## 1 Introduction  
In the era of the Fourth Industrial Revolution, many countries aspire to embrace the digital transformation of manufacturing or production plants. However, many Small and Medium Industries (SMI) or Small and Medium Enterprises (SME) are not ready for the industrial revolution, especially in Malaysia [1]. In recent years, there are many Industry 4.0-related programs and incentives offered by the government, such as the Readiness Assessment, Intervention Fund, Domestic Investment Fund (DIFS), and Automation Capital Allowance (Automation CA), with the aim of assisting the SMIs and SMEs to work towards Industry 4.0 [2]. Industry 4.0 growth is inevitable and may impact the traditional business manufacturing industry that is labor-intensive and cost-sensitive [3]. In fact, market demand is very dependent on the economic conditions, the rise of new markets, and product diversity [4]. The integration of planning and scheduling not only satisfies customers' requirements but also optimizes the resources and achieves on-time delivery. Insufficient modal and lack of advanced technology are some of the main reasons that many old players in the industry resist the digital transformation [5]. First of all, when SMEs begin the transition to Industry 4.0, they refuse to start from scratch, and secondly, SMEs tend to become overwhelmed by the multitude of technologies that are currently available on the market [6]. Ultimately, most of the processes in the manufacturing industry are similar as in Figure 1.  
Figure 1 shows a simple process of production. It shows that proper production planning is important because each step of the process affects the others. In conjunction with this, the main goal of production planning is to achieve high customer satisfaction with a low cost [8]. Production planning can be divided into the product structure, material requirement planning, and uncertainty management [9]. As the customer demand is increasing, the manufacturers realize that most of them are facing the same problem in the production stage. This resulted in the emergence of modern production planning tools and methods in the late 19 th century. In the past, traditional manufacturing systems focused on reducing cost and improving efficiency. However, they have not been able to follow the current trends, such as agile manufacturing, networked manufacture, mass customization, intelligent manufacture, and et cetera [10]. In fear of being eliminated by the competitors, a lot of SMIs and SMEs were forced to transform their business model. The transformation allowed them to cater to fluid customer demand. However, their demands may change often and dramatically in some cases [11]. Resource limitation is one of the common challenges. Hence, scheduling is essential to handle the allocation of resources based on the job sequence [12]. The output of the schedules is a production plan to estimate the start and end times for every job and machine. The jobs are planned according to certain optimization criteria while the scheduling performance is evaluated based on performance indicators such as costs, resource utilization, and adherence to deadlines. To strengthen the competitiveness of Malaysia as a production location, the manufacturing industry has to respond quickly and more flexibly according to the business models. The objectives of production planning are to reduce inventory levels, increase machine utilization, and improve customer responsiveness [13]. Based on these objectives, many manufacturers are willing to provide customization services to improve customers' satisfaction [14]. For example, Make-To-Order (MTO) is a business model that refers to a just-in-time philosophy due to the zero-inventory level between the stage of production and distribution [15]. This is to cater to personal customization, as shown in Figure 2.  
Due to the different requirements of every customer, production has to produce products according to the needs of the customer.