L5 Software Architecture

Question 1

What is the software architecture of a system?

Answer 1

Is a set of structures needed to reason about the system, which comprise software elements, relations amongst and properties of both

Software elements may be software sub-system or components

Question 2

What is the implications of “architecture is a set of structures”?

Answer 2

A structure is a simple set of elements held together by relations

Architecture consists of structures and structures consist of elements and relations

Architecture purposely omits certain information that is not useful for reasoning about the system

Question 3

What are the different levels of abstraction in a software design process

Answer 3

• Architectural Design
• Interface Design
• Component Design
• Data Structure Design
• Algorithm design

Question 4

What does it mean by Architectural Design level of abstraction?

Answer 4

The sub-systems making up the system and their relationships are identified and documented. Model of control is identified.

Question 5

What does it mean by Interface Design level of abstraction?

Answer 5

For each sub-system, its interface with other sub-systems is designed and documented.

Question 6

What does it mean by Component Design level of abstraction?

Answer 6

Services (i.e. functional requirements) are allocated to different components and the interfaces of these components are designed.

Question 7

What does it mean by Data structure design?

Answer 7

The data structures used in the system implementation are designed in detail

Question 8

What does it mean by Algorithm design?

Answer 8

The algorithms used to provide services are designed in detail and specified.

Question 9

What are the steps in architectural design?

Answer 9

1. System structuring - the system is structured into a number of principal sub-systems or components and the communication between sub-systems is identified.
2. Control Modelliing - a general model of control relationships between the parts of the system is established.
3. Modular decomposition - each sub-system is decomposed into modules

Question 10

What are the ways to describe architecture?

Answer 10

1. Formal notations
2. Architecture Description Languages
3. Semi Formal
4. Box and Line notation

Question 11

What are the 4 steps to identify a sub-systems & their interfaces

Answer 11

1. Identify MAJOR functions the system is required to provide (From textual requirements and use cases)
2. Associate the major system functions with sub-systems by grouping together logically similar functions
3. Sub-systems may be decomposed further using the same principle
4. Specify sub-system interfaces

Question 12

Architectural representation: box and line diagram

Answer 12

● At its most abstract level, an architectural design may be depicted as a block diagram (using box and line diagram):
○ The block diagram comprises the system’s major sub-systems, their components and relations
○ Sub-systems are often identified by clustering logical functionality (e.g. UI, computation, management, storage, etc.)
○ Boxes within boxes indicate that the sub-system itself has been decomposed to components (we often keep decomposition to two levels)
○ Links represent function calls, data or control signals that are passed from component to component in the direction of the arrow

Question 13

Identifying sub-systems: generic example
Partitioning requirements into sub-systems

Answer 13

Question 14

Alarm System example

● Consider an intruder alarm system that activates a siren and notifies the local control centre when its sensors are tripped. The alarm system has:
○ Movement (motion), door and window sensors
○ A video system that is activated when potential intruders are detected. The date, time, property address and video of potential intruders is relayed to an external control centre
○ A messaging system that sends a text message together with a preconfigured number of still images of potential intruders to the home owner

● Task
○ Identify possible sub-systems for the alarm system and their interaction. Hint: Group together related functionality to form sub-systems

Answer 14

Alarm system: identifying sub-systems
I. Read through the system description to establish what the system is required to do OR what the system does

II. List the system functions (sensing intruders, activating siren, notifying local control, activating video, messaging home owner)

III. Group related functionality (sensor, siren, video relay, video, message)

IV. Associate functionality of sub-systems

V. For each sub-system establish if there is need to partition further

Question 15

Alarm system: identifying sub-systems

Answer 15

Question 16

Alarm system: identifying sub-system interaction (adding “glue” component, i.e. “Controller”)

Answer 16

Question 17

Alarm system: label calls & control flows

Answer 17

Question 18

Specifying sub-system interfaces
- The description of a sub-system or component interface should include the following elements:

Answer 18

○ Interface name: a unique identifier for the interface (e.g. ISensor)
○ Operations: the name of each operation, together with input parameters and output/return, e.g. motionDetected(:int)
■ Operation name: motionDetected
■ Input: None
■ Output/return: int (indicating location of sensor or sensor number)

○ Exceptions: the name and data context for operation exceptions (i.e. what causes the exception, e.g. invalid input parameter)
○ Service quality (optional): non-functional properties associated with the service provided at the interface

Question 19

Specifying sub-system interfaces
Continued

Answer 19

• An interface may have several operations, each with its own input and output parameters
  
  • An output parameter represents what is returned by operation
• For example, the ISensor interface may have the following operations:
  
  • motionDetected(): int
  • windowOpened(): bool
  • doorOpened(): bool
• I is convention used to denote “interface”: used less and less
• Common alternatives to I are names that end in er or able, e.g., Reader, Writer, Executer, Moveable, Destroyable, Closeable etc.

Question 20

Specifying sub-system intefaces: ball and socket

Answer 20

Question 21

Guidelines for interface

Answer 21

Question 22

Alarm system with interfaces (ball and socket)

Answer 22

Question 23

Interface operations and parameters

Answer 23