Systems Engineering. Ch.4: Architectures

Chapter 4: § 1. Structuring Systems § 2. Fundamental Principles Systems Architectures § 3. Archetypes

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° ISO/IEC 42010: Systems and – Architecture can in fact refer to: Architecture Description ° The architecture of a system, i.e., a model to ° The architecture of a system is the set of fundamental describe/analyze a system concepts or properties of the system in its ° Architecting a system, i.e., a method to build the environment, embodied in its elements, relationships , architecture of a system and the principles of its design and evolution ° A body of knowledge for "architecting" systems while ° Systems architecture is a response to the conceptual meeting business needs, i.e., a discipline to master and practical difficulties of the description and the systems design design of complex systems

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° A structure ° Elements are pieces that constitute a system ° Properties (of various elements involved) ° object, module, component, partition, subsystem, … ° Relationships (between various elements) ° Architecturally significant pieces of a system: clearly identifiable and self-meaningful ° Behaviors & dynamics ° Elements and relations between them define the ° Multiple views of the system (complementary and structure of the system consistent) ° Static structure: organization of design-time elements ° Dynamic structure: organization of runtime elements

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1 . Ch.4: Systems Architectures

° Definition of internal design-time elements and their ° Definition of runtime elements and their interactions arrangement in the system ° Software elements: programs, object-oriented classes ° Information flows: A sends messages to B or packages, services, any self-contained code unit ° Tasks execution (parallel or sequential): ° Data elements: classes, entities, data files A invokes a routine on B ° Hardware elements: computers or their constituent ° Effect on data: data item A is created, updates many parts (disk, CPU), networking elements (cables, routers) times, and finally destroyed ° Arrangement: ° Hierarchy: A is built from B and C Interacting components ° Dependency: A relies on services of B ° Data relationship: data item is linked to another one

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° Externally visible behavior: what a system does from ° Architectural principle: the standpoint of an external observer Fundamental approach or intent that guides the ° Defines the functional interactions between the system and definition of architecture its environment ° Architecture requires definition of a set of principles ° Black box model ° Principles expose underlying assumptions, making ° State model: internal state changes in response to external “implicit explicit” stimuli ° Quality properties: an externally visible, nonfunctional property ° Performance, security, scalability

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1.1. The objects of the reality are modeled as systems 1. The objects of the reality are modeled as systems 2.2. A system can be broken down into a set of smaller subsystems 3.3. A system must be considered in interaction with other systems i.e. a box performing a function and defined by its 4. A system must be considered through its whole lifecycle perimeter, inputs, outputs and an internal state 5.5. A system can be linked to another through an interface, which Example: a mobile phone is a system which takes in input a voice will model the properties of the link & keystrokes and outputs voices & displays. Moreover, it can 6. A system can be considered at various abstraction levels, be on, off or in standby. Overall, the phone allows to make allowing to consider only relevant properties and behaviors phone calls (among other functions) 7. A system can be viewed according to several layers 8. A system can be described through interrelated models with given semantics 9. A system can be described through different viewpoints http://www.lix.polytechnique.fr/~golden/systems_architecture.html

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2. A system can be broken down into a set of smaller 3. A system must be considered in interaction with other subsystems systems, i.e., its environment A subsystem is less than the whole system Example: a mobile phone is in interaction with users. All contacted systems constitute its environment and shall be considered during Example: a mobile phone is in fact a screen, a keyboard, a body, a its design. 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.

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4. A system must be considered through its whole lifecycle 5. A system can be linked to another through an interface, which will model the properties of the link Example: a mobile phone will be designed, prototyped, tested, approved, manufactured, distributed, selled, used, repaired, and finally Example: when phoning, our ear is in direct contact with the phone, recycled. All these steps are important (and not only the moment and there is therefore a link between the two systems (the ear and when it is used). 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).

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6. A system can be considered at various abstraction 7. A system can be viewed according to several layers levels, allowing to consider only relevant properties Usually three: its sense, its functions, and its composition and behaviors Example: a phone accomplishes several missions for its environment : making Example: do you consider your phone as a device to make phonecalls phone calls, being a fashionable object, offering various features of (and other functions of modern phones), a set of material and personal digital assistants, etc. But it is also a set of functions organized electronics components manufactured together, or a huge set of to accomplish these missions (displaying on the screen, transmitting atoms ? All these visions are realistic, but they are just at different signal, delivering power supply, looking for user inputs, making noise if abstraction levels, whose relevancy will depend on the context. necessary, etc). And finally, all these functions are implemented through physical components organized to perform these functions.

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8. A system can be described through interrelated models 9. A system can be described through different with given semantics viewpoints properties, structure, states, behaviors, data, etc. Corresponding to various actors concerned by the system Example: phone is a device expected to meet requirements like "a phone Example: commercials, designers, engineers , users, repairers... All they have must resist to falls from a height of one meter". But a phone will also different visions of the phone. When the designer will see the phone as change state : when a phone is off and that the power button is pressed, an easy-to-use object centered on the user, the engineer will see it as a the phone shall turn on. Function dynamics of the phone are also technological device which has to be efficient and robust. A commercial relevant: when receiving a call, the screen will display the name and the may rather see it as a product which must meet clients' needs and speaker will buzz, but if the user presses no button the phone will stop market trends to be sold. All these visions are important and define the system in multiple and complementary ways. after 30 seconds...

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°° System archetypes are patterns of behavior of a system ° The basic idea of system thinking is that every action triggers a reaction – °° Circles (loops) of : similar system structure ° Reinforcing feedback (or amplifying feedback) °° Identifying a system archetype and finding the leverage accelerates the given trend of a process enables efficient changes in a system ° Balancing feedback (or stabilizing feedback) works if any °° A fundamental property of nature is that no cause can goal-state exists affect the past ° Delays in systems cause people to perceive a response to an action incorrectly ° under- or overestimation of the needed action and results http://en.wikipedia.org/wiki/System_archetype in oscillation, instability or even breakdown

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1. Balancing process with delay 2. Limits to growth Explains the system in which the response to action is delayed The unprecedented growth is produced by a reinforcing feedback process until the system reaches its peak

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3. Shifting the burden 4. Eroding goals The problem is handled by a simple solution with immediate A kind of shifting the burden effect, thereby "healing the symptoms" archetype. As current problems need to be handled immediately, the long-term goals continuously decline

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5. Escalation 6. Success to successful Both players suppose that just one of them can win Two activities need the same limited They are responding to actions of the other player in order to resources “defend themselves”. The aggression grows and can result As one of them becomes more in self-destructive behavior successful, more resources are assigned it. The second one becomes less and less successful due to lacking resources, and “prove the right decision” to support the first one

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7. 8. Agents use common limited The problem is solved by some fix (a resource to profit individually specific solution) with immediate As the use of the resource is not positive effect controlled, the agents would like to continuously raise their The “side effects” of this solution turn benefits out in the future. The best remedy seems to apply the same solution The resource is therefore used more and more and the revenues of the agents are decreasing

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9. Growth 1. Supply chain (The Beer Game) and underinvestment 2. Dieting; Learning foreign languages 3. Drug addiction; Paying debts by borrowing The limit to growth is 4. Balancing the public debt; Sliding limits of environmental the current production capacity. pollution It can be removed by sufficient 5. Arms race investment in new capacities. 6. Two products of one company; Work vs. family If the investment is not aggressive enough (or it is too low), the capacities are 7. Fish stocks (The Fishing Game) overloaded, the quality of services declines and 8. Saving costs on maintenance; Paying by other loans the demand decreases (with other ) 9. Small, but growing company

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