Sebastian Vastag

A Framework for Simulation Models of Service-Oriented Architectures

Service-Oriented Architectures (SOA) are one of the main paradigms for future software systems. Since these software systems are composed of a large number of different components it is non trivial to assure an adequate Quality of Service (QoS) of the overall system and performance analysis becomes an important issue. To consider performance issues early in the development process, a model based approach becomes necessary which has to be embedded into the development process of SOA to avoid overhead and assure consistency. In particular the specification of the software system should be used as a base for the resulting performance model. However, since common specification techniques for SOA are very high level, many details have to be added to come to an executable simulation model which is often needed for a detailed analysis of performance or dependability. This paper presents an approach which combines an extended version of process chains to describe the SOA components and some quantitative specifications at the higher levels. For the modelling of the detailed architecture and protocols the simulation tool OMNeT++ is used. Both modelling levels are combined resulting in an executable simulation model for the whole architecture.

Simulating process chain models with OMNeT

This paper presents an approach to simulate complex hierarchical process chains resulting from large logistics networks in OMNeT++, a discrete event simulation environment designed for communication networks. For this purpose OMNeT++ has been integrated as a new simulation engine into the ProC/B toolset which is designed for the analysis and optimization of large logistics networks. The paper highlights the main steps of the automatic transformation of a hierarchical process chain model into a hierarchical model in OMNeT++. Furthermore it shows how the transformation has been validated and how detailed performance figures can be evaluated with OMNeT++.

A Simulation Environment for Hierarchical Process Chains Based on OMNeT

OMNeT++ is a discrete event simulation environment primarily designed for communication networks. In this paper we present an approach to enable OMNeT++ to simulate complex hierarchical process chains. Process chains are a common modeling paradigm in the logistics area for analysis and optimization, and have been used intensely in many practical applications. Their evaluation is supported by the ProC/B toolset, a collection of software tools for modeling, analysis, validation and optimization of process chains. Here we describe how OMNeT++ has been integrated as a new simulation engine into the toolset. The integration has to overcome some core problems to allow a smooth interaction between OMNeT++ and the other tools: in particular, the OMNeT++ model description of the logistics network should be kept manageable, it should reflect the entire model structure and non-standard performance figures, being relevant for an economic evaluation should be ascertainable in order to satisfy the specific needs of the application area. This paper highlights the main steps of the automatic transformation of a hierarchical process chain model into a hierarchical model in OMNeT++. Furthermore, we show how the transformation has been validated and how detailed performance figures can be evaluated with OMNeT++.

Simulation based validation of quantitative requirements in service oriented architectures

Large Service Oriented Architectures (SOAs) have to fulfill qualitative and quantitative requirements. Usually Service Level Agreements (SLAs) are defined to fix the maximal load the system can accept and the minimal performance and dependability requirements the system has to provide. In a complex SOA where services use other services and thus performance and dependability of a service depend on the performance and dependability of lower level services, it is hard to give reasonable bounds for quantitative measures without performing experiments with the whole system. Since field experiments are too costly, model based analysis, often using simulation is a reliable alternative. The paper presents an approach to model complex SOAs and the corresponding SLAs hierarchically, map the model on a simulator and analyze the model to validate or disprove the different SLAs.

A calculus for SLA delay properties

Service Providers in Service-Oriented Architectures (SOA) often specify system performance values with the help of Service Level Agreements (SLAs) that do not specify details of how the system realizes services. Analytic modeling of SOA to estimate performance values is thus made difficult without knowledge of service rates. Service components are characterized by quantitative requirements in SLAs only, that are not supported by most modeling methods. We propose a calculus to model and evaluate SOA with quantitative properties described in SLAs. Instead of defining a system by its service capacity we will use flexible constraints on delays as found in SLAs. From these delays approximate service rates to fulfill the delay will be derived.

Arrival and delay curve estimation for SLA calculus

An algorithm and selection method to estimate Network Calculus arrival bounds for systems with concurrent arrivals is presented. Concurrent job arrivals are common for Service-Oriented Architectures. Their performance is described in Service Level Agreements including quantitative requirements for load and response times. SLA Calculus, a variant of Network Calculus, can be used for service performance modeling and validation with SLAs. Functions called curves are used to bound job arrivals as well as their delay. Due to the concurrent nature of job arrivals curve estimation methods used for successive packet arrivals in Network Calculus cannot be applied in SLA Calculus. We present a method to estimate unknown SLA Calculus arrival and delay bounds from input and output traces. This paper introduces an algorithm for the estimation of the curves. Optimal selection of a curve model based on several fitting criteria is performed using candidates from trace sets.

ProC/B for networks: integrated INET models

The ProC/B toolset [1] is used to model an analyze process chain models in application fields ranging from logistics systems to service oriented architectures. The language comes with an GUI supporting users with different levels of experience. Simulative analysis can be performed with OMNeT++ [2]. A different but also OMNeT++-based class of models are communication networks created with the popular INET framework. Network and process chain models have been two separate worlds. Recently we presented a hybrid approach for modeling SOA systems using ProC/B combined with INET models [3].

Toward an analytical method for SLA validation

Quantitative properties of modern software systems are often defined as a part of a service-level agreement (SLA) that fixes the maximal load to be submitted to a system and guarantees bounds for the response time or delay. The evaluation of software architectures in order to validate SLAs is a challenging task since the systems tend to be complex, highly dynamic and to some extent unpredictable. Thus, there is a need for fast and abstract techniques to evaluate the performance of modern software architectures based on the information available in the SLAs. The paper presents an efficient approach to compute bounds on the delay of composed systems based on available bounds for the load and the response times of components. The technique can be used by a user of a software architecture to validate SLAs of composed services based on SLAs of the components. It can also be used by a provider of a software architecture to validate whether additional users can be accepted or to compute required service capacities to fulfill an SLA.