ETSF01 - Lecture 1 · 10/27/2014 1 MTAT.03.094/ Lecture 07 / © Dietmar Pfahl 2014 MTAT.03.094...

10/27/2014

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MTAT.03.094 / Lecture 07 / © Dietmar Pfahl 2014

MTAT.03.094

Software Engineering

Lecture 07: Architecture

and Design

Dietmar Pfahl

email: [email protected] Fall 2014


Exams

Date – Time – Place:

• Exam 1: Friday, 09-Jan-2015, 14:15-16:15, room 2-405

• Exam 2: Friday, 16-Jan-2015, 14:15-16:15, room 2-405

• Re-take Exam: Friday, 23-Jan-2015, 14:15-16:15, room 2-405

• Capacity limit of 90 students

• You must register: First-come-first-serve (FIFO) principle


Schedule of Lectures

Week 01: Introduction to SE

Week 02: Requirements Engineering I

Week 03: Requirements Engineering II

Week 04: Analysis

Week 05: Dev. Infrastructure I

Week 06: Dev. Infrastructure II

Week 07: ICS Day / ATI Päev 2014

Week 08: Architecture and Design

Week 09: Refactoring

Week 10: Verification and Validation

Week 11: Software Quality

Management

Week 12: Agile/Lean Methods

Week 13: Measurement & Process

Improvement

Week 14: Course wrap-up, review and

exam preparation

Week 15: no lecture

Week 16: no lecture


Acknowledgements

Textbooks/Slides:

• Ian Sommerville: Software Engineering, 9th edition, 2010 (http://www.softwareengineering-9.com/)

• Hans van Vliet: Software Architecture, Free University of Amsterdam, Lecture 2008

• Richard Taylor et al.: Software Architecture, University of California at Irvine, Lecture 2011

• Alexander Serebrenik: Software architecture: Domain-Specific Software Architecture and Architectural Patterns, TU Eindhoven, Lecture 2013

• George Fairbanks: Just Enough Software Architecture, 2012 (Video: https://www.youtube.com/watch?v=x30DcBfCJRI)


Structure of Lecture 07

• What is it? Why bother?

• Terminology: Architect, Architecting, Architecture

• Viewpoints and View Models

• Notation

• Patterns, Styles and DSSAs

• Analysis/Assessment

• Wrap-up


A tale of two systems

https://www.youtube.com/watch?v=x30DcBfCJRI

https://www.youtube.com/watch?v=x30DcBfCJRI

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Let’s design a system!


The system is real – Rackspace


Rackspace: Architecture 1

CSR: Customer Service Request







(https://www.usenix.org/legacy/publications/library/proceedings/osdi04/tech/full_papers/dean/dean_html/index.html)


Rackspace: Quality Attribute Trade-offs

https://www.usenix.org/legacy/publications/library/proceedings/osdi04/tech/full_papers/dean/dean_html/index.html

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What if you don’t think architecturally?

Remember: All programs have an architecture ... But not every architecture suits the program!


Virtuosos and Roman Engineers






• Notation



• Wrap-up


Terminology

• Architect – Person

• Architecting – Process

• Architecture – Product


The Role of the Architect

client,

usersarchitect developers

appearance,

behaviour

construction,

co-operation

architectural

design

visualises prescribes

requirements solutions

createsassess assess


Pre-Architecture Life-Cycle

functionality

agreement

quality

development

stakeholders (few)

Characteristics:

• Iteration mainly on functional requirements

• Few stakeholders involved

• No balancing of functional and quality requirements

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Adding Architecture: The Easy Way

architecture

detailed design

implementation

functionality

agreement

quality

development

stakeholders (few)


Architecture in the Life-Cycle

functionality

architecture

quality

agreement

stakeholders (many)

development

Characteristics:

• Iteration on both functional and quality requirements

• Many stakeholders involved

• Balancing of functional and quality requirements


Architecture Drivers Importance

Difficulty


Rational Architecture Decisions


Design Issues, Options and Decisions

A designer is faced with a series of design issues

These are sub-problems of the overall design problem.

For example, a design issue can be the type and level of security.

Security can be decomposed into authentication, authorization, and privacy.

Each issue normally has several alternative solutions (or design options)

The designer makes a design decision to resolve each issue.

