Nicolas Repp

Heuristics for QoS-aware Web Service Composition

This paper discusses the quality of service (QoS)-aware composition of Web services. The work is based on the assumption that for each task in a workflow a set of alternative Web services with similar functionality is available and that these Web services have different QoS parameters and costs. This leads to the general optimization problem of how to select Web services for each task so that the overall QoS and cost requirements of the composition are satisfied. Current proposals use exact algorithms or complex heuristics (e.g. genetic algorithms) to solve this problem. An actual implementation of a workflow engine (like our WSQoSX architecture), however, has to be able to solve these optimization problems in real-time and under heavy load. Therefore, we present a heuristic that performs extremely well while providing excellent (almost optimal) solutions. Using simulations, we show that in most cases our heuristic is able to calculate solutions that come as close as 99% to the optimal solution while taking less than 2% of the time of a standard exact algorithm. Further, we also investigate how much and under which circumstances the solution obtained by our heuristic can be further improved by other heuristics

Dynamic Replanning of Web Service Workflows

The composition of Web Services to workflows is one of the major challenges in the area of service-oriented computing. To meet the business and user requirements, it is crucial to manage the quality of service (QoS) of Web Service workflows. In our approach, we calculate the execution plan of workflows on the QoS attributes ex ante based on predictions. However, due to the volatile nature of the Internet and the web servers, the runtime behavior of Web Services is likely to differ from the predictions. Therefore, we propose replanning as a mechanism to adapt the execution plan to the actual behavior of already executed services by a dynamic service selection at runtime, ensuring that the QoS and cost requirements will still be met. In this paper, we discuss replanning strategies, show how replanning leads to cost-savings in most cases, and evaluate the additional overhead caused by the adaptation of the execution plan at runtime.

A Cross-Layer Approach to Performance Monitoring of Web Services

An increasing amount of applications are currently built as Web Service compositions based on the TCP/IP+HTTP protocol stack. In case of any deviations from desired runtime-behavior, problematic Web Services have to be substituted and their execution plans have to be updated accordingly. One challenge is to detect deviations as early as possible allowing timely adaption of execution plans. We advocate a cross-layer approach to detect bad performance and service interruptions much earlier than by waiting for their propagation through the full protocol stack. This position paper describes an approach to gain detailed real-time information about Web Service behavior and performance based on a cross-layer analysis of the TCP/IP+HTTP protocols. In this paper we focus especially on TCP. The results are used to make decisions supporting service selection and replanning in service-oriented computing scenarios. Furthermore, generic architectural components are proposed implementing the functionality needed which can be used in different web-based scenarios.

Towards automated monitoring and alignment of service-based workflows

Using Web services and service-oriented architectures to implement cross-organizational workflows has become state-of-the-art for the realization of collaborations between enterprises. Here, a key issue is the monitoring of workflows and services based on given business requirements and the handling of deviations from those requirements to fulfil service level agreements. In this paper we present an approach to the automated monitoring and alignment of service-based workflows. We describe a policy language for the specification of requirements and deviation handling as well as a distributed architecture supporting automated monitoring and alignment in service ecosystems.

Structuring SOA Governance

Companies’ IT Systems are confronted with constantly changing market conditions, new competitive threats and a growing number of legal regulations. The service-oriented architecture (SOA) paradigm provides a promising way to address these challenges at the level of a company’s IT infrastructure. These challenges, as well as the management of the newly introduced complexity and heterogeneity, are targeted by SOA Governance approaches. In recent years, a number of concrete frameworks for SOA Governance addressing these issues have been proposed. There is no holistic approach considering all proposed elements, consolidating them in order to form a universally applicable model. In this contribution, we motivate SOA Governance, investigate and compare different approaches, identify common concepts, and derive a generic model for governance of Service-oriented Architectures.

Challenges of governance approaches for service-oriented architectures

IT Systems in companies nowadays are confronted with constantly changing market conditions, new competitive threats and an increasing number of legal regulations. The service-oriented architecture (SOA) paradigm provides a promising way to address these challenges at the level of the companys IT infrastructure. These challenges and the management of the introduced complexity and heterogeneity are targeted by SOA Governance approaches. Hereby, the basic structure of IT Governance frameworks is applicable to SOA; however, they lack applicability concerning some SOA-specific challenges. In this paper, we discuss deficiencies and provide insights of what regulation challenges a SOA Governance approach is actually required to be capable of, in particular in the area of service lifecycles and service marketplaces.

Resource Planning Heuristics for Service-Oriented Workflows

Resource allocation and resource planning, especially in a SOA and grid environment, become crucial. Particularly, in an environment with a huge number of workflow consumers requesting a decentralized cross-organizational workflow, performance evaluation and execution-management of service-oriented workflows gain in importance. The need for an effective and efficient workflow management forces enterprises to use intelligent optimization models and heuristics to compose workflows out of several services under real-time conditions. This paper introduces the required architecture workflow performance extension - WPX.KOM for resource planning and workload prediction purposes. Furthermore, optimization approaches and a high-performance heuristic solving the addressed resource planning problem with low computational overhead are presented.

WS-Re2Policy: A Policy Language for Distributed SLA Monitoring and Enforcement

Web service technology and the Service-oriented Architecture (SOA) paradigm have become state of the art for the integration of systems across enterprise boundaries. Here, a strong need for policies exists, which describe the Quality of Service delivered by third parties.Current policy languages in the area of Web services and SOAs allow the specification of requirements with respect to the Quality of Service as well as the parameters, which should be monitored. They do not cover the countermeasures needed and accepted in case of requirement violations. Especially, in distributed scenarios it is helpful to provide the monitoring units with information about possible reactions to violations in order to enforce policies at the monitoring units. Therefore, we developed the Web service requirements and reactions policy language (WS-Re2Policy), which overcomes those issues by specifying requirements and reactions in a single policy to be distributed to independent monitoring units.

Worst-Case Workflow Performance Optimization

Performance evaluation and execution management of service-oriented workflows became quite important in order to avoid performance degradation. Performance measurement is crucial to ensure that workflow execution remains feasible and that SLA violations due to overload are avoided. Network calculus as a well-known system theory for deterministic queuing systems can be used to describe the worst-case performance behavior of a workflow in order to plan workflow control in advance. Concerning business processes with high repetition rates the workflow controller has to be able to serve all incoming requests with an optimal composition of Web services. Thus, this paper presents a formal worst-case calculation model using the concepts of network calculus. Furthermore, optimization problems based on the worst-case scenario are introduced in order to minimize the worst-case delay and to maximize the throughput of the Web services invoked with minimal costs.

Process reliability in service oriented architectures

The creation of business processes by composing Web services has attracted considerable efforts with respect to integrating elements of Service Oriented Architectures within enterprise applications. If one of those atomic services fails, the whole business process does not necessarily have to fail, too, as it is possible to exchange the faulty service during runtime with another one which has the same functionality. In this paper we present a service monitoring and exchanging approach that aims to ensure a successful execution of crucial processes in case of faulty services. It consists of three parts which supervise the process execution and try to recover it if an error occurs. In case process reliability is not fore grounded, our approach can also serve as basis for process optimization.