4+1 view model

4+1 View Model

Designed by : Philippe Kruchten

Designed for : Describing the architecture of
software-intensive systems, based
on the use of multiple, concurrent
views

What is View Model ?

 A view model in systems engineering or software
engineering is a framework.

 It defines a coherent set of views to be used in the
construction of a system architecture or software
architecture.

 A view is a representation of a whole system from the
perspective of a related set of concerns.

 Viewpoint modeling has become an effective approach for
dealing with the inherent complexity of large distributed
systems.

Intent of 4+1 view model
 To come up with a mechanism to separate the different
aspects of a software system into different views of the
system.

 But why???? -> Different stakeholders always have different
interest in a software system.

DEVELOPERS – Aspects of Systems like classes

SYSTEM ADMINISTRATOR – Deployment, hardware and network
configuration.

 Similar points can be made for Testers, Project Managers
and Customers.

Solution came from 4+1 View Model
 The views are used to describe the system from the viewpoint
of different stakeholders, such as end-users, developers and
project managers.

 The four views of the model are:
-> Logical View
-> Development View
-> Process View
-> Physical View

 In addition selected use cases or scenarios are utilized to
illustrate the architecture serving as the 'plus one' view.

The 4+1 View model

End user Development Programmers
Logical view & software
view
managers

Scenarios

Process View Physical View

Integrator System Engineer

 Describes software architecture using five concurrent views.

Logical View
(Object-oriented Decomposition)

Viewer : End-user
Considers : Functional requirements- What the system
should provide in terms of services to its
users.

 This view shows the components (objects) of the system as
well as their interactions / relationships.

 UML diagrams – Class, State, Object, Sequence,
Communication diagrams.

Process View
(The process decomposition)

Viewer : Integrators
Considers : Non - functional requirements (concurrency,
performance, scalability)

 The process view shows the processes / workflow rules of a
system and how those processes communicate with each
other.

 UML diagrams – Activity diagram

Development View
(Subsystem decomposition)

Viewer : Programmers and Software Managers
Considers : software module organization
(Hierarchy of layers, software management,
reuse, constraints of tools)

 It gives a building block view of the system.
Eg: Packages Used, Execution Environments, Class
Libraries and Sub systems utilized.

 UML diagrams – Component, Package diagrams.

Physical Views
(Mapping the software to the Hardware)

Viewer : System Engineers
Considers : Non-functional req. regarding to underlying
hardware (Topology, Communication)

 This view shows the systems execution environment

 UML diagram – Deployment diagram.

Scenario / Use case View
(Putting it all together)

Viewer : All users of other views and Evaluators.
Considers : System consistency, validity

 Validation and illustration to show the design is complete

 It is redundant with other views.

 UML diagram – Use case diagrams.

Relationship among the views

 Concurrency is not addressed in the logical view, to achieve the
process view, we need to map classes and their objects onto tasks
and processes addressing concurrency and synchronization.

 The processes and process groups are mapped onto the
processing nodes of a physical computer network to obtain the
physical view. For each dependency between components, there
must be a corresponding link between nodes.

 The logical view is also the basis for the development view.
Normally each class corresponds to a module. Large classes may
be decomposed into packages. Collections of classes are grouped
into subsystems.

Modeling a system
 Decide which views are needed to best express the
architecture of the system and to expose the technical risks
to the project.

 For each of these views, decide the artifacts(eg. UML
Diagrams) needed to be created to capture the essential
details of that view.

 As part of process planning, decide which of the UML
diagrams require frequent reviews and which have to be
documented.

 Allow room for diagrams that are thrown away. Such
transitory diagrams are useful for exploring the implications
of decisions taken and for experimenting with changes.

Views – UML Diagrams
 A simple Monolithic Application that runs on a
single machine requires…

• Use case view Use case Diagrams

• Design view Class Diagrams
Interaction Diagrams

• Process view None

• Implementation view None

• Deployment view None

Views – UML Diagrams
 A complex, distributed system requires…
• Use case view Use case Diagrams
Activity Diagrams

• Design view Class Diagrams
State chart Diagrams

• Process view Class Diagrams

• Implementation view Component Diagrams

• Deployment view Deployment Diagrams

Why is it called the 4 + 1 instead of just 5?
 The use case view has a special significance.

 When all other views are finished, it's effectively
redundant.

 However, all other views would not be possible without it.

 It details the high levels requirements of the system.

 The other views detail how those requirements are
realized.

4+1 View model came before UML
 It's important to remember the 4 + 1 approach was put
forward two years before the first introduction of UML
which did not manifest in its first appearance until 1997.

 UML is how most enterprise architectures are modeled and
the 4 + 1 approach still plays a relevance to UML today.

 UML 2.0 has 13 different types of diagrams - each diagram
type can be categorized into one of the 4 + 1 views.

 UML is 4 + 1 friendly!

Is it important?
 It makes modeling easier.

 Better organization with better separation of concern.

 The 4 + 1 approach provides a way for architects to be able
to prioritize modeling concerns.

 The 4 + 1 approach makes it possible for stakeholders to get
the parts of the model that are relevant to them.

4+1 view model

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