This process involves choosing the best option from among the alternatives.

Attribute-Driven Design (ADD)


ADD Example Iterations

Top-level:

• usability separate UI 3-tier architecture

Lower-level, within user interface:

• security authenticate users

Lower-level, within data layer:

• availability active redundancy

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Decision Space

• The space of possible designs that can be achieved by choosing different sets of alternatives.

Issues that can be relevant in the decision process could be: - level of flexibility; - outsourcing/external

acquisition of client technology (that mans need for separate presentation – also relevant for the budget);

- if using the Web/Internet; - performance; ...


Tree or Graph?

• Issues and options are not independent ...

A number of options become invalid due to a desired NFR (quality). For example, flexibility could be achieved through certain architectural patterns, like MVC which facilitates separation of concerns and layered architecture restricting client-server interactions. If we choose any of the two, we exclude the 'monolithic' sub-tree and we need a separate GUI layer.


More than just IT

• Technical and non-technical issues and options are intertwined

• Architects deciding on the type of database

versus

• Management deciding on new strategic partnership

or

Management deciding on budget


Once again: Why is documenting design

decisions important?

• Prevents repeating (expensive) past steps

• Explains why this is a good (better: suitable) architecture

• Emphasizes qualities and criticality for requirements/goals

• Provides context and background

Design rationale example:


Architecture in Construction of Buildings


Software Architecture

• Architecture is conceptual.

• Architecture is about fundamental things.

• Architecture exists in some context.

Architectural descriptions are concrete, but the architecture itself is inherently conceptual, and cannot be captured in any (set of) views – nor in the code. Abstraction !!! We can only understand qualities in context. -> Views !!!

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Software Architecture – Definition (1)

The architecture of a software system defines that system in terms of computational components and interactions among those components.

(from Shaw and Garlan, Software Architecture, Perspectives on an Emerging Discipline, Prentice-Hall, 1996)

statement

procedure

module

(design) pattern

architecture

2



The software architecture of a 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.

(from Bass, Clements, and Kazman, Software Architecture in Practice, SEI Series in Software Engineering. Addison-Wesley, 2003)

3



Architecture is the fundamental organization of a system embodied in its components, their relationships to each other and to the environment and the principles guiding its design and evolution

(from IEEE Standard on the Recommended Practice for Architectural Descriptions, 2000)

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Architecture – Sloppy Definitions

• Architecture is high-level design

• Architecture is overall structure of the system

• Architecture is components and connectors

• Architecture is

• the structure,

• the behavior (properties),

• and the principles and guidelines governing its design and evolution over time






• Notation



• Wrap-up


Analogy with Building Architecture

• Overall picture of building (client)

• Front view (client, “beauty” committee)

• Separate picture for water supply (plumber)

• Separate picture for electrical wiring (electrician)

• etc

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IEEE Model for Architectural Descriptions

Mission

Sy stemEnv ironment Architecture

RationaleArchitecture Description

Concern

Library Viewpoint

Viewpoint

Stakeholder

Model

View

Mission

Sy stemEnv ironment Architecture

RationaleArchitecture Description

Concern

Library Viewpoint

Viewpoint

Stakeholder

Model

View

System stakeholder: an individual, team, or organization (or classes hereof) with interests in, or concerns relative to, a system.

View: a representation of a whole system from the perspective of a related set of concerns.

Viewpoint: A viewpoint establishes the purposes (concerns) and audience (stakeholders) for a view and the techniques or methods employed in constructing a view.


Kruchten’s 4+1 View Model

Logical Viewpoint

Implementation Viewpoint

Process Viewpoint

Deployment Viewpoint

Scenarios

End-user Functionality

Programmers Software management

Integrators Performance Scalability

System engineers Topology

Communications



Logical Viewpoint


Process Viewpoint


Scenarios





Communications

Supports the functional requirements, i.e., the services the system should provide to its end users. Typically, it shows the key abstractions (e.g., classes and associations amongst them).



Logical Viewpoint


Process Viewpoint


Scenarios





Communications

Takes into account some nonfunctional requirements, such as performance and system availability. It addresses concurrency and distribution, system integrity, and fault-tolerance. The process view also specifies which thread of control executes each operation of each class identified in the logical view. So the process view describes the mapping of functions to runtime elements. It concerns the dynamics of the system. A process is a group of tasks which form a logical unit. A process can be started, stopped, resumed, etc., and there is communication between processes.



