Each part of the Guide to the SE Body of Knowledge (SEBoK) is divided into KAs, which are groupings of information with a related theme. This KA contains the following topics: What is a System? - Types of Systems - Groupings of Systems – Complexity - Emergence
Introduction
The word system is used in many areas of human activity and at many levels. But what do systems researchers and practitioners mean when they use the word system? Is there some part of that meaning common to all applications?
The concepts of open system and closed system are explored. Open systems, described by a set of elements and relationships, are used to describe many real world phenomena. Closed systems have no interactions with their environment. Two particular aspects of systems, complexity and emergence, are described in this KA. Between them, these two concepts represent many of the challenges which drive the need for systems thinking and an appreciation of systems science in SE.
Some systems classifications, characterized by type of element or by purpose, are presented.
Within the SEBoK an engineered system is defined as encompassing combinations of technology and people in the context of natural, social, business, public or political environments, created, used and sustained for an identified purpose. The application of the Systems Approach Applied to Engineered Systems requires the ability to position problems or opportunities in the wider system containing them, to create or change a specific engineered system-of-interest, and to understand and deal with the consequences of these changes in appropriate wider systems.
The concept of a system context allows all of the system elements and relationships needed to support this to be identified.
The discussions of engineered system contexts includes the general idea of groups of systems to help deal with situations in which the elements of an engineered system are themselves independent engineered systems. To help provide a focus for the discussions of how SE is applied to real world problems, four engineered system contexts are introduced in the KA:
1. Product System (glossary) context
2. Service System (glossary) context
3. Enterprise System (glossary) context
4. System of Systems (SoS) (glossary) capability context
What is a System?
This article forms part of the Systems Fundamentals Knowledge Area (KA). It provides various perspectives on systems, including definitions, scope, and context. The basic definitions in this article are further expanded and discussed in the articles Types of Systems and What is Systems Thinking?. This article provides a guide to some of the basic concepts of systems developed by systems science and discusses how these relate to the definitions to be found in systems engineering (SE) literature. The concept of an engineered system is introduced as the system context of most relevance to SE.
A Basic Systems Science View
The most basic ideas of a system whole can be traced back to the thinking of Greek philosophers such as Aristotle and Plato. Many philosophers have considered notions of holism, that ideas, people or things must be considered in relation to the things around them to be fully understood (M’Pherson, 1974). One influential systems science definition of a system comes from general system theory (GST) "A System is a set of elements in interaction." (von Bertalanffy 1968)
The elements of a system may be conceptual organizations of ideals in symbolic form or real objects. GST considers abstract systems to contain only conceptual elements and concrete systems to contain at least two elements that are real objects, e.g. people, information, software and physical artifacts, etc. GST starts with the notion of a system boundary defined by those relationships which relate to membership of the system. The setting of a boundary and hence the identification of a system is ultimately the choice of the observer.
For closed systems all aspects of the system exist within this boundary. This idea is useful for abstract systems and for some theoretical system descriptions. The boundary of an open systems (glossary) defines those elements and relationships which can be considered part of the system and those which describe the interactions across the boundary between system elements and elements in the environment (glossary).
Open Systems
The relationships between the various elements of an open system can be related to a combination of the system's structure and behavior. The structure of a system describes a set of system elements and the allowable relationships between them. System behavior refers to the effect produced when an instance of the system interacts with its environment. An allowable configuration of the relationships between elements is referred to as a system state and the set of allowable configurations as its state space.
A system may be made up of a network of system elements and relationships at a single level of detail or scale. However, many systems evolve or are designed as hierarchies of related systems. Thus, it is often true that the elements of a system can themselves be considered as open systems. A “holon” was defined by Koestler as something which exists simultaneously a whole and as a part (Koestler 1967). Natural systems are real world phenomena to which systems thinking is applied to help better understand what those systems do and how they do it. A truly natural system would be one that can be observed and reasoned about, but over which people cannot exercise direct control, such as the solar system.
Social systems are purely human in nature, such as legislatures, conservation foundations, and the United Nations Security Council. These systems are human artifacts created to help people gain some kind of control over, or protection from, the natural world.
Engineered systems may be purely technical systems, such as bridges, electric autos, and power generators. Engineered systems which contain technical and either human or natural elements, such as water and power management, safety governance systems, dams and flood control systems, water and power safety assurance systems are often called sociotechnical systems (glossary). The behavior of such systems is determined both by the nature of the engineered elements and by their ability to integrate with or deal with the variability of the natural and social systems around them. The ultimate success of any engineered system is thus measured by its ability to contribute to the success of relevant sociotechnical system contexts.