## 2 Literature Review
### Order Releasing and Scheduling Using Heuristic Based on Drum Buffer (DRB)  
[PERSON] et al. [45] explored the application of the theory of constraints (TOC), specifically the DBR technique, to improving process performance in healthcare services. Customising DBR for scheduled patient flows and introducing DBR for unscheduled patient flows, the study provided rules to control patient throughput, considering the trade-off between minimising flow times and maximising throughput volumes. Simulation experiments demonstrated the impact of TOC DBR rules on this trade-off. [PERSON] and [PERSON] [46] examined bottleneck shifitness and order release methods, revealing that downstream bottleneck shifts had adverse effects on performance while upstream shifts had minimal impact. The distance between the actual and assumed bottlenecks had negligible performance implications, offering valuable insights for DBR and similar release methods. [PERSON] and [PERSON] [47] further explored different sequencing and dispatching rules to enhance the performance of the DBR scheduling mechanism. The researchers found that prioritising the shortest bottleneck processing time during high load periods significantly improved performance. Researchers are now exploring flow shop configurations, focusing on bottleneck management and connections to the theory of constraints (TOC). The drum buffer rope (DBR) methodology has been developed to address these challenges as a production planning and control tool within this context [48]. [PERSON] et al. [49] presented a heuristic approach based on the DBR method to address the challenges of order releasing and multi-item scheduling in MTO production systems. The proposed approach aims to enhance productivity and efficiency in MTO companies by considering dynamic customer demands and capacity-constrained resources. Through experimentation on different problem types based on due date tightness and product demand, the performance of the proposed heuristic is compared with other well-known heuristic methods from the literature. The results indicate that the proposed DBR-based heuristic outperforms the competitors, particularly when an optimal buffer size is adopted, offering significant improvements in order release and scheduling effectiveness. Multi-Level Rolling Horizon Planning and Scheduling Using DBR Approach Considering Material Constraints  
The advent of Industry 4.0 concepts has led to a growing trend of digitization in industries. Consequently, traditional planning and scheduling approaches need to be revised to address the challenges posed by this new digital landscape. [PERSON] et al. [50] proposed a drum buffer rope-based heuristic algorithm (DBR-HA) for efficient planning and scheduling in mixed-model production industries aiming to implement Industry 4.0. The algorithm considered shifting bottleneck resources and maximised the use of capacity constraint resources, providing effective plans and schedules for each planning horizon. [PERSON] et al. [51] integrated TOC and ERP in production planning to improve quality and cost efficiency. Utilising the DBR method, the study synchronised production, identified bottlenecks, allocated resources effectively, and enhanced production performance. A practical case demonstrated the successful integration and implementation of TOC and ERP in production planning. [PERSON] et al. [52] addressed an imbalance problem in a shoe company's production lines by applying the TOC approach, utilising linear programming for production planning optimisation, and DBR for flow control. By considering the output gap between production lines, buffers were determined, and three solution alternatives were proposed, resulting in a substantial reduction of the imbalance issue at the shoe production lines. [PERSON] et al. [53] examined the effects of implementing DBR on the efficiency of three engineering-to-order (ETO) production lines in an aerospace manufacturing context.