Logical Viewpoint


Process Viewpoint


Scenarios





Communications

Focuses on the organization of the actual software modules in the software-development environment. The software is packaged in small chunks (program libraries or subsystems) that can be developed by one or more developers.



Logical Viewpoint


Process Viewpoint


Scenarios





Communications

=> Physical view: Defines how the various elements identified in the logical, process, and implementation views (networks, processes, tasks, and objects) must be mapped onto the various nodes. Takes into account the system's non-functional requirements such as system availability, reliability (fault-tolerance), performance (throughput), and scalability.

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4 + 1: Scenario Viewpoint

• Consists of a small subset of important scenarios (e.g., use cases) to show that the elements of the four views work together seamlessly.

• This view is redundant with the other ones (hence the "+1"), but it plays two critical roles:

• Acts as a driver to help designers discover architectural elements during the architecture design;

• Validates and illustrates the architecture design, both on paper and as the starting point for the tests of an architectural prototype.

Logical Viewpoint


Process Viewpoint


Scenarios





Communications






• Notation



• Wrap-up


Standard Notation: UML


Architecture presentations in practice

• By and large two flavors:

• Powerpoint slides – for managers, users, consultants, etc

• UML diagrams, for technicians

• A small sample …


Conceptual View Customer, Users


More technical view Developers (same system as on previous slide)

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Runtime Component Deployment View Component clientArs User machine Search App. Server 1, App Server 2 (WebLogic 7.0) ReserveAndBuy App. Server 1, App Server 2 (WebLogic 7.0) Authenticate App. Server 1, App Server 2 (WebLogic 7.0) ArsStartup App. Server 1, App Server 2 (WebLogic 7.0) LocalRestart App. Server 1, App Server 2 (WebLogic 7.0) ARS Database DB Server (MS SQL Server) CreditCard Database DB Server (MS SQL Server) CreditCard Naming server & Replication mgr God Naming server & Replication mgr

Runtime View Deployment View


Application layer This Application layer has all the boundary classes that represent the application screens that the user sees. Business Services layer The Business Services process layer has all the controller classes that represent the use case managers that drive the application behavior. This layer represents the client-to-mid-tier border. Middleware layer The Middleware layer supports access to Relational DBMS and OODBMS. Base Reuse package The Base Reuse package includes classes to support list functions and patterns.

High-level overview of the architecture (Logical view – Implementation view)

A University Course Catalogue System

(see doc on course wiki)


Process view of the architecture. Shows the tasks (processes and threads) involved in the system's execution, their interactions and configurations. Processes exist to support student registration, professor functions, registration closing, and access to the external Billing System and Course Catalog System.


Deployment view of the architecture. Shows the various physical nodes for the most typical platform configurations. Also shows the allocation of tasks (from the Process View) to the physical nodes.


So, …

• Different representations

• For different people

• For different purposes

• These representations are both descriptive and prescriptive






• Notation



• Wrap-up

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Learning from Others:

Patterns, Styles, and DSSAs

Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John

Wiley & Sons, Inc. Reprinted with permission.


How to solve a problem?


Examples of Domains

• Compilers for programming languages

• Consumer electronics

• Electronic commerce system/Web stores

• Video game

• Business applications

• Basic/Standard/“Pro”

We can subdivide, too: • Avionics systems -> Boeing Jets -> Boeing 747-400


Domain-Specific Software Architectures

• A DSSA is an assemblage of software components

• specialized for a particular type of task (domain),

• generalized for effective use across that domain, and

• composed in a standardized structure (topology) effective for building successful applications.

• DSSAs are the pre-eminent means for maximal reuse of knowledge and prior development.


Domain-Specific Software Architectures

A DSSA comprises:

• A reference architecture, which describes a general computational framework for a significant domain of applications

• A component library, which contains reusable chunks of domain expertise, and

• An application configuration method for selecting and configuring components within the architecture to meet particular application requirements

Examples:

• ADAGE for avionics, AIS for adaptive intelligent systems, and MetaH for missile guidance, navigation, and control systems


Reference Architecture – Example

• Structural view of Lunar Lander DSSA

• Invariant with explicit points of variation

• Satellite relay

• Sensors

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Reference Architecture





MURA: Microsoft Upstream Reference Architecture


DSSAs also include …


Extreme Case of DSSA ...


