Element: Relations: Topology: no constraints.

Transcription

1 The Module Viewtype The Module Viewtype Element: Elements, Relations and Properties for the Module Viewtype Simple Styles Call-and-Return Systems Decomposition Style Uses Style Generalization Style Object-Oriented Systems Layered Style Virtual Machines Relations: Module an implementation unit of software. Properties: Name, Responsibilities, implementation information is-part-of Properties: visibility depends-on Properties: constraints is-a Topology: no constraints. Properties: implementation property 1 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 2 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering Call-and-Return Systems Main Program and Subroutines Synchronous execution: A component calls another component ceases execution and waits until the other component is ready may get a return value continues execution A main program acts as the controller. One or more Subroutines perform specific functions when called by the main program. A single thread of control. The correctness of execution is dependant on the correctness of he main program and all subroutines that are called. is used when the order of computation is fixed. Common forms: Main program and subroutines Client-server systems Object-oriented systems Hierarchical layers 3 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 4 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering

2 Main Program and Subroutines Main Program and Subroutines Hierarchical decomposition Single thread of control Subroutines typically aggregated into subsystems Hierarchical reasoning Old-fashioned, popular in the 1970s to 90s: Jackson Structured Programming, (JSP!) Advantages: Simple to learn and understand. Disadvantages: Correctness depends on the subroutines that are called Tends to have different subroutines doing the same thing, spaghetti code. Does not scale easy to bigger systems. 5 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 6 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering Decomposition Style Decomposition Style Modules with focusing on is-part-of. UML: Examples of uses: Subsystem1 A Hardware-Hiding module Behavior-Hiding module B C D E Software Decision module: Application Data Type module Filter Behavior module Physical Models module Topology... No loops A module can not be part of more than one module. 7 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 8 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering

3 Uses Style Generalization Style Modules with focusing on depends-on. is useful for planning incremental development, system extensions and subsets, debugging and testing, and gauging the effects of specific changes. UML Modules with focusing on is-a. is useful for object-oriented designs, extension and evolution, local change or variation, reuse UML Subsystem1 Subsystem2 A No topology constraints B C 9 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 10 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering Object-Oriented Systems Object-Oriented Systems Advantages Special case of Main Program and Subroutines. 1-1 mapping of the problem domain makes the system easier to understand. Encapsulation Restricted access to information. This is the most important quality of O-O! Structure that follows the Problem Domain. Natural understanding of the system. Dynamic binding Actual operation to call is invoked at runtime. Inheritance Share definition of functionality. Encapsulation Drawbacks 11 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 12 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering

4 Layered Style Hierarchical layers Dividing into modules that are layers. Topology constraint: The layers are partly ordered, If A is above B, B cannot be above A. A hierarchically layered system is partitioned into layers that are put above each other. Each layer act as a virtual machine for the layers above it a client to the layers below it Communication between the layers occurs through well-defined APIs. Special useful for Large Systems that can have several layers 13 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 14 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering Hierarchical layers Example Hierarchical layers Considerations Presentation logic Business logic Middleware System software Special function General function Visibility Dependencies only within current layer and the layer below. Volatility Upper layer affected by requirement changes. Lower layers affected by environment changes. Generality More abstract model elements in lower layers Number of layers Small system: 3-4 layers. Complex system: 5-7 layers. not more than 7 ± 2 15 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 16 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering

5 Hierarchical layers UML Hierarchical layers Example <<layer>> Presentation Logic Five layers: Database Management System, DBMS Set oriented interface (ODBC, SQL,... <<layer>> Business Logic <<layer>> Middleware Library Implementation of the set oriented DB-Interface Implementation of the record oriented DB-Interface Realization of storage structures System buffer management External memory management Record oriented interface Internal record interface Segment interface File interface (UNIX, Windows,..) Device interface 17 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 18 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering Hierarchical layers Advantages Virtual Machines Support designs based on increasing levels of abstraction. Suitable design method. Support maintenance: Only interaction with closest layers. A virtual machine is an emulator that executes on top of a hardware or software platform. Gives the same services as a given hardware but using software rather than hardware. Examples: Supports reuse: Possibility to standard layer APIs. Disadvantages Java VM PC emulator on (old) MAC. Not all systems are appropriate to structure in levels. Even, if so, it might be good for performance to have closer coupling with the basic functions. e.g. the ISO s OSI had to many layers to work well in real-time applications. 19 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering 20 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering

6 Provides complete protection of system resources. Still permits sharing of resources. A virtual machine is difficult to implement. Virtual Machines Advantages Disadvantages Must follow a complicated specification. e.g. emulating Windows on a Linux machine. (worse with old MAC!) 21 (21) - SOFTWARE ARCHITECTURE The Module Viewtype - Sven Arne Andreasson - Computer Science and Engineering