Hindawi Publishing Corporation Mathematical Problems in Engineering Volume 2013, Article ID 703414, 15 pages http://dx.doi.org/10.1155/2013/703414 Research Article A Hybrid Distributed Mutual Exclusion Algorithm for Cluster-Based Systems Moharram Challenger, 1,2 Elif Haytaoglu, 1 Gorkem Tokatli, 1 Orhan Dagdeviren, 1 and Kayhan Erciyes 3 1 International Computer Institute, Ege University, 35100 Izmir, Turkey 2 Department of Computer Engineering, Shabestar Branch, Islamic Azad University, 53815 Shabestar, Iran 3 Department of Computer Engineering, Izmir University, 35140 Izmir, Turkey Correspondence should be addressed to Moharram Challenger; m.challenger@gmail.com Received 26 April 2013; Accepted 10 June 2013 Academic Editor: Guanghui Wen Copyright © 2013 Moharram Challenger et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Distributed mutual exclusion is a fundamental problem which arises in various systems such as grid computing, mobile ad hoc networks (MANETs), and distributed databases. Reducing key metrics like message count per any critical section (CS) and delay between two CS entrances, which is known as synchronization delay, is a great challenge for this problem. Various algorithms use either permission-based or token-based protocols. Token-based algorithms ofer better communication costs and synchronization delay. Raymond’s and Suzuki-Kasami’s algorithms are well-known token-based ones. Raymond’s algorithm needs only O(log 2 ()) messages per CS and Suzuki-Kasami’s algorithm needs just one message delivery time between two CS entrances. Nevertheless, both algorithms are weak in the other metric, synchronization delay and message complexity correspondingly. In this work, a new hybrid algorithm is proposed which gains from powerful aspects of both algorithms. Raysuz’s algorithm (the proposed algorithm) uses a clustered graph and executes Suzuki-Kasami’s algorithm intraclusters and Raymond’s algorithm interclusters. Tis leads to have better message complexity than that of pure Suzuki-Kasami’s algorithm and better synchronization delay than that of pure Raymond’s algorithm, resulting in an overall efcient DMX algorithm pure algorithm. 1. Introduction Using shared resources among diferent processes is a pri- mary need in distributed systems. For this reason, distributed mutual exclusion (DMX) has drawn great attention over the years and a good number of algorithms have been proposed in this area. Tese algorithms are used in distributed systems such as mobile ad hoc networks (MANETs), sensor networks [1, 2], grids, and distributed databases. Messages sent for acquiring and releasing CS are an important measure for DMX algorithms and have a great efect on system’s overall performance. Safety, liveness, and fairness are the main requirements for any mutual exclusion algorithm. Lamport’s algorithm [3] and Ricart-Agrawala’s (RA) [4] algorithm are considered as two of the most important fair distributed mutual exclusion algorithms in the literature. Generally, DMX algorithms can be classifed into two major groups, token-based algorithms and permission-based ones. In the frst case, a node enters a CS afer receiving permission from all of the nodes in its set for the critical section. For token- based algorithms, however, processes are on a logical ring and possession of a system-wide unique token provides the right to enter a critical section. Suzuki-Kasami’s algorithm [5] and Raymond’s tree-based algorithm [6] are milestone token- based algorithms. Suzuki-Kasami’s algorithm has a low synchronization delay of one message between each two consecutive CS entrances, meanwhile its message communication number per any CS is  and that is fairly high for vast distributed sys- tems which can restrict system scalability. On the other hand, Raymond’s algorithm requires low message communication