The “4+1” View Model of Software Architecture

In November 1995, while working as Lead software architect at Hughes Aircraft Of Canada Philippe Kruchten published a paper entitled: "Architectural Blueprints—The “4+1” View Model of Software Architecture". The intent was to come up with a mechanism to separate the different aspects of a software system into different views of the system. Why? Because different stakeholders always have different interest in a software system. Some aspects of a system are relevant to the Developers; others are relevant to System administrators. The Developers want to know about things like classes; System administrators want to know about deployment, hardware and network configurations and don't care about classes. Similar points can be made for Testers, Project Managers and Customers. Kruchten thought it made sense to decompose architecture into distinct views so stakeholders could get what they wanted. In total there were 5 views in his approach but he decided to call it 4 + 1. We'll discuss why it's called 4 + 1 later! But first, let's have a look at each of the different views.

• The logical view
  This contains information about the various parts of the system. In UML the logical view is modelled using Class, Object, State machine and Interaction diagrams (e.g Sequence diagrams). It's relevance is really to developers.
  
• The process view
  This describes the concurrent processes within the system. It encompasses some non-functional requirements such as performance and
  availability. In UML, Activity diagrams - which can be used to model concurrent behaviour - are used to model the process view.
  
• The development view
  The development view focusses on software modules and subsystems. In UML, Package and Component diagrams are used to model the development view.
  
• The physical view
  The physical view describes the physical deployment of the system. For example, how many nodes are used and what is deployed on what node. Thus, the physical view concerns some non-functional requirements such as scalability and availability. In UML,
  Deployment diagrams are used to model the physical view.
  
• The use case view
  This view describes the functionality of the system from the perspective from outside world. It contains diagrams describing what the system is supposed to do from a black box perspective. This view typically contains Use Case diagrams. All other views use this view to guide them.
  

Why is it called the 4 + 1 instead of just 5?
Well this is because of the special significance the use case view has. 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 realised.

4 + 1 came before UML
It's important to remember the 4 + 1 approach was put forward two years before the first the introduction of UML which did not manifest in its first guise until 1997. UML is how most enterprise architectures are modelled 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 categorised into one of the 4 + 1 views. UML is 4 + 1 friendly!

So is it important?
The 4 + 1 approach isn't just about satisfying different stakeholders. It makes modelling easier to do because it makes it easier to organise. A typical project will contain numerous diagrams of the various types. For example, a project may contain a few hundred sequence diagrams and several class diagrams. Grouping diagrams of similar types and purpose means there is an emphasis in separating concerns. Sure isn't it just the same with Java? Grouping Java classes of similar purpose and related responsibilities into packages means organisation is better. Similarly, grouping different components into different jar files means organisation is better. Modelling tools will usually support the 4 + 1 approach and this means projects will have templates for how to split the various types of diagrams. In a company when projects follow industry standard templates again it means things are better organised.

The 4 + 1 approach also provides a way for architects to be able to prioritise modelling concerns. It is rare that a project will have enough time to model every single diagram possible for an architecture. Architects can prioritise different views. For example, for a business domain intensive project it would make sense to prioritise the logical view. In a project with high concurrency and complex timing it would make sense to ensure the process view gets ample time. Similarly, the 4 + 1 approach makes it possible for stakeholders to get the parts of the model that are relevant to them.


References:

1. Architectural Blueprints—The “4+1” View
   Model of Software Architecture Paper
   http://www.cs.ubc.ca/~gregor/teaching/papers/4+1view-architecture.pdf


2. Learning UML 2.0 by Russ Miles & Kim Hamilton. O'Reilly