Product Line Architecture

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Product Line Architecture – Why?


How do Product Lines come to be?


Example: Lunar Lander Game


Example: Lunar Lander Game


Product Lines in the Lunar Lander Game


Product Lines: Components, Features, Products

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Better Representation: Variability Model


DSSAs vs. Product Lines


Architectural Patterns


State-Logic-Display (a.k.a. Three-Tier Pattern)


State-Logic-Display (a.k.a. Three-Tier Pattern)


State-Logic-Display in Web Development

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Tiers and Layers

Note: The middle tier might be multi-tiered (resulting in an n-tier architecture)


Model-View-Controller (MVC)


Model-View-Controller (MVC)


Do you recall ...?

Boundary = View Entity = Model Control = Controller


Two Flavors of MVC: Passive Model

• The passive model is employed when one controller manipulates the model exclusively.

• The controller modifies the model and then informs the view that the model has changed and should be refreshed.

• The model is completely independent of the view and the controller, i.e. there is no means for the model to report changes in its state.

• The HTTP protocol is an example of this. There is no simple way in the browser to get asynchronous updates from the server. The browser displays the view and responds to user input, but it does not detect changes in the data on the server.

• Only when the user explicitly requests a refresh is the server interrogated for changes.


Two Flavors of MVC: Active Model

Solution: Observer Design Pattern!

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Two Flavors of MVC: Active Model

Figure shows the structure of the active MVC using Observer and how the observer isolates the model from referencing views directly.

Java code example: http://en.wikipedia.org/wiki/Observer_pattern

Observer Pattern:

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems.


Benefits and Liabilities of MVC

Benefits

Liabilities


Patterns, Styles, and DSSAs

Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John

Wiley & Sons, Inc. Reprinted with permission.

• Pipes and filters • Data abstraction and

object-oriented organization

• Layered systems • Repositories • Event-based, implicit

invocation • ... and many more


Patterns – Styles – Patterns

• Architectural patterns <> Architectural styles <> Design patterns.

• While architectural styles define the components and connectors possible within the system (more asking ‘what?’), architectural patterns define the implementation strategies of those same components and connectors (a bit more on ‘how?’).

• Example: Model-View-Controller (MVC) pattern

• At the same time, a good architecture will make use of design patterns (on a more fine-granular level)

• Example: Observer pattern used to implement (active) MVC pattern

• Different architectural styles will find different architectural and design patterns more or less helpful.


Design Patterns

• A design pattern is a way of reusing abstract knowledge about a problem and its solution.

• A pattern is a description of the problem and the essence of its solution.

• It should be sufficiently abstract to be reused in different settings.

• Pattern descriptions usually make use of object-oriented characteristics such as inheritance and polymorphism.

ELEMENTS:

Name

A meaningful pattern identifier.

Problem description.

Solution description (might have an example)

Not a concrete design but a template for a design solution that can be instantiated in different ways.

Benefits and Consequences

The results and trade-offs of applying the pattern.


The Observer Pattern

Name: Observer

Problem description

• Situations where multiple displays of state are needed.

Solution description

• Separates the display of object state from the object itself.

• See UML description.

Consequences

• Optimisations to enhance display performance are difficult.

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Three Types of

Patterns

Creational patterns:

• Deal with initializing and configuring classes and objects

Structural patterns:

• Deal with decoupling interface and implementation of classes and objects

• Composition of classes or objects

Behavioral patterns:

• Deal with dynamic interactions among societies of classes and objects

• How they distribute responsibility

Observer


Benefits of Design Patterns

• Design patterns enable large-scale reuse of software architectures and also help document systems

• Patterns explicitly capture expert knowledge and design tradeoffs and make it more widely available

• Patterns help improve developer communication

• Pattern names form a common vocabulary






• Notation



• Wrap-up


Architecture Evaluation/Analysis

• Assess whether architecture allows system to meet certain quality goals

• e.g. regarding maintainability, modifiability, reliability, performance, ...

