1. Figure 30.2 Layers in NextGen
They only have three layers in this architecture
Each layer is shown as a UML Package
No separate Application Layer is shown - Register manages the UI
not all Classes are shown - the big picture is illustrated
March 2002
R McFadyen

2. Figure 30.3 Architecture drawing showing noteworthy coupling
Dependency lines depict coupling
Figure 30.4 reinforces the idea that you show the level of detail appropriate for the audience
March 2002
R McFadyen

3. Figure 30.5 An architecturally significant interaction diagram
Illustrates inter-package and inter-layer connections
shows a subsystem as an object
March 2002
R McFadyen

4. Use of Façade in POS - Fig 30.7, p 459
The Domain layer is only exposing one object (Register) to the UI - the above represents an application of Controller or Façade
As the system grows to handle many Use Cases, an Application Layer is likely
March 2002
R McFadyen

5. Figure 30.8 The system evolves ... Application Layer
Collaboration via Façade is usually from a higher level to a lower level - downward collaboration
March 2002
R McFadyen

6. Figure 30.11 Upward communication in case of Observer pattern
When the application layer needs to communicate with the presentation layer it is usually via the Observer Pattern
the lower layer objects send messages to the higher layer objects, but the coupling is not to direct classes, but rather to any class implementing an interface such as PropertyListener
March 2002
R McFadyen

7. Classic View of 3-Tier Architecture - Fig 30.14, P 470-1
3-tier architectures appeared in the 90s
its prominence was partly due to its promotion by the Gartner Group
March 2002
R McFadyen

8. Different Deployments of 3-Tier - Fig 30.15, p 471
Thick Client Thin Client
There are many ways of slicing the 3-tier architecture across one or more hardware components
March 2002
R McFadyen

9. Model-View Separation Principle - P 471-3
Concept originated with SmallTalk development (MVC)
The general concern is: what kind of visibility should other packages have to the Presentation Layer?
Model is synonymous with Domain Layer
View is synonymous with Presentation Layer
The Model-View Separation principle states that model objects should not have direct knowledge of View objects
Window classes are relatively thin; they are responsible for input/output, catching GUI events, but do not maintain data or provide application functionality
e.g. Register or Sale objects should not send messages directly to a GUI window object
(Note Observer does not involve direct coupling)
March 2002
R McFadyen

10. Model-View Separation Principle - P 471-3
How do windows obtain information to display?
Two approaches:
Polling/Pull-from-above
Push-from-below
needed when polling is too inefficient
two solutions:
Observer
Presentation Façade object
One is deciding how two layers are going to interact - this is a big decision, an architectural decision
March 2002
R McFadyen

11. Figure 30.16 Presentation Facade
March 2002
R McFadyen