1. An Introduction to Software Architecture Pejman Salehi

2. Table of content Introduction Architecture Business Cycle
Architectural Structures and Views
Quality Attributes
Achieving Qualities
Attribute Driven Design

3. Introduction

4. Definition
The software architecture of a program or computing system is the structure or structures of the system, which comprise software elements, the externally visible properties of those elements, and the relationships among them.

5. Externally Visible vs. Internal Properties
Externally visible properties are what assumption other elements can make of an element
Provided services (and interface to access those services)
Performance
Fault handling
Shared resource usage …
SA intentionally abstracts away internal properties of elements (to better encounter complexity)

6. Some Points
Every software system has an architecture (SA ≠ specification of SA)
Specification of architecture can comprise more than one structure
Behavior of elements and relationships are defined in SA (abstractly)
The definition do not talk about Good and Bad architectures: SA evaluation methods
In the literature:
Component = Element
Connector = Relationship

7. Why is architecture important?
Handling complexity
Communication among stakeholders
Requirements and concerns of stakeholders
Time
Budget
Other Resources

8. Why is architecture important? (cont)
Early Design Decisions
Constraints implementation and implementers
Organizational structure
Enables predicting and ensuring quality attributes
Makes it possible to reason about and manage change
Helps evolutionary prototyping (risk reduction)

9. Why is architecture important? (cont)
SA is a transferable, reusable model
Software product lines
Component-based development
Automatic generation of lower-level models

10. Hazards With regards to SA changes are categorized to
Local (a single component)
Non-local (a few components)
Architectural (architectural style)
Once decided architecture is extremely hard to change
It impossible to reach to some quality attribute if architecture disallows

11. Software Arch. vs. System Arch.
System Arch. is the overall architecture of system including hardware and software architecture
In assuring quality attributes the architect needs to think about system architecture too (e.g. performance or reliability)
But architect has more freedom in software architecture than hardware (hardware choices is less under the architects control)

12. Architecture Business Cycle

13. Who influences SA?

14. Summary: Influences on the Architect

15. Architecture Business Cycle

16. Architectural Structures and Views

17. Architectural Structures and Views
In construction, there are blueprints of
Plan
Different sides of construction
Electrical wiring
Plumbing …
Each of these views specifies a single entity (i.e. the construction) from a different perspective (used by a different person, for a different goal).
Similarly there are different structures and views in SA.

18. Structures and Views (cont)
Structures is a set of coherent elements and the relations among them. For each structure these we can specify:
Types of elements
Types of relations
A set of syntactic constraints
Semantics of the diagram
Rationale, principles, and guidelines
For what purposes it is useful
View is a representation of software architecture based on an structure as written by the architect and read by stakeholders (an instance of the structure)
SA is documented by a number of views.

19. Categorization of Structures
Module Structures
Component and Connector Structures
Allocation Structures

20. 1 Module Structures
Elements: modules (units of implementation). Modules are a code-based way of considering the system
Specifies:
Functional responsibility of modules
Other elements a module is allowed to use
Generalization and specialization relations
Run-time operation of software is not a concern from this view

21. 1.1 Decomposition Structure
Elements: modules in a hierarchy
Relations: is a sub-module of, shares secret with
Function Example:
Contributes to system's modifiability, by ensuring that likely changes fall within the scope of at most a few small modules.

22. Uses Structure
Elements: modules, procedures, or resources on the interfaces of modules
Relations: uses: one unit uses another if the correctness of the first requires the presence of a correct version.
Function Example:
Allows incremental development

23. 1.3 Layered Structure Is a subclass of uses structure
Elements: layers: a coherent set of related functionality
Relations: uses (ideally layer n may only use the services of layer n – 1), provides abstraction to
Function Example:
Layers are often designed as abstractions (virtual machines) that hide implementation specifics below from the layers above, engendering portability.

