this material is approved for public release. distribution is limited by the software engineering...

This material is approved for public release. Distribution is limited by the Software Engineering Institute to attendees.

Sponsored by the U.S. Department of Defense© 2002 by Carnegie Mellon University

Version 1.5 page 1

Pittsburgh, PA 15213-3890

1

Software Architecture Software Architecture in Practicein PracticePaul C. Clements

Software Engineering InstituteCarnegie Mellon UniversityPittsburgh, PA 15213-3890

Sponsored by the U.S. Department of Defense© 2002 by Carnegie Mellon University

Carnegie Mellon University

Software Engineering Institute

© 2002 by Carnegie Mellon University Version 1.5 page 2

The SEI: Who we are -1

The Software Engineering Institute (SEI) is

• a federally funded research and development center

• sponsored by the U. S. Department of Defense

• hosted by Carnegie Mellon University in Pittsburgh

• staffed by about 200 people

The SEI’s core purpose is to help others make measured improvements in their software engineering capabilities.


The SEI: Who we are - 2

First objective:

• Accelerate the introduction and widespread use of high-payoff software engineering practices and technology by identifying, evaluating, and maturing promising or underused technology and practices.

We are a small organization. We have to pick these practices carefully.

One we have chosen isSoftware architecture


The Ascendance of Software ArchitectureOver the past 10 years, software architecture has emerged as the prominent paradigm in large-system development.

There are:

• worldwide conferences devoted to it

• books devoted to it

• defined “architect” roles in organizations

• courses and training for it


And yet...

It is still not well understood in some circles.

Some organizations have no “architect” position. Others have the position but it is informally defined.

Some organizations are still proceeding to development without an architecture in place.

The tools of the trade -- styles and patterns, views, evaluation -- are used sparingly if at all.


Today’s talk

What is software architecture and why is it important?

A benefit of architecture: Software product lines

Evaluating software architectures

Documenting software architectures


Today’s talk






What Is Software Architecture?

Software architecture is the structure or structures of

the system, which comprise software elements, the

externally visible properties of these elements, and

the relationships among them.

The exact structures to consider

and the ways to represent

them vary according to

engineering goals.


Implications of this Definition – 1

A software architecture is an abstraction of a system.

• Architecture defines elements and how they interact.

• Architecture suppresses purely local information about elements; private details are not architectural.

Externally-visible properties of elements are assumptions that one elements can make about another:

• provided services, required services, performance characteristics, fault handling, resource usage



Every system has an architecture.

• Every system is composed of elements and there are relationships among them.

• In the simplest case, a system is composed of a single elements, related only to itself.

Just having an architecture is different from having an architecture that is known to everyone:

• The architecture versus specification of the architecture

If you don’t explicitly develop an architecture, you will get one anyway – and you might not like what you get.



This means that box-and-line drawings alone are not architectures; but they are just a starting point.

• You might imagine the behavior of a box labeled “database” or “executive” -- but that’s all

• You need to add specifications and properties.

Systems have many structures (views).

• No single structure can be the architecture.

• The set of candidate structures is not fixed or prescribed: choose whatever is useful for analysis, communication, or understanding.


Example of Multiple ViewsSoftware Architecture for A-7E Corsair II Aircraft • U. S. carrier-based, light attack aircraft• Used from the 1960s through the 1980s• Small computer on board for navigation, weapons

delivery


Module View (2 Levels)

Har

dw

are-

Hid

ing

Mo

du

le

Deviceinterfacemodule

Extendedcomputermodule

Beh

avio

r-H

idin

g M

od

ule

Functiondrivermodule

Sharedservicesmodule

Data bankermodule

Physicalmodels module

Applicationdata types mod.

Filterbehavior module

Softwareutilities module

Systemgeneration mod.S

oft

war

e -D

ecis

ion

-Hid

ing

Mo

du

le


Data Flow View

Device interfaces

Data banker

Shared services

Function drivers Filter behaviors

Physical models

sensor inputs

computed values stored values

computed values

storedvalues

valuesto display

filteredvalues

sensorvalues

calculatedcalculatedreal-worldreal-world

valuesvalues

Pilot, external world


Layers View

Function drivers

Extended computer

Application data types

Device interfaces

Databanker

Physicalmodels

Filterbehaviors

Shared services

Softwareutilities


Views -1

An architecture is a very complicated construct -- too complicated to be seen all at once.

Views are a way to manage complexity.• 1974: Parnas observed that software is composed of

many structures• 1992: Perry and Wolf recognize that, similar to

buildings (with plumbing and electrical and wall diagrams), different views of a system are required.

• 1995: Kruchten defined the “4+1 views” approach to software architecture.

• 2000: Hofmeister, Nord, and Soni defined the “Siemens Four Views” approach to software.


Views -2

A view is a representation

of a set of architectural

elements and therelations associatedwith them.

Not all architectural

elements -- some of them.

A view binds element

types and relation types

of interest, and shows

those.

All information

Some information


Views -3

In box-and-line diagrams, a way of asking what the boxes and lines mean is:

“What element types and relation types are you showing?

In other words,“What view are you showing?”

Which view shows “the” architecture?• None of them.• All of them.


