# Resilience in the Cyberworld: Definitions, Features and Models
## 2 Definitions
In summary, a partial failure of a system should not have any effect on an individual user, so the system can be assumed to be highly reliable. Should nevertheless a partial failure or a defect occur, the best possible continuation of the services provided should be guaranteed. In extreme cases, this continuation can also be achieved by performing graceful degradation of services. The approach of continuing a service of a system even under transient effects, permanent load or failures is also described in [11]:  
_\"A resilient system keeps processing transactions, even when there are transient impulses, persistent stresses, or component failures disrupting normal processing. This is what most people mean when they just say stability. It's not just that your individual servers or applications stay up and running but rather that the user can still get work done\"_. According to [PERSON] [11], a system must remain stable in case of tensions or stress situations or failures. As a consequence, involved (sub-) systems, or possibly also users, can still continue their work. The system must also be able to continue fulfilling at least its rudimentary functions despite any restrictions that may occur. The scope of these rudimentary functions may have been defined as part of the Risk Management, for example. Risk management also shows at what level of functional loss the entire system function according to its specifications can no longer be provided. The paper by [PERSON] [13] proposes two definitions of resilience. The first definition is as follows:  
_\"The persistence of service delivery that can justifiably be trusted, when facing changes\"_. According to [PERSON], this definition corresponds in principle to the original definition of reliability. In a second definition, [PERSON] offers an alternative which provides a more detailed description:  
_\"The persistence of the avoidance of failures that are unacceptably frequent to severe, when facing changes\"_. In [13], resilience is described as the persistence of service delivery when changes occur that have system-wide effects. These changes can be functional, environmental or technological. In addition, changes can either be planned (for example: initialized by an update), the timing of their occurrence can be unpredictable, or they can be completely unexpected. The duration of changes is also taken into account: short-term, medium-term, long-term. This refers to the duration of the impact of the change on the system or a subsystem. In the collection of papers from 1985 [5], robustness was already mentioned in connection with resilience. About 30 years later, in [19], this connection is concretized. Resilience is defined as the trustworthiness of a software system to adapt to adverse conditions. The software system should accept and tolerate the consequences of failures, attacks and changes inside and outside the system boundaries. This is defined as an approach for robustness:_\"Software resilience refers to the robustness of the software infrastructure and may be defined as the trustworthiness of a software system to adapt itself so as to absorb and tolerate the consequences of failures, attacks, and changes within and without the system boundaries\"._  
The definition of resilience was further specified in [19]. [PERSON] [19] refers to the definition already mentioned in [3] and another definition in [44]. This definition states that resilience can be characterized as a measure of the persistence of both functional and non-functional features of a system under certain and unpredictable disturbances. After analyzing these two definitions, according to [PERSON], resilience is the ability to act and balance between two main behaviors:  
1. Continuous readjustment with the aim of improving the fit of the systems' environment, and compensating for both foreseeable and unforeseeable changes in the system environment. 2. Ensure that the said changes and adjustments from 1) do not affect the identity of the system.