24. 1.4 Class Structure Elements: classes /objects
Relations: inherits from, is an instance of
Function Example:
Allows us to reason about reuse and the incremental addition of functionality

25. 2 Component and Connector Structures
Elements: run-time components (principal units of computation) and connectors (communication vehicle among components.)
Specifies:
Major executing components and how they interact
Major shared data-stores
Which part of system is replicated
Flow of data through the system
What parts can run in parallel
How can system structure change as it executes

26. 2.1 Process Structure Elements: processes or threads
Relations: attachment (that allow communication, synchronization, and/or exclusion operations)
Function Example:
Engineering a system's execution performance and availability.

27. 2.2 Shared Data or Repository Structure
Elements: data stores, data producers, and data consumers
Relations: data-flow
Function Example:
To ensure good performance and data integrity.

28. 2.3 Client-Server Structure
Elements: clients and servers
Relations: protocols and message passing infrastructure.
Function Example:
Separation of concerns (supporting modifiability)
Load balancing (supporting runtime performance)

29. 2.4 Concurrency Structure
Elements: Component
Relations: Logical Thread.
Function Example:
managing the issues associated with concurrent execution.
resource contention

30. 3 Allocation Structures
Show the relationship between the software and the elements in one or more external environment in which software is created and executed.
Specifies:
The processor that executes each software element
The file that stores each software element during development
Assignments of software to development team

31. 3.1 Deployment Structure Shows how software is assigned to hardware
Elements: software (usually a process from a component and connector view), hardware entities, and communication pathways
Relations: is-allocated-to and migrates-to (for dynamic allocations)
Function Example:
Allows reasoning about performance, data integrity, availability, and security.

32. 3.2 Implementation Structure
Shows how software elements (usually modules) are mapped to the file structure(s) in the system's development, integration, or configuration management environments.
Elements: any logical unit (e.g. module)
Relations: implemented in
Function Example:
management of development activities and build process

33. 3.3 Work Assignment Structure
Assigns responsibility for implementing and integrating the modules to the appropriate development teams
Elements: any logical unit (e.g. module), team structure
Relations: is assigned to
Function Example:
The architect will know the expertise required on each team
The means for factoring functional commonalities and assigning them to a single team, rather than having them implemented by everyone who needs them.

34. Notes
Each structure is useful on its own right but not all structures are used in all projects.
Structures are not independent and must be considered together
e.g. relationship of modules with components (many to many)
Some structures may be the same in some systems
Some structures may be combined (e.g. all component and connector structures may be combined in a single structure)

35. Structures

36. Quality Attributes

37. Traditional Classification of Requirements
Functional
Non-Functional (Quality Attributes)
A popular software myth: first we build a software that satisfies functional requirements, then we will add or inject non-functional requirements to it.
This idea leads to loss of resources and finally poor quality.
So we should design for qualities from the very beginning (architecture level).

38. Functionality and Architecture
Functionality and quality attrs are orthogonal [in theory]. But not all qualities are achievable to any level desired with any functionality.
Functionality may be achieved in many ways (it is not so architectural.)
Architecture is a means of achieving quality attributes by structuring functionality into elements.

39. Architecture and Qualities
Achieving qualities must be considered throughout design (including SA), implementation, and deployment.
Qualities have both architectural and non-architectural aspects. For example
In usability: selecting form elements vs. supporting undo operation
Performance: amount of communication among components vs. algorithms

40. Architecture an Qualities
Quality attributes are not independent and may be achieved in isolation.
Positive Correlation; e.g.
Modifiability and Buildability (in many cases)
Negative Correlation (conflict); e.g.
Reliability vs. Security
Performance vs. All Other Qualities

41. Classification of Quality Attributes
Qualities of the system: availability, modifiability, performance, security, testability, and usability.
Business qualities (such as time to market) that are affected by the architecture.
Architecture qualities, such as conceptual integrity.