Architecture As Communication VehicleArchitecture provides a common frame of reference in which competing interests may be exposed and negotiated.

• negotiating requirements with users and other stakeholders

• keeping the customer informed of progress and cost

• implementing management decisions and allocations


Architecture Constrains Implementation

An architecture defines constraints on an implementation.

• Architectures are descriptive and prescriptive

- descriptive for communication

- prescriptive for design and implementation

• Global resource allocation decisions constrain implementations of individual components

• System tradeoffs regarding quality attributes are in the architectural realm.


Architecture Organizes the Development Project

The architecture influences the organizational structure for development/maintenance efforts. Examples include• division into teams• units for budgeting, planning• basis of work breakdown structure• organization of documentation• organization of CM libraries• basis of integration• basis of test plans, testing• basis of maintenance• Incremental deployment


Architecture Permits/Precludes Achievement of Quality Attributes

For example

If you desire Examineperformance inter-component communicationmodifiability component responsibilitiessecurity inter-component communication,

specialized components (e. g., kernels)scalability localization of resourcesability to subset inter-component usagereusability inter-component coupling


Architecture is Key to Managing Change

An architecture helps reason about and manage change.

• important since 80% of effort in systems occurs after deployment

Architecture divides all changes into three classes:

• local: modifying a single component

• non-local: modifying several components

• architectural: modifying the gross system topology, communication, and coordination mechanisms

A “good” architecture is one in which the most likely changes are also the easiest to make.


Architecture is Basis for Incremental Development

An architecture helps with evolutionary prototyping and incremental delivery.

• Architecture serves as a skeletal framework into which components can be plugged.

• By segregating functionality into appropriate components, experimentation is easier.

• Risky elements of the system can be identified via the architecture and mitigated with targeted prototypes.


Architecture is a Reusable Model

An architecture is an abstraction: enables a one-to-many mapping (one architecture, many systems).

Systems can be built from large, externally developed components that are tied together via architecture.

Architecture is the basis for product (system) commonality.

Entire software product lines can share a single architecture.


Today’s talk






What is a Software Product Line?

A software product line is a set of software-intensive systems sharing a common, managed set of features that satisfy the specific needs of a particular market segment or mission and that are developed from a

common set of core assets in a prescribed way.


Software Product Lines

Market strategy/ Application domain

Architecture

Components

pertain to

share an

are built from

is satisfied by

used to structureProducts

COREASSETS

Product lines • take economic advantage of commonality • bound variability

Product lines • take economic advantage of commonality • bound variability


How Do Product Lines Help?

Product lines amortize the investment in these and other core assets:

• requirements and requirements analysis

•domain model

•software architecture and design

•performance engineering

•documentation

• test plans, test cases, and data

• people: their knowledge and skills

•processes, methods, and tools

•budgets, schedules, and work plans

•components

earlier life- cycle

reuse

more benefit


The Key Concepts

Use of a common asset base

in production

of a related set of products


Economics of Product Lines

Derived from data supplied byLucent Technologies

Bell Laboratories Innovations

With Product Line Approach

CurrentPractice

CumulativeCost

Number of Products


Example: Cummins, Inc.

World’s largest manufacturer of large diesel engines

(over 200 hp)

25,000 employees

350 controls and

electronics engineers

$7B annual sales


Complex Domain Of Variation

Today’s diesel engines are driven by software.

• Micro-control of ignition timing to achieve optimum mix of power, economy, emissions.

• Conditions change dynamically as function of road incline, temperature, load, etc.

• Must also respond to statutory regulations that often change.

• Reliability is critical! Multi-million dollar fleets can be put out of commission by a single bug.

• 130KSLOC -- C, assembler, microcode.• Different sensors, platforms, requirements.


Changing the Way to Do Business

In 1994, with six engine projects under way and 12 more planned, Cummins

• realized that it could not continue to do business the old way

• realized hiring to fill the need was out of the question

• launched a software product line

• built a core asset base from the most successful of its projects

• turned the other projects into product development projects using the core assets


Cummins’ Results

In early 1995, the first product was launched on time (relative to re-vamped schedule) with high quality. Others followed -- on time and with high quality.

Cummins achieved a product family capability with a breathtaking capacity for variation, or customization• 9 basic engine types• 4-18 cylinders• 3.9 - 164 liter displacement• 12 kinds of electronic control modules• 5 kinds of microprocessors• 10 kinds of fuel systems• diesel fuel or natural gas


Quantitative Results - 1

20 product groups launched, which account for over 1000 separate engine applications

75% of all software, on average, comes from core assets

Product cycle time has plummeted. Time to first engine start went from 250 person-months to a few person-months. One prototype was built over a weekend.

Software quality is at an all-time high, which Cummins attributes to product line approach.


• Customer satisfaction is high. Productivity gains enable new features to be developed (more than 200 to date).

• Projects are more successful. Before product line approach, 3 of 10 were on track, 4 were failing, and 3 were on the edge. Now, 15 of 15 are on track.

• Widespread feeling that developers are more portable, and hence more valuable.




Achieving this flexibility without the product line approach would have required 3.6 times the current staff.

Supported Components

1992 1993 1994 1995 1996 1997 1998

Electronic control modules (ECMs)

3 3 