Flexible job shop scheduling is an important issue in the integration of research area and real-world applications. The traditional flexible scheduling problem always assumes that the processing time of each operation is fixed value and given in advance. However, the stochastic factors in the real-world applications cannot be ignored, especially for the processing times. We proposed a hybrid cooperative co-evolution algorithm with a Markov random field (MRF)-based decomposition strategy (hCEA-MRF) for solving the stochastic flexible scheduling problem with the objective to minimize the expectation and variance of makespan. First, an improved cooperative co-evolution algorithm which is good at preserving of evolutionary information is adopted in hCEA-MRF. Second, a MRF-based decomposition strategy is designed for decomposing all decision variables based on the learned network structure and the parameters of MRF. Then, a self-adaptive parameter strategy is adopted to overcome the status where the parameters cannot be accurately estimated when facing the stochastic factors. Finally, numerical experiments demonstrate the effectiveness and efficiency of the proposed algorithm and show the superiority compared with the state-of-the-art from the literature.Mathematics 2019, 7, 318 2 of 20 optimization problem with two sub-problems, i.e., operation sequence and machine assignment, and was proved as an NP-hard problem [9].
MotivationVarious researchers concentrated on the effective algorithms for minimizing the maximum completion time which named makespan of FJSP. Lin and Gen provided a survey in 2018 and presented the information of scheduling, e.g., mathematical formulation and graph representations [10]. Chaudhry and Khan reviewed solution techniques and methods published for flexible scheduling and presented an overview of the research trend of flexible scheduling in 2016 [11]. However, most of the existing research assumes that the processing time of each operation on the corresponding machine is fixed and given in advance. In actual, the stochastic factors in the real-world applications cannot be ignored. In addition, the processing time appears to be particularly important [12]. Therefore, in this paper, we consider the stochastic FJSP (S-FJSP), and the stochastic processing time is modeled as three probability distributions, i.e., the normal distribution, the Gaussian distribution and the exponential distribution.In recent years, various research was studied for stochastic scheduling problems. As listed in Table 1, Lei proposed a simplified multi-objective genetic algorithm (MOGA) for minimizing the expected makespan of JSP [13]. Hao et al. proposed an effective multi-objective estimation of distribution algorithm (moEDA) for minimizing the expected makespan and the total tardiness of JSP [14]. Kundakc and Kulak proposed a hybrid evolutionary algorithm for minimizing the expected of JSP makespan as well [15]. Represented by the above studies, some researchers modelled the stochastic processing timess by the unifor...