, the visual notions for the structural aspects of object-oriented analysis and design should be combined with techniques for handling concurrency and distribution. A novel approach and language for the visual design of distributed software systems is introduced and illustrated by means of an example. The language of OCoNs (Object Coordination Nets) is integrated into the structuring mechanisms of the UML (Unified Modeling Language) standard for object-oriented analysis and design. Such an object-oriented notation is crucial for handling complex software systems and can be extended with the graphical expressive power of Petri nets to also describe concurrency and coordination. The same visual language is used to specify the interfaces and contracts of software components, the resource handling within a component as well as the control flow of services.

Describing Services for Service Ecosystems

Service ecosystems are electronic market places and emerge as a result of the shift toward service economies. The aim of service ecosystems is to trade services over the internet. There are still obstacles that impede this new form of market places. Two of these challenges are addressed in this paper: (1) identification of appropriate service properties to specify service descriptions, and (2) a need of a clear classification for service description notations. Therefore, service properties and their relationship are introduced and an adaption for the Zachman Framework is presented to classify service description notations depending on the relative perspective.

Extending UML with Workflow Modeling Capabilities

Nowadays application systems are modeled in object-oriented design languages like the Unified Modeling Language (UML). Although workflow management has to deal with such environments, typically propriatory modeling languages are used. We try to remedy the use of seperate languages by proposing an extension of the UML to allow workflow modeling, thereby providing a seamless integration of workflow functionality in object-oriented application systems.