• Note: the architecture is assessed, while we hope the results will hold for a system yet to be built

Software architecture

System

Properties Qualities

implementation

properties ?


Analysis Techniques and Scenarios

• Questioning techniques:

• how does the system react to various situations;

• often make use of scenarios

• Measuring techniques:

• rely on quantitative measures; architecture metrics, simulation, etc.

• Different types of scenarios, e.g. use-cases, likely changes, stress situations, risks, far-into-the-future scenarios

• Which stakeholders to ask for scenarios?

• When do you have enough scenarios?


Exercise: Analyse of Alarm Clock Architectures

Consider standard alarm clocks that you have seen.

For this exercise, consider each of the following to be representative of an architectural style of alarm clocks:

• An LED alarm clock for a bedroom,

• A LCD travel alarm,

• An analog alarm clock (there are several varieties; choose one).

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List of Analysis Methods

• SAAM (Scenario-based Architecture Analysis Method)

• SAAMER (Software Architecture Analysis Method for Evolution and Reusability)

• ATAM (The Architecture Trade-Off Analysis Method)

• SBAR (Scenario-Based Architecture Reengineering)

• ... and many more ...


Scenario-based Architecture Analysis

Method (SAAM)

• Describe architecture(s)

• Based on desired system qualities, develop scenarios for

• kinds of activities the system must support

• kinds of changes anticipated

• Classify scenarios in direct & indirect

• direct -- execution of scenario requires no change of architecture

• indirect – execution of scenario requires change of architecture

• Evaluate indirect scenarios: list changes and estimate cost

• Reveal scenario interaction

• Overall evaluation



Method (SAAM)

• List of scenarios

• Examples:

• User U1 wants to have added another feature

• Maintainer M1 wants to port the system to platform XYZ

• Maintainer M2 wants to change the look-and-feel of the GUI

U1 and M1 interact because they require changes to the same components



Method (SAAM)

Overview: SAAM inputs and activities

Goal: By analysing the extend and difficulty (cost) of required changes (indirect scenarios) as well as the extent of scenario interactions, SAAM allows an insight to the future product capabilities, if the given architecture is chosen.


Conditions for Successful Architecture

Analysis/Evaluation

• Clear goals and defined quality requirements for the architecture

• Controlled scope – focus on most important goals and quality requirements

• Cost-effectiveness

• Key personnel availability – e.g. for scenario dev.

• Competent evaluation team – ideally independent from developers/owners of architecture

• Managed expectations






• Notation



• Wrap-up

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Why is Architecture Important?

• Architecture is the vehicle for stakeholder communication

• Architecture manifests the earliest set of design decisions

• Constraints on implementation

• Dictates organizational structure

• Inhibits or enables quality attributes

• Architecture is a transferable abstraction of a system

• Product lines share a common architecture

• Allows for template-based development

• Basis for training


Further Reading

• Len Bass et al, Sofware Architecture in Practice, 2008 (3rd edition).

• Jan Bosch, Design & Use of Software Architectures, 2000.

• Frank Buschmann et al., Pattern-Oriented Software Architecture: A System of Patterns, 1996. Part II: 2001.

• George Fairbanks: Just Enough Software Architecture, 2012.

• Erich Gamma et al., Design Patterns: Elements of Reusable Object-Oriented Software, 1995.

• C. Hofmeister et al., Applied Software Architecture, 1999.

• Philippe B. Kruchten, The 4+1 view model of architecture, IEEE Software, 12(6):42-50, November 1995.

• Mary Shaw and David Garlan, Software Architecture; Perspectives of an Emerging Discipline, 1995.

• Richard Taylor et al.: Software Architecture, University of California at Irvine, Lecture 2011.

• Ian Sommerville: Software Engineering, 9th edition, 2010. (http://www.softwareengineering-9.com/)

• Hans van Vliet: Software Architecture, Free University of Amsterdam, Lecture 2008.


Next Lecture

• Date/Time:

• Friday, 31-Oct, 14:15-16:00

• Topic:

• Refactoring (and TDD)

• For you to do:

• Finish and submit Lab Task 4

ETSF01 - Lecture 1 · 10/27/2014 1 MTAT.03.094/ Lecture 07 / © Dietmar Pfahl 2014 MTAT.03.094...

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