42. System Quality Attributes
Availability (related to Reliability)
Modifiability (includes Protability and Reusability, Scalability)
Performance
Security
Testability
Usability (includes Self-Adaptability and User-Adaptability)

43. Business Qualities Time to market Cost and benefit
Predicted lifetime of the system
Targeted market
Rollout schedule
Integration with legacy systems

44. Qualities of the Architecture
Conceptual Integrity
Conceptual integrity is the most important consideration in system design. It is better to have a system omit certain anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas. [Brooks 75]
Correctness and Completeness
Buildability

45. System Quality Attributes
System quality attributes have been of interest to the software community at least since the 1970s
Shortcoming of the previous work:
The definitions for an attribute are not operational.
Modifiability with regards to which aspect?
Which quality a particular aspect belongs to.
Is a system failure an aspect of availability, an aspect of security, or an aspect of usability?
Each attribute community has developed its own vocabulary.
Performance community events, security community attacks, availability community failures, and usability community user input may actually refer to the same occurrence.

46. Quality Attribute Scenario
Solution:
Quality Attribute Scenarios as a means of characterizing quality attributes
consists of six parts:
Source of stimulus.
Stimulus.
Environment.
Artifact.
Response.
Response measure.

47. Quality Attribute Scenario (cont.)
General Scenario

48. Quality Attribute Scenario (cont.)
Sample Scenario

49. Achieving Qualities

50. The Whole Story Business Requirements Quality Requirements
Tactics Selection
Tactics Implementation:
Architectural Patterns

51. Tactics
A tactic is a design decision that influences a quality attribute.
e.g. using redundancy to increase availability
Tactics can be refined to other tactics to become more concrete; e.g. redundancy: redundancy of data + process

52. Availability Tactics

53. Modifiability Tactics

54. Patterns
A pattern is a common abstract solution to a common abstract problem that
Can be tailored to a given situation
Has predefined characteristics
Abstraction level of patterns
Business
Analysis
Architecture
Design
Implementation (Idioms)
Test Patterns (or guideline to testing patterns)

55. Relationship of Tactics to Patterns
An architect usually chooses a pattern or a collection of patterns designed to realize one or more tactics.
However, each pattern implements multiple tactics, whether desired or not.

56. Famous Pattern (Style) categories
Data-centered
Repository
Blackboard (publisher-subscriber)
Structural solution to integrability of data
Scalability
Modifiability
Client
Client
Shared Data
Client
Client

57. Famous Pattern (Style) categories
Dataflow
Bach sequential
Pipes and filters
Reusability
Modifiability
Not interactive
Poor performance

58. Famous Pattern (Style) categories
Virtual Machine
Interpreter (e.g. Adaptive Object Model) …
Portability
Simulation
Adaptability
Low performance

59. Famous Pattern (Style) categories
Call and Return
Main program and sub-routine
Modifiability
Remote procedure call
Performance tuning
Object-oriented or abstract data type
Reuse
Layered
Portability

60. Famous Pattern (Style) categories
Independent components
Communicating Processes
Scalability
Event Systems
Modifiability

61. Example: ATM Software
Develop 3 different architectures for ATM software and compare them regarding fulfillment of quality attributes.
ATM = Automatic Teller Machine
User operations:
Insert card and enter PIN
Withdraw money
Check Balance

62. Shared-Memory Style

63. Abstract Data Type Style

64. Layered Style

65. Analysis and Comparison
Shared-Mem
ADT
Layered
Performance 3 2 1
Change account record format
New service: close account and withdraw the remained balance
Portability
Availability and Reliability
Buildability and Integrability
Sum 9 13 15

66. Attribute-Driven Design
bases the decomposition process on the quality attributes.
quality requirements should be expressed as a set of system-specific quality scenarios.
Steps
Choose the module to decompose
Refinement
Choose the architectural drivers
Choose an architectural tactic and pattern
Instantiate modules and allocate functionality
Define interfaces of the child modules
Verify and refine use cases and quality scenarios
Repeat the steps above for every module