Encapsulation
of system structure and behaviour
Copyright 2017 Graham Berrisford.
One of about 300 papers at http://avancier.website. Last updated
21/02/2017 17:14
“Systems concepts include: system-environment boundary, input, output, process, state….” Principia Cybernetica
This table arranges these concepts in a generic structure.
|
General System Theory |
|
A bounded structure of |
|
actors/organs interact by playing |
|
roles in |
|
processes to meet |
|
goals, by maintaining |
|
system state and exchanging |
|
inputs/outputs with each other and with |
|
entities outside the boundary, using |
|
system
resources. |
The environment outside a system’s boundary contains structures/entities and behaviors/events that can change the state of the system, or be changed by the system.
“Every
living organism is essentially an open system. It maintains itself in a
continuous inflow and outflow…” Bertalanffy
TOGAF regards
a business as a system that maintains itself in a continuous inflow from
suppliers and outflow to customers.
There are usually feedback loops between a system and its environment.
To encapsulate a
system means defining its input-process-output (IPO) boundary.
The inputs and outputs can be flows of information,
material or energy.
TOGAF defines a business and each application/technology component it uses by the I/O services it offers.
“Any
suggestion that we should study "all" the facts is unrealistic, and
actually the attempt is never made.
What is
necessary is that we should pick out and study the facts that are relevant to
some main interest that is already given.” Ashby
A system
describer defines the inputs and outputs with reference to an already-given interest or aim.
In
thermodynamics, the inputs and outputs are matter and energy.
An open
system exchanges energy and matter with its environment.
A closed
system exchanges energy, but not matter, with its environment.
In
cybernetics, the primary inputs and outputs are materials and information.
“In this discussion, questions of energy play almost no part; the
energy is simply taken for granted.” Ashby
In social,
business and software systems, the primary inputs and outputs are information
or data flows.
“EA is about “information intensive
organisations”, ArchiMate 2.1
“closely connected with system theory is… communication.
The general notion in communication theory is that of information.
A second central concept of the theory of communication and control is that of feedback.” Bertalanffy
SIPOC is an acronym
that captures GST ideas used in business systems analysis, design, process and
quality improvement.
The
conventional design process defines the system-environment boundary thus:
-1- Define Customers - entities in the environment that
need outputs to meet their goals
-2- Design Outputs - that customers
need from the system
-3- Design Inputs – that the
system needs to produce the outputs
-4- Define Suppliers - entities in the environment that
will supply the inputs
-5- Design Processes - to produce the outputs from the
inputs.
To which one
might reasonably add:
-6- Define Roles - in which actors can perform the
process steps
-7- Hire and/or make Actors - to play the roles
-8- Organise, deploy, motivate and manage the actors – to
perform the processes.
Of course, the process is iterative in practice.
Systems and processes can be, and are, composed and decomposed.
Decomposition continues until individual actors can be hired or made to perform the required behaviors.
At every level of composition, a describer defines cross-boundary input/output flows.
If you expand the system-environment boundary then external events become internal events that pass between subsystems.
If you contract the system-environment boundary
then internal events become external events, crossing that boundary.
The system-environment boundary is a choice made by system describer(s).
It meaningless to point to an entity (say IBM) and call it a system.
Because it isn’t a system until those in the discussion agree on its boundary and which of its properties are relevant to some interest already given.
“Every
living organism is essentially an open system. It maintains itself in a
continuous inflow and outflow…” Bertalanffy
TOGAF regards
a business as a system that maintains itself in a continuous inflow from
suppliers and outflow to customers.
|
General System Theory |
In EA/TOGAF terms |
|
A bounded structure
of |
A bounded organisation
of |
|
actors/organs interact
by playing |
actors/components
interact by playing |
|
roles in |
roles in |
|
behaviors
to meet |
processes to
meet |
|
goals, by
maintaining |
goals/objectives/requirements
by maintaining |
|
system state
and exchanging |
data entities
and providing |
|
inputs/outputs
with each other and with |
input/output services
to each other and to |
|
entities outside
the boundary, using |
entities outside the boundary, using |
|
system resources. |
platform technology components. |
EA is concerned with coupling not only of the enterprise system to its environment, but of subsystems to each other.
It takes the holistic view; it looks to integrate subsystems to the benefit of the wider business system.
Enterprise architects encapsulate whole systems and internal actors/components - to describe their roles at an abstract level.
Encapsulated actors/components interact with each other and with external entities via input and output feedback loops.
Each system/actor/component can be seen as an input-process-output mechanism.
It receives messages about the state or needs of entities in its environment
It processes information in the light of the current state of the business and/or environment.
It sends messages to help actors produce desired effects or outcomes.
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