What is Systems Architecture ?

• Systems Architecture is a generic discipline to handle objects (existing or to be created) called "systems", in a way that supports reasoning about the structural properties of these objects.

• Systems Architecture is a response to the conceptual and practical difficulties of the description and the design of complex systems.

On this page, you will find three sections:

• a unified definition of Systems Architecture (outlining what are systems, their architecture and the justifications for Systems Architecture)
• fundamental principles underlying Systems Architecture (in all acceptions of the term)
• socio-cognitive aspects of Systems Architecture to take into account the reality of men (cognitive limitations) and teams (social behaviors).

>> A Unified Definition

• You have said "architecture" ?

  Systems Architecture is a generic discipline to handle objects (existing or to be created) called "systems", in a way that supports reasoning about the structural properties of these objects.

  Depending on the context, Systems Architecture can in fact refer to:

  • the architecture of a system, i.e. a model to describe/analyze a system
  • architecting a system, i.e. a method to build the architecture of a system
  • a body of knowledge* for "architecting" systems while meeting business needs, i.e. a discipline to master systems design. 
    * consisting in: concepts, principles, frameworks, tools, methods, heuristics, practices

  At this point, we can only say that the "architecture of a system" is (similarly to the one of a building) a global model of this system consisting of:

  • a structure
  • properties (of various elements involved)
  • relationships (between various elements)
  • behaviors & dynamics
  • multiple views of the system (complementary and consistent).

• But what is a "complex system" ?

  Systems are typically objects designed by men, involving heterogeneous components (e.g. hardware, software, humans) working together to perform a mission. The complexity of a system will mainly come from two aspects:

  • integration of components: there are many interrelations between a possibly huge number of components, and there are recursive levels of integration
  • heterogeneity of components: several specialized fields are involved in the design of a complex system, making it difficult to keep a unified vision of this system and to manage its design.

  You can read further explanations on complex systems. We will not describe here the numerous issues raised (at every level of a company: corporate strategy, marketing, product definition, engineering, manufacturing, operations, support, maintenance, etc) by the design and management of such complex systems. But every experienced business leader, manager, consultant or engineer will perfectly know what we are talking about. These issues can be summarized as:

  • going from local to global, i.e. mastering integration and emergence
  • building an invariable architecture in a moving environment.

• And so, Systems Architecture is ... ?

  In this context, Systems Architecture is a response to the conceptual and practical difficulties of the description and the design of complex systems. Systems Architecture helps to describe consistently and design efficiently complex systems such as:

  • an industrial system (the original meaning of Systems Architecture)
  • an IT infrastructure (Enterprise Architecture)
  • an organization (Organizational Architecture)
  • a business (Business Architecture)
  • a project (Project Architecture?).

  Systems Architecture will often rely on a tool called an architecture framework, i.e. a reference model to organize the various elements of the architecture of a system into complementary and consistent predefined views allowing to cover all the scope of Systems Architecture. Famous architecture frameworks are for example DoDAF, MoDAF or AGATE.

  Finally, Systems Architecture will consider any system with a socio-technical approach (even when dealing with a "purely technical" system). In particular, during the design (or transformation) of a system, the systems in the scope of this design (or transformation) can be divided in two separated systems in interaction :

  • the product, i.e. the system being designed or transformed
  • the project, i.e. the socio-technical system* in charge of the design or transformation of the product. 
    * teams, tools, other resources and their organization following strategies & methods

>> Fundamental Principles

Whatever the type of system and the acception considered (model, method or discipline), Systems Architecture is based on 9 fundamental principles :

