## 2 Literature Review
### SC Network Resilience and SC Capabilities  
SC network resilience can be represented with two characteristics that are often shaped by the interconnected suppliers' capabilities: SC network complexity (i.e., number of suppliers and their distances within a supply chain) and node criticality (the degree of dependence by other suppliers) collectively affect supply network rerouting and communication times in a disruption [48]. These characteristics and their degrees of complexity and criticality are significantly shaped by four SC capabilities: flexibility, agility, efficiency, and alertness. Flexibility is defined as the ease of altering the number and range of possible alternatives (i.e., number of possible alternatives and the degree of differences between alternatives) in the SC to cope with a variety of market changes and events while delivering acceptable performance [20]. Specifically, [PERSON] et al. [42] defined flexibility as the SC's ability to modify or adapt without trade-offs such as increased costs and lead times. Similarly, flexibility has been often described as the number of stable states a supply could readily take in response to a number of changes that might arise [20; 45; 49]. From another perspective, how a supplier manages disruption and responds to uncertain demands are core capabilities of flexibility [50]. From the buyer's perspective, [PERSON] et al. [44] emphasized the pertinence of obtaining flexibility through backup suppliers, production systems and distribution channels [19; 51; 52]. Lastly, [PERSON] et al. [53] distinguished four main properties of SC flexibility: sourcing flexibility, product flexibility, process flexibility, and transportation flexibility [8; 54; 55; 56; 57]. Increasing network complexity not only enhances vulnerability, but also reduces SC agility [37]. Agility is defined as the ability to respond to changes in a timely way by adapting SC processes [41]. Several studies have interlinked agility and flexibility. For example, [PERSON] et al. [58] defined agility as a higher-level SC capability (compared with flexibility) consisting of visibility (i.e., communication and information sharing), velocity (i.e., acceleration and responsiveness), and redesign (i.e., supply chain redesign) elements. [PERSON] et al. (2016) described agility as a function of flexibility and noted that \"agility tends to be used at a more encompassing, business wide level, with a focus on satisfying demand while flexibility tends to be used at a lower, more operational level\" [59]. However, previous articles stated that while an agile supply chain had to be flexible, a flexible supply chain did not necessarily guarantee agility [44; 60]. Thus, flexibility and agility must be treated as two distinct capabilities. Specifically, several scholars have considered response speed as inherent to SC agility [61]. [PERSON] et al. [62] contended that agile firms had to be able to respond to actual events or disruptions in a timely manner, and [PERSON] and [PERSON] [63] stated that SC agility was a critical capability for the global sourcing process. As an example of agility practice, agile firms must carry safety stocks to buffer against uncertain events to reduce the probabilities of stock outs and lost sales [44]. Efficiency refers to lean suppliers containing little or no excess inventory which then enable utilization of extra capacity in the post-disruptive phase [42]. Efficiency is often viewed alongside robustness, as robustness requires redundancy. However, during the response and recovery phases of disruptions, efficiency plays a key role in minimizing the overall financial loss and complementing other SC capabilities that contribute to resilience. The balanced level of inventories with efficiency has been identified in several studies of SC capabilities. For example, balancing redundant resources with the level of efficiency is referred to as a prerequisite of resilience [45]. Moreover, flexibility attributes have needed to be efficient to ensure an effective response to disruptions [19].