Strong Consensus in Cloud Computing Farah Habib Chanchary and Samiul Islam Abstract – In the context of distributed system it is very important that there exists a common agreement among all participating nodes of the system. Over a long period of time, Byzantine agreement (BA) and consensus problem have been used in a fully connected network, broadcasting network and generalized connected network for achieving the goal of agreement. These agreement protocols maintain a set of rules to allow the healthy nodes of the distributed system to agree on a common value to aid the reliable execution of tasks. Recently cloud computing has emerged as a new paradigm of large-scale distributed computing. It refers to the use and access of multiple server-based computational resources via a digital network where users may access the server resources using any kind of computing devices. In order to achieve successful cloud storage management, massive data processing and resource scheduling, a cloud computing infrastructure often requires that all processing nodes of the underlying network should reach a common agreement. In this paper, we focused on the application of strong consensus problem on the cloud computing infrastructure to ensure smoother communication among all the healthy nodes in the distributed system to reach a common consensus with minimum round of message exchange. Keywords – Arbitrary Fault, Cloud Computing, Manifest Fault, Secure Transmission, Strong Consensus Protocol, Symmetric Fault. I. INTRODUCTION LOUD computing system is an Internet based system development in which large scalable computing resources are provided “as a service” over the Internet to users. In cloud system, applications are provided and managed by the cloud server and data is also stored remotely in the cloud configuration. In most of the cases, cloud computing infrastructures consist of services delivered through parallel or distributed data centers, which appear to consumers as a single point of access for their computing needs. For successful execution of users' requests a cloud infrastructure needs excellent teamwork by several clusters of computing nodes and servers those can be originally located in different places and it is expected that all nodes in a cluster exchange information with one another. It is very common that some of these components suffer from faulty symptoms and act maliciously. So it is required that a common agreement should be established among all fault-free computing nodes prior to executing some tasks. Farah Habib Chanchary is with Department of Computer Science, Najran University, Najran, Kingdom of Saudi Arabia (e-mail: chanchary@gmail.com). Samiul Islam is with Department of Computer Science, Najran University, Najran, Kingdom of Saudi Arabia (e-mail: samiul80@gmail.com). Over a long period of time, two standard protocols Byzantine Agreement (BA) problem and consensus problem have been extensively studied and used for fully connected network, broadcasting network and generalize connected network. Since no standard topology is available for cloud systems and this issue demands more careful consideration on how cloud computing resources can be managed with minimum effort. In this paper authors focuses on the application of strong consensus protocol in a proposed cloud topology so all processing nodes can reach an agreement with minimum round of message exchange. The rest of this paper is organized as follows. Section II introduces strong consensus problem and its conditions. Section III presents related works done by others. Section IV describes the cloud topology used in this study. In section V authors discussed the proposed protocol and its properties, behaviors of faulty nodes and allowable number of faulty nodes. Section VI illustrates complete process of the protocol with an example. Conclusions are presented in section VII. II. STRONG CONSENSUS PROTOCOL Pease, Shostak and Lamport first proposed and solved the BA problem in [1]. Consensus problems and BA are two closely related fundamental problems in agreement on a common value in distributed system. A variant of distributed consensus, called the strong consensus (SC) problem, was introduced by G. Neiger [2]. The SC problem assumes the followings, 1. There are n processors in the synchronous network of which at most F processors are subjected to Byzantine fault. 2. Each processor P starts with an initial value v i , that belongs to a finite set, V, of all possible values (|V| = m). 3. Each fault free processor sends the initial value v i to all other processors. 4. On receipt of the value v i , each processor exchanges the received value with other processors. 5. In addition, there is an adversary that controls up to F (n > max{mF, 3F}) of the processors and can arbitrarily deviate from the designated protocol specification. 6. After (n-1)/max{m,3}+1 rounds of message- exchange, a common value can be obtained. The protocol for the SC problem is to enable all fault-free processors to obtain a common value. After execution of the protocol, the common value obtained by the fault-free processors shall be the value that satisfies the following C International Conference on Emerging Trends in Computer and Electronics Engineering (ICETCEE'2012) March 24-25, 2012 Dubai 36