• "Thinking with a systemic approach"
  1. the objects of the reality are modelled as systems (i.e. a box performing a function and defined by its perimeter, inputs, outputs and an internal state)
     Ex: a mobile phone is a system which takes in input a voice & keystrokes and outputs voices & displays. Moreover, it can be on, off or in standby. Overall, the phone allows to make phone calls (among other functions).
  2. a system can be broken down into a set of smaller subsystems, which is less than the whole system (because of emergence)
     Ex: a mobile phone is in fact a screen, a keyboard, a body, a microphone, a speaker, and electronics. But the phone is the integration of all those elements and cannot be understood completely from this set of elements.
  3. a system must be considered in interaction with other systems, i.e. its environment
     Ex: a mobile phone is in interaction with users, relays (to transmit the signal), reparators (when broken), the ground (when falling), etc. All these systems constitue its environment and shall be considered during its design.
  4. a system must be considered through its whole lifecycle
     Ex: a mobile phone will be designed, prototyped, tested, approved, manufactured, distributed, selled, used, repaired, and finally recycled. All these steps are important (and not only the moment when it is used).
• "Reasoning according to an architecture paradigm"
  1. a system can be linked to another through an interface, which will model the properties of the link
     Ex: when phoning, our ear is in direct contact with the phone, and there is therefore a link between the two systems (the ear and the phone). However, there is a hidden interface : the air! The properties of the air may influence the link between the ear and the phone (imagine for example if there is a lot of noise).
  2. a system can be considered at various abstraction levels, allowing to consider only relevant properties and behaviors
     Ex: do you consider your phone as a device to make phonecalls (and other functions of modern phones), a set of material and electronics components manufactured together, or a huge set of atoms ? All these visions are realistic, but they are just at different abstraction levels, whose relevancy will depend on the context.
  3. a system can be viewed according to several layers (usually three: its sense, its functions, and its composition)
     Ex: a phone is an object whose sense is to accomplish several missions for its environment : making phone calls, being a fashionable object, offering various features of personal digital assistants, etc. But it is also a set of functions organized to accomplish these missions (displaying on the screen, transmitting signal, delivering power supply, looking for user inputs, making noise if necessary, etc). And finally, all these functions are implemented through physical components organized to perform these functions.
  4. a system can be described through interrelated models with given semantics (properties, structure, states, behaviors, datas, etc)
     Ex: from the point of view of properties, the phone is a device expected to meet requirements like "a phone must resist to falls from a height of one meter". But a phone will also change state : when a phone is off and that the power button is pressed, the phone shall turn on. Function dynamics of the phone are also relevant: when receiving a call, the screen will display the name and the speaker will buzz, but if the user presses no button the phone will stop after 30 secondes... This will typically be described with diagrams in SysML (an evolution of UML).
  5. a system can be described through different viewpointscorresponding to various actors concerned by the system.
     Ex: commercials, designers, engineers (in charge of software, electronics, acoustics, materials, etc) users, repairers... All these people will have different visions of the phone. When the designer will see the phone as an easy-to-use object centered on the user, the engineer will see it as a technological device which has to be efficient and robust. A commercial may rather see it as a product which must meet clients‘ needs and market trends to be sold. All these visions are important and define the system in multiple and complementary ways.

>> Socio-Cognitive Aspects

Systems Architecture involves multiple views (sometimes partial or conflictual) of the same system by multiple actors. These views can be understood as "projections" of the system in the spaces of those different actors:

• this is the set of all those views (themselves involving several interrelated models at different abstraction levels) which define the system. But it is in general impossible to define a system in an objective, unified and exhaustive way.
• each view is an analytical description of the system. However, complexity of systems and their architecture cannot be grasped by an analytical decomposition. Considering multiple viewpoints allows to compensate the weaknesses of analytical decomposition (which is the only one we can handle as humans), following the ideas of Edgard Morin and Jean-Louis Le Moigne.

Moreover, social and cognitive aspects of Systems Architecture are absolutely critical to carry out a successful design. Indeed, Systems Architecture is key to make individual and collective work more efficient in projects, in order to meet business needs (quality, delays, performances, costs, risk):

• for individuals, Systems Architecture is a powerful tool helping to overcome the complexity of systems and to keep a vision of their work. It allows to describe, model and design systems in a rich and diverse way, while keeping a good usability of the objects handled and improving decision making. For instance, cognitive rules as "7x7x7" (i.e. for one model, there must be at most 7 elements per level and at most 3 recursive levels) will allow people to work efficiently with their cognitive limitations.
• for teams, Systems Architecture proposes a common language to understand and be understood. It is also a strong tool & method to facilitate collaboration in projects and to create transversality between departments of a company, while allowing to make the right questions emerge in the discussion. In particular, it can help actors to create a shared vision of the system and to converge on various issues. For example, the rule "Every element of the architecture of a system must have an owner" will help teams to advance their work without losing traceability of responsabilities.

Finally, Systems Architecture is not only a model or a method to design complex systems. It is more of a discipline, allowing to consider at the same time the system and the project in charge of it, while overcoming the difficulties related to the complexities (technical, social and cognitive) of the system and its design. 
As a discipline, Systems Architecture has its own practical rules & heuristics, as much as powerful best practices coming from various fields. These points have not been addressed in the scope of this introduction, and can be found for instance in this book.

Click here to learn more about my research on Systems Architecture.

For more information on Systems Architecture as a discipline, you can check this book chapter (in French). For a quality academic introduction to Systems Architecture (centered on industrial systems), you can read this paper from MIT.

***https://www.lix.polytechnique.fr/~golden/systems_architecture.html

原文地址：https://www.cnblogs.com/feng9exe/p/10251670.html