1. System Design ECE 417/617: Elements of Software Engineering
Stan Birchfield
Clemson University

2. Decomposition ... ... ... System Subsystem1 SubsystemN Class1a Class1n
ClassNa
ClassNn

3. Coupling and cohesion Coupling -- # of dependencies between subsystems
Cohesion -- # of dependencies within subsystem
Goal: low coupling, high cohesion
7 +/- 2 rule – keep number of concepts at any given layer of abstraction bounded

4. Layers and Partitions Layer – group of related subsystems
Layer knows about layers below it, but not layers above it
Top layer: no one else knows about it
Closed architecture – can only access layer immediately below
Open architecture – can access any layer below
Partition – peer subsystems, each with different responsibilities

5. Hierarchical decomposition
Application
Presentation
Session
Transport
Network
DataLink
Physical
Socket
CORBA
TCP/IP
Object
Ethernet
Wire
Frame
Packet
Bit
Connection
Format
Message
Level of abstraction
Example: Open Systems Interconnection (OSI)

6. Architectural Styles
Repository – subsystems interact through single repository
Model / View / Controller
Client / Server
Three-tier
Four-tier
Pipe and filter

7. Model / View / Controller (MVC)
Model subsystems maintain domain knowledge
View subsystems display it to the user
Controller subsystems manage sequence of interactions with user
M doesn’t depend upon V or C
Changes propagated via subscribe/notify protocol, using Observer design pattern
Well-suited for interactive systems

8. MVC Details Controller Model subscriber notifier initiator *
repository 1
View

9. MVC Example

10. MVC Example Details 2:enterNewFileName(file,newName)
3:setName(newName)
:Controller
:Model
1:subscribeToFileEvents(file)
5:getName()
:InfoView
4:notifySubscribedViews(file)
4:notifySubscribedViews(file)
:FolderView
1:subscribeToFileEvents(file)
5:getName()