4+1 View Model of Software Architecture

1. 4+1 View Model of Software Architecture Architectural Blueprints – The “4+1” View Model of Software Architecture Paper Published in IEEE Software 12 (6), pp. 42-50(November 1995)AuthorPhilippe KruchtenRational Software Corporation

2. About Kruchten and his paper • Philippe Kruchten • Have more than 16-years experience as the leader of Rational Unified Process (RUP) development team in Rational Corporation (now owned by IBM). • Have valuable experiences in industry (Telecom, Air traffic control system) which he used them for confirmation of his model. • Paper Citations • More than 1822 citations according to ACM portal site

3. Abstract This article presents a model for describing the architecture of software-intensive systems, based on the use of multiple, concurrent views. This use of multiple views allows to address separately the concerns of the various ‘stakeholders’ of the architecture: end-user, developers, systems engineers, project managers, etc., and to handle separately the functional and non functional requirements. Each of the five views is described, together with a notation to capture it. The views are designed using an architecture-centered, scenario driven, iterative development process.

4. General Concept of Software Architecture • Software Architecture “deals” with the • design and • implementation of the high level structure of software. • Software Architecture is the “result” of • assembling a number of architectural elements • in some well chosen form to satisfy major functionality as well as non-functional requirements such as reliability, scalability, portability, and availability Software architecture deals with abstraction, with decomposition and composition, with style and esthetics. To describe a software architecture, we use a model composed of multiple views or perspectives

5. 4+1 View Model of Architecture Philippe Kruchten Proposed a Model which is Composed of 5-Views or Architectures Logical view Development view End user Programmers & software managers Scenarios Process View Physical View System Engineer Integrator The description of an architecture can be organized around these four views, and then illustrated by a few selected use cases, or scenarios which become a fifth view. The architecture is in fact partially evolved from these scenarios

6. General Concept of Software Architecture Perry & Wolfe Software Architecture = {Elements, Form, Rationale/Constraints} We apply Perry & Wolf’s equation independently on each view, i.e., for each view we define the set of elements to use (components, containers, and connectors), we capture the forms and patterns that work, and we capture the rationale and constraints, connecting the architecture to some of the requirements. Each view is described by a blueprint using its own particular notation. For each view also, the architects can pick a certain architectural style, hence allowing the coexistence of multiple styles in one system.

7. In describing each of 5 Views, Kruchten uses a common methodology • Notations that will be used for the view are defined: • Elements to express: components, containers, connectors • Form to express: “pattern” that worked • Rationale/Constraints to express: some requirements to be satisfied • For each “view” also showed: • An example of blueprint, with the view’s notation • The architectural styles that may be utilized for that view

8. Logical View (Viewer = End-user) • Logical view/architecture primarily describes the services provided to the users. The system is decomposed into key abstractions (Classes) that describe the functional requirements. • May include the identification of common components across the system. • Uses OO Approach & Class Diagrams, which shows • Classes and • Relationships (inheritance, composition, usage, etc.) among Classes • Alternative to OO, may use E-R diagrams for data driven systems

9. Logical View • Notationused for the OO approach of logical view is derived from Booch and include: • Components: Class, Class Utility, Parameterized Class, Class Category • Connectors: Association, Containment & Aggregation, Usage, Inheritance, Instantiation • Style: • Object Oriented Style • A Logical Blueprint • Some pictorial representation, using Booch Notation, of the logical view of a design. class usage inheritance

10. Logical View Example

11. Process View (Viewer = Integrator) • Process view/architecture primarily describes the non-functional aspectsof the requirementsand execution controlsuch as performance or integrity and addresses issues such as concurrency or distribution • Describes the thread of control of an operation of how a process (i.e. set of tasks) is executed. • Process is a set of tasks that form an executable unit which can be started, controlled, terminated, etc. • Task is a thread of control that can be individually scheduled on one processing node • Tasks also communicate via some well defined mechanism • Process loads and flow of messages can be estimated via studying a blueprint of process view.

12. Process View • Notation used for process view is derived from Booch’s proposal for Ada tasking and includes: • Components: Process, Simplified Process, Periodic Process adornment • Connectors: Unspecified, Message (unidirectional and bidirectional), Remote Procedure Call, Event Broadcast • Style: (several may be used) • Pipe and filter, client server • Process Blueprint: • Some diagram, using the defined notation, to depict the flow of messages from process to process. message message process simplified process RPC

13. Process View Example

14. Development View (Viewer = Programmers & Software Managers) • Development view/architecture focuses on the software module organization as they are packaged into small units of subsystems or libraries that can be developed by a small unit of developers. • Subsystems are organized in a hierarchy of layers • The rules that govern the development architecture may include: partitioning, grouping, visibility • Takes into account of the “internal requirements” related to development, project management, re-use, toolset constraints, development platform and language constraints, etc.

