## 2 Related Works
### Robustness Enhancement Strategies and Recovery Measures  
The specific robustness enhancement and defense strategies vary from network to network, but from the perspective of a network structure, establishing redundant nodes or links is one of the most common methods to improve network robustness. As early as 2005, [PERSON] et al. [21] proposed that people are usually faced with the problem of improving the robustness of an existing network that cannot be substantially modified or redesigned: only minor modifications are allowed, such as adding new nodes, reconnecting partial edges, or adding new edges. [PERSON] [22] stated that the robustness of the whole network can be improved by increasing the number of reliable nodes in the network or improving the reliability of a small number of nodes in the network. [PERSON] et al. [23] argued that it is impossible to enhance the reliability of every node in the network, so it is instead necessary to specify the priority of nodes and links and focus on their protection.  
Regarding the study of reliable nodes, [PERSON] [24] proposed a heuristic scheduling rule based on controlling critical nodes for the scheduling optimization of the job shop in a disturbed environment, and [PERSON] [25] used both the improved node shrinkage method and the triangular fuzzy number method to comprehensively evaluate the importance of nodes in the network, proposing corresponding improvement measures in terms of critical node protection. Regarding the study of adding links (i.e., connected edges), [PERSON] et al. [23] proposed two new strategies: low inter degree-degree difference addition and random inter degree-degree difference addition. They verified the effectiveness of these proposed strategies through a comparison with four existing link-addition strategies (i.e., random addition, low degree addition, low betweenness addition, and algebraic connectivity-based addition). [PERSON] et al. [26] proposed a preferred connectivity strategy based on the structure of inter-dependent networks. They applied this strategy to three existing link-addition strategies (i.e., random addition, low degree addition, and low inter degree-degree difference addition), finding that each improved strategy was significantly better than the previous one in terms of robustness improvement. The effects of link-addition strategies on the improvement of the robustness of different networks have been further investigated [27; 28; 29; 30].  
Measures such as adding redundant nodes and connected edges can effectively improve network robustness. As such, these measures are receiving increasing attention from scholars, and they also provided ideas for the design of the cloud manufacturing-based robustness improvement strategy in this paper. Targeting the problem of frequent cloud resource node failure, this paper attempts to establish redundant nodes so that appropriate alternative resources [31] can be used to avoid the complete breakdown of the manufacturing process or system when the original manufacturing resource nodes fail. Based on this and the background characteristics of cloud manufacturing, this paper presents three kinds of robustness improvement strategies: internal resource replacement, external resource replacement, and internal-external integration replacement.