2/20 - Software Architecture

Question 1

Model View Controller (MVC) Architecture

Answer 1

• A method for separating the user interface of an application from its domain logic.
• UI and domain logic have different drivers for change and rates of change.

Question 2

The Model

Answer 2

1. The model is the functional core of the application
2. Encapsulates the state of the application
3. Model is independent of the GUI
4. Active Model
5. Passive Model

Question 3

Functional core of the application

Answer 3

1. Business logic

2. Domain logic

Question 4

Business logic

Answer 4

business layer in enterprise applications

Question 5

Domain logic

Answer 5

domain-specific logic

Question 6

The State of the application

Answer 6

• Example: databases
• Invariants (e.g. a transaction in a zero-sum operation)
• If model is correct the data will continue to make sense

Question 7

Model is independent of the GUI

Answer 7

• It cannot direct references to the view or controller
• This gives it flexibility, testability and robustness
  
  • Model can be tested using automated controller and machine checkable views

Question 8

Active Model

Answer 8

• Defines a change notification mechanism that view and controller can subscribe to
• Common in desktop GUI apps

Question 9

Passive Model

Answer 9

• Model is called by view and controller to get data

* Common in web UI

Question 10

The View

Answer 10

• What data to show and how to present it based on the state of the model
• View informs the controller of user actions
• View has access to the model but should not change the model
• For web apps, view contains HTML + Javascript

Question 11

The Controller

Answer 11

• View deals with output, Controller deals with input
• Controller decides what model actions are needed, selects the next view and allows user to drive the app
• In web apps, controller will be handler for incoming HTTP requests

Question 12

Pros of MVC

Answer 12

• Allows to separate and modularize concerns. Leads to ease of development, testing and maintenance
• Separation between data layer and interface is the key

Question 13

Why is separation between data layer and interface key?

Answer 13

• View is easily replaced or expanded
• Model data changes are reflected in views
• Better scalability
• Distribution over network is greatly simplified

Question 14

Cons of MVC

Answer 14

• View and Controller depend heavily on model API
• Close coupling between view and controller
• Additional complexity

Question 15

Architectural Configurations

Answer 15

Arrangements of components and connectors to form an architecture

Question 16

Examples of Architectural Configurations

Answer 16

MVC, Pipe and Filter, Layered Systems, Event based Systems

Question 17

Example of Pipe and Filter Architecture

Answer 17

Unix Shell commands

Question 18

Components of Pipe and Filter Architecture

Answer 18

filter transform input into output

Question 19

Connectors of Pipe and Filter Architecture

Answer 19

data stream

Question 20

Other examples of Pipe and Filter Architecture

Answer 20

compilers, batch data processing

Question 21

Pros of Pipe and Filter Architecture

Answer 21

• Overall behavior is understood as a simple composition of the behaviors of the individual filters
• Support reuse. Existing filters can be hooked up to form a system
• Modular testing
• Easy maintenance and enhancement
• Supports specialized analysis (throughput or deadlock analysis)
• Supports parallel or distributed execution

Question 22

Cons of Pipe and Filter Architecture

Answer 22

• Not good for handling interactive apps
• Sometimes we need tighter coupling of filter for better performance
• Extra overhead to parse and unparsed data to an external format

Question 23

Example of a special case of pipe -filter

Answer 23

Map-Reduce

Question 24

Layered Architecture

Answer 24

Each layer provides a higher level of abstraction to the layer above

Question 25

Examples of Layered Architecture

Answer 25

Operating systems, ISO Network Protocol

Question 26

Interesting properties of Layered Architecture

Answer 26

1. Support increasing levels of abstraction during design 2. Reuse 3. Support Enhancement (add functionality)

Question 27

Reuse (interesting property of Layered Architecture)

Answer 27

Can define standard layer interfaces and interchange implementations of same interface.

Question 28

Support Enhancement (interesting property of Layered Architecture)

Answer 28

Change in one layer affects at most two layers.

Question 29

Cons of Layered Architecture

Answer 29

* May not be able to identify layers | * One layer may not want to communicate with a non-adjacent layer for performance reasons

Question 30

Event-Based Architecture

Answer 30

Independent reactive objects or processes

Question 31

Components of Event-Based Architecture

Answer 31

objects that register interest in “events” and objects that signal events

Question 32

Connectors of Event-Based Architecture

Answer 32

* Explicit method invocation to register event listener * Implicit listener invocation when event fires * Control: decentralized, de-coupling of sender and receiver