15. Development View • Notationused for development view is again derived from Booch: • Components: Module, Subsystem, Layer • Connectors: References, Compilation Dependency • Style: • Layered (4 to 6 layers) • Development Blueprint: • Some diagram depicting the layers of a software system that matches the logical view of the same system. • Contained within each layer are several subsystems, which in turn may contain several modules. layer subsystem module reference

16. Physical View (Viewer = System Engineer) • Physical view/architecture focuses on the non-functional requirements of the “hardware” systemon top of which the software resides. • The various elements identified are parts of the physical configuration needed to run the software • These may includenodes, processors, devices, lines, etc. • The configurations may be for development purpose, for testing purpose, deployment purpose, etc.

17. Physical View • Notationsused are pretty much box and line: • Components: processors, devices • Connectors: communication lines, high bandwidth bus • Style: • There is no specific style except possibly hierarchical • Physical Blueprint: • Diagram depicting the process architecture mapped onto the physical architecture. comm. line processor high bandwidth communications device

18. Physical View Example

19. Physical View Example (cont.)

20. Comments on Views • It is good to realize that architectural design is a result of considering many different perspectives. Clearly, we need to somehow combine these differing views into one cohesive architecture ---- or do we? • Also, do we always need the logical, process, development, and physical views? • Do we need more? ---- how about Data View? • Can we do with less?

21. Use Scenarios to Converge and Test Views • Come up with scenarios of “most important” or “most frequently” encountered use cases and run through the different views. • Note that Logical view describes the “components”, and Process view describes the “execution” control and synchronization. So use the scenarios to converge (map) the Logical and Process Views together so that we can: • Discover if we defined all the needed “components” • Discover if the components can interact in the expected manner Thus the “major” scenarios form the 5th View or the “4+1” Views!

22. Scenario Example

23. Mapping the Views is Hard • Do all the views have to be carried to the same level of depth before we can map them together? • How do we map the views? • Logical View to Process view • Basically this is looking at the “objects” or “components” and asking how the execution path will look like. • So we need to know the characteristics of these objects or components: Autonomy; Persistence; Subordination; Distribution • We need to know the execution thread of control in the process view: inside-out locus of control; outside in locus of control • Logical View to Development View • Looking at implementing a “class” as a module and packaging modules together. Should the modules be implemented and packaged in a specific pattern such as “layered”? • Process View to Physical View • Looking at the processing or execution control, we may choose to deploy different processes on different hardware nodes.

24. Iterative Approach to Designing an Architecture • The phases of architecting (sketching, organizing, specifying, and Optimizing) should not be a linear, single pass approach --- a more iterative “scenario driven approach” should be taken: • Start with a number of main scenarios and run them through the “strawman” architecture. • The architectural elements discovered through this scenario driven approach is documented via logical, process, development, and physical views. • Use more scenarios toiterate through the views and capture the architecture (including the rationale part) until satisfied ----

25. The Architecture must be Documented • Architectural Document should include: • Architectural goals & constraints • Software Architectural Views: • Logical Views (representing user functionalities) • Process views (representing system execution ) • Development Views (representing implementation breakdown ) • Physical Views (representing deployment/hardware assignments) • Data view (representing the key files and tables) • Scenarios • Rationale • Software Architecture (combined all the views and rationale) = {elements, forms, rationale} The author includes more such as: change history, scope, references, performance & size, quality, etc.

26. Further Comments • Addressing multiple views make sense. • Seems that Data View should be included as one of the “required” and Development View may be an “option”. • The hardest part of moving forward with multiple views is “how do you combine” them: • The views must be at the same level of depth • The views must not have any conflicting information • The views must be clearly expressed in languages that allows “combination” into a software architectural representation language.

27. Final Remarks • Methodology successfully used in the industry • Air Traffic Control • Telecom • Model is comprehensive as all other views are reducible to one of the 4 views • In this paper there are no tools to integrate views. So there is an inconsistency problem in this model which is more tangible in the maintenance of the architecture.