Oliver Heckmann

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

On realistic network topologies for simulation

Simulations are an important tool in network research. As the selected topology often influences the outcome of the simulation, realistic topologies are needed to produce realistic simulation results. We first discuss the different types of topologies and present our collection of real-world topologies that can be used for simulation. We then define several similarity metrics to compare artificially generated topologies with real world topologies. We use them to find out what the input parameter range of the topology generators of BRITE, TIERS and GTITM are to create realistic topologies. These parameters can act as a valuable starting point for researchers that have to generate artificial topologies.

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.

Generating realistic ISP-level network topologies

Simulations are an important tool in network research. As the selected topology often influences the outcome of the simulation, realistic topologies are needed to produce realistic simulation results. Using several similarity metrics to compare artificially generated topologies with real world topologies this paper gives hints on how to use the widespread topology generators BRITE, TIERS, and GT-ITM to create realistic topologies.

TCP: Local stability and Hopf bifurcation

In this paper we analyze a fluid model of TCP with an approximation of drop tail using tools from control and bifurcation theory. The focus of our analysis and experiments lies in a regime where the buffer sizes are small, as recently advocated by Appenzeller, Keslassy and McKeown [G. Appenzeller, I. Keslassy, N. McKeown, Sizing router buffers, in: Proceedings of ACM SIGCOMM, 2004]. We find that to ensure local stability of TCP with drop tail it is necessary and sufficient that the arrival rate be greater than capacity by a certain factor, which does not depend on the round-trip time. This factor is found to be 1.1415. The next natural question to ask is: what if these conditions of local stability are just violated? This entails conducting a local bifurcation theoretic analysis (at the point of linear instability), from which we conclude that the corresponding nonlinear system undergoes a supercritical Hopf bifurcation. So as stability of the equilibrium is just lost, it is regained by a stable limit cycle. The analysis is complemented by simulations at the packet level performed using the Network Simulator, ns2.

Eine Dienstgüte unterstützende Webservice-Architektur für flexible Geschäftsprozesse

Web services as a technology to enable distributed business processes gain in importance. However, the support of Quality of Service (QoS) is crucial in this context. Thus, we present an integrated web service architecture with comprehensive QoS support. The architecture we introduce in this paper supports the assessment of web services to assure that only web services will be used in critical business processes that satisfy the requirements defined by the user. The selection and execution of a certain web service depends on its QoS-properties that must be guaranteed in Service Level Agreements (SLAs). The compliance with SLAs is monitored by the system as well. Furthermore we introduce a prototypical implementation of our web service architecture.

Location Awareness—Improving Distributed Multimedia Communication

Multimedia creation and consumption is highly intensive and makes up the majority of Internet traffic nowadays. End-users are able to share their digital content with each other and to build communities based on interests, which often differ drastically according to location. Distributing these media using a central server can be quite expensive for a content provider. Distributed (peer-to-peer like) systems share costs evenly among participants. Thus, distributed multimedia systems will be more important in the future. The global distribution of end-users aggravates high-quality delivery of multimedia content. In this paper, we argue that geographical location-awareness greatly helps distributed multimedia communication. It increases the quality of multimedia content delivery and at the same time satisfies the growing need for more personalized, location-based services. In this paper, as a proof of concept, we introduce an overlay structure for distributed multimedia systems (and similar systems), which is location-aware and uses the locations of its nodes to optimize node-to-node communication for performance and delay. At the same time, the system enables location-based services.

Charging in peer-to-peer systems based on a token accounting system

Today, Peer-to-Peer applications are predominant on the internet when considered in terms of its traffic consumption. However apart from Skype, their commercial success is still very limited. This is due to the difficulties faced when trying to implement crucial functionality such as accounting and charging without violating the Peer-to-Peer paradigm. A fully decentralized accounting scheme based on tokens was presented by the authors last year. In this paper we analyse the interactions between token-based accounting and charging in order to enable peers to charge for their services. We present three different charging schemes using tokens as (1) pure receipts, as (2) Micropayment, and as (3) bill of exchange. These schemes are evaluated based on the provided security and the overhead traffic introduced into a Peer-to-Peer system.

Management of Service-oriented Architecture (SoA)-based Application Systems

Flexible business processes are a key success factor for enterprises to succeed in globalized markets. The Service-oriented Architecture (SoA) concept is very well suited to support flexible business processes and their underlying application systems because services can be composed in an efficient way to achieve a high level of agility. However, the management of SoA-founded application systems is often neglected. Thus, in this paper we conclude and extend our previous work on the enhancement of the generic SoA-based concept by additional management functionality, e.g. monitoring mechanisms and SLA management. As a validation of our concept we introduce WSQoSX, a prototypical implementation based on Web Services.