## 2 Literature Review
### System-of-Systems Resilience  
Resilience, first introduced to the academic research by [PERSON] [9], is a multi-faceted capability that encompasses avoiding, absorbing, adapting to, and recovering from disruptions [3]. [PERSON] and [PERSON] [3] proposed a conceptual framework for resilience engineering that covers disruptions, system attributes, methods, and metrics. This is consistent with the recent research focuses in resilience engineering. Based on our understanding, the current resilience literature mostly concentrates on one or a few of the following aspects--developing resilience metrics, building models for calculating resilience, and developing resilience-enhanced strategies.  
A variety of resilience metrics have been developed in the literature, but there is no consistent one yet. While some studies aim for a generic metric of quantifying resilience across disciplines [10], the majority have developed resilience metrics for a particular type of system, such as infrastructure systems [11] and transportation systems [12]. In the military and defense domain, a resilient system is expected to be trusted and effective in a wide range of mission contexts, be easily adapted to many others through reconfiguration and/or replacement, and have predictable, graceful degradation of function [13]. It is often equivalent to survivability in the aerospace and military context [14]. Due to the fact that a large body of SoS research revolves around the aerospace and defense context, many scholars have developed resilience metrics for combat SoSs that are also applicable to other fields. For instance, [PERSON] et al. [15; 16] proposed a resilience metric for command and control architecture based on information entropy-based battlespace awareness, and later developed an improved metric that accounts for the repeated disruptions and performance volatility that commonly exist on a battlefield. [PERSON] et al. [17] recognized the interfaces as the most potent SoS leverage point and proposed a new resilience metric based on graph theory to compare SoS architectures. [PERSON] et al. [18] developed a bio-inspired metric based on ecological network analysis that can help balance SoS resilience and affordability. [PERSON] et al. [19] indicated that instead of using a single metric for evaluating SoS resilience, it would be more practical to use a set of System Importance Measures (SIMs) that not only assess the impact of disruptions on SoS performance but also evaluate the usefulness of the mitigation strategies. A few researchers have developed resilience metrics purely for combat SoSs. For example, [PERSON] et al. [20] proposed a combat SoS resilience index based on combat effectiveness considering the number of combat loops, combat capability, and time efficiency.  
In addition to metrics development, researchers have employed a variety of modeling methods to support resilience quantification, such as system dynamics [17], agent-based modeling [19], network theory [21], Bayesian network [22], and Petri net [23]. Regarding the resilience enhancement strategies, [PERSON] et al. [3] and [PERSON] et al. [24] indicated 14 representative resilience heuristics, namely functional redundancy, physical redundancy, reorganization, human backup, human-in-loop, predictability, complexity avoidance, context spanning, graceful degradation, drift correction, neutral state, inspectability, intent awareness, and learning/adaptation. However, the suggested design heuristics are qualitative descriptions and lack a quantitative framework to support informed decision-making. Considering the characteristics of SoS, this paper plans to develop an architecture-level reconfiguration framework to enhance SoS resilience.