March R McFadyen1 Architecture Architecture involves the set of significant decisions about the organization of a software system, decisions concerning: its structural elements and their interfaces its behaviour as specified in the collaborations its composition into progressively larger subsystems

March R McFadyen2 Layers Pattern The Layers Pattern organizes the large-scale logical/conceptual structure of a system into discrete layers with distinct, but related responsibilities. Each layer exhibits high cohesion. Lower layers are low-level providing general services; higher levels are more application specific. Collaboration and coupling is from higher to lower layers Layers defines an N-tier model Page 450-1

March R McFadyen3 Layers Pattern Benefits/goals: strong cohesion within layers clean/clear definition of interface between layers promotes re-usability, replaceability

March R McFadyen4 Typical Layers in an Information System - Fig 30.1, P. 451 Layers is a pattern where the structure of a system is organized into discrete layers where each layer is highly cohesive, and where higher layers are more application specific, lower layers are more low-level and service oriented.

March R McFadyen5 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 R McFadyen6 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 R McFadyen7 Figure 30.5 An architecturally significant interaction diagram Illustrates inter- package and inter-layer connections shows a subsystem as an object

March R McFadyen8 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 Use of Façade in POS - Fig 30.7, p 459

March R McFadyen9 Figure 30.8 The system evolves... Application Layer Collaboration via Façade is usually from a higher level to a lower level - downward collaboration

March R McFadyen10 Figure 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 R McFadyen11 Classic View of 3-Tier Architecture - Fig 30.14, P tier architectures appeared in the 90s its prominence was partly due to its promotion by the Gartner Group

March R McFadyen12 Different Deployments of 3-Tier - Fig 30.15, p 471 Thick ClientThin Client There are many ways of slicing the 3-tier architecture across one or more hardware components

March R McFadyen13 Model-View Separation Principle - P 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 R McFadyen14 Model-View Separation Principle - P 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 R McFadyen15 Figure Presentation Facade