Hatem Hadj Kacem

Workflow Scheduling in Cloud Computing: A Survey

Workflow systems have become a major vehiclefor easy and efficient development of scientific applications. This type of systems can benefit from the resource provisioningtechnology offered by the cloud computing. In fact, the latteroffers on-demand virtualized resources to its users. These virtualresources can be added and released dynamically. Also, usersare charged on a pay-per-use basis. How to make appropriatedecisions when allocating resources to the tasks and dispatchingthe computing tasks to resource pool has become the main issuein cloud computing. The amount of allocated resources affects theexecution time of the applications and the cost incurred by the user. In fact, resource under-provisioning will necessarily affectthe performance. In contrast, over-provisioning can result in idleinstances and cause additional costs. Then, efficient scheduling algorithms are required for selection of best suitable resources for task execution. This paper focuses on some of the important workflow scheduling strategies. It brings out an exhaustive survey of such strategies in cloud computing and includes a detailed classification of them. Then, it presents a comparative analysis of the studied approaches. Finally, it stands out a critical challenge for further research.

Elastic Multi-tenant Business Process Based Service Pattern in Cloud Computing

Elasticity is an essential property of cloud computing. It helps service providers to efficiently exploit cloud resources and reduce servicing costs. Therefore, the multitenant business processes are long-running and they are concurrently accessed by dynamic requests from tenants. However, ensuring business process elasticity at the infrastructure and the platform levels may only result in significant resources waste due to the partner services autonomy and variability. For this purpose, we tackle the problem of handling elasticity at the process and services levels to scale-out and scale-in their service instances whenever possible. To do this, we propose an auto-scaling approach to hold the promise of ensuring the elasticity of the multitenant business process. This research is based on service patterns for integrating multi-tenancy architecture and making ecisions on the execution of the elasticity mechanisms. Furthermore, we encapsulate our approach into a middleware layer (Middleware as a Service) between the application layer and the platform layer in the cloud architecture. Provided experimental evaluations show that our approach is efficient for ensuring elasticity under various workloads variation of the business process in cloud computing.

A Formal Approach for the Validation of Web Service Orchestrations

Service Oriented Architecture (SOA) is an emerging computing model that aims to build new software, by assembling independent and loosely coupled services. This composition can be described from both a local or a global perspective by respective orchestration or choreography. In fact, the main focus of this paper is to validate a web service orchestration through the verification of properties. This verification must take into account both generic and specific properties. The generic properties can be checked for any invoked web services when the specific properties are different interdependence relationships between activities within an orchestration process. These properties cannot be directly verified on the process, so, we have to use formal techniques. Doing so, we will present, in this paper, our formal approach to validate a web service orchestration. The paper adopts WS-BPEL 2.0 as the language to describe the web service orchestration and uses the SPIN model-checker for the verification engine. The WS-BPEL specification is translated into Promela code which is the input language for SPIN model-checker, in order to check generic and specific properties expressed with LTL (Linear Temporal Logic). A tool named Bpel VT (BPEL Verification Tool) is developed to support the proposed approach. It provides the WS-BPEL manager, the automated process translation of WS-BPEL to Promela code and model-checking views.

Randomized broadcasting in wireless mobile sensor networks

Wireless sensor networks are a new generation of networks that need specific models and algorithms. We are interested specifically in mobile wireless sensor networks that are considered as anonymous asynchronous distributed mobile systems. As broadcast is one of the most important applications for such networks, and as it depends on the communication model, we tried to find the most suitable one to make a distributed broadcast algorithm. We adopted the population protocols, the Angluins model of pairwise interactions of anonymous finite‐state agents, to broadcast an information. We tried to modify this model to avoid the information duplication and then calculated the complexity of the algorithm. Then, we extended the model with the rendezvous one that made the stabilization of the algorithm faster. The implementation, the simulation, and the validation of these algorithms and results have been done with Visidia. Copyright

Simulation tools for cloud computing: A survey and comparative study

Today, cloud computing has become a promising paradigm that aims at delivering computing resources and services on demand. The adoption of these services has been rapidly increasing. One of the main issues in this context is how to evaluate the ability of cloud systems to provide the desired services while respecting the QoS constraints. Experimentation in a real environment is a hard problem. In fact, the financial cost and the time required are very high. Also, the experiments are not repeatable, because a number of variables that are not under control of the tester may affect experimental results. Therefore, using simulation frameworks to evaluate cloud applications is preferred. This paper presents a survey of the existing simulation tools in cloud computing. It provides also a critical and comparative analysis of the studied tools. Finally, it stands out a major challenge to be addressed for further research.

Controlling Elasticity Dependencies for Multi-tenant Business Process

One of the key properties driving the popularity of cloud computing is the elasticity of multi-tenant business process. However, performing elasticity at the business process and the SaaS (Software as a Service) levels independently may only result in significant resources waste relating to the scalability, complex logic and long-running of the cloud applications. In this paper, we propose a holistic approach to dynamically manage elasticity mechanisms based on functional dependencies between the multi-tenant process and its involved services. This research is based on a service pattern for enabling multi-level elasticity control and analyzing the elasticity impact of invoked services on quality of the affected processes. Furthermore, we define an auto-scaling algorithm for enforcing multi-criteria dependencies for the elasticity control. Evaluation of our approach on a realistic situation shows its efficiency.

An efficient validation approach for quasi-synchronous checkpointing oriented to distributed diagnosability

Quasi-synchronous checkpointing algorithms are classified into: SZPF, ZPF and ZCF.We propose a validation approach to detect the previously mentioned properties.We model an algorithm execution into the HBR graph and IDR graph.We designed two sets of validation rules to work over the HBR graph and IDR graph.A lower cost is achieved using the IDR graph which is the minimal causal graph. The autonomic computing paradigm is oriented towards enabling complex distributed systems to manage themselves, even in faulty situations. The diagnosability analysis is a priori a study through which a system can be self-aware about its current state. It is from the determination of a consistent state that a system can take some action to repair or reconfigure itself. Nevertheless, in a distributed system it is hard to determine consistent states since we cannot observe simultaneously all the local variables of different processes. In this context, the challenge is to efficiently monitor the system execution over time to capture trace information in order to determine if the system accomplishes both functional and non-functional requirements. Quasi-synchronous checkpointing is a technique that collects information from which a system can establish consistent snapshots. Based on this technique, several checkpointing algorithms have been developed. According to the checkpoint properties detected and ensured, they are classified into: Strictly Z-Path Free (SZPF), Z-Path Free (ZPF) and Z-Cycle Free (ZCF). Generally, the method adopted for the performance evaluation of checkpointing algorithms involves simulation. However, few works have been designed to validate their correctness. In this paper, we propose an efficient validation approach based on a graph transformation oriented towards the automatic detection of the previously mentioned properties. To achieve this, we took the vector clocks resulting from an algorithm execution, and we modeled them into the happened-before graph and the immediate dependency graph (which is the minimal causal graph). Then, we designed a set of transformation rules to verify if in these graphs, the algorithm is exempt from non-desirable patterns, such as Z-paths or Z-cycles, according to the case.

A Graph Transformation-Based Approach for the Validation of Checkpointing Algorithms in Distributed Systems

Autonomic Computing Systems are oriented to prevent the human intervention and to enable distributed systems to manage themselves. One of their challenges is the efficient monitoring at runtime oriented to collect information from which the system can automatically repair itself in case of failure. Quasi-Synchronous Check pointing is a well-known technique, which allows processes to recover in spite of failures. Based on this technique, several check pointing algorithms have been developed. According to the checkpoint properties detected and ensured, they are classified into: Strictly Z-Path Free (SZPF), Z-Path Free (ZPF) and Z-Cycle Free (ZCF). In the literature, the simulation has been the method adopted for the performance evaluation of check pointing algorithms. However, few works have been designed to validate their correctness. In this paper, we propose a validation approach based on graph transformation oriented to automatically detect the previous mentioned check pointing properties. To achieve this, we take the vector clocks resulting from the algorithm execution, and we model it into a causal graph. Then, we design and use transformation rules oriented to verify if in such a causal graph, the algorithm is exempt from non desirable patterns, such as Z-paths or Z-cycles, according to the case.

Towards error-handling-aware choreography to orchestration transformation approach

In this work, we are interested in the problem of service composition and particularly in the choreography to orchestration automatic transformation problem. In this context, several parameters play a crucial role, such as message exchanges and operations invocation. Indeed, during service executions, faults can arise and can fail the composition. Thus, the automatic transformation of choreographies to orchestrations must be enhanced by error-handling primitives. In this paper, we propose an automatic transformation approach of choreographies to orchestrations in which we consider, in addition to correct executions, error-handling features. To do so, we first identify a classification of faults that may occur. So, we propose an enriched choreography meta-model that integrates several aspects such as messages and errors handling. Such a meta-model defines a transformation process to generate correct executable orchestrations. The generated orchestrations are finally validated using a model checking based process to check that orchestration conforms to the choreography.

BpelVT: A Tool for Formal Validation of Web Service Orchestrations

To validate a web service composition, we have developed, in a previous work, a formal approach which takes into account both generic and specific properties. Generic properties can be checked for any invoked web services when the specific properties are different interdependence relationships between activities within an orchestration process. In this work, we present BPELVT, a tool to support the proposed approach. We adopt WS-BPEL as the language to describe the web service orchestration. This specification is translated to Promela which is the input language for SPIN model-checker, in order to check generic and specific properties expressed with LTL (Linear Temporal Logic). The BPELVT tool provides the WS-BPEL manager, the automated process translation of WS-BPEL to Promela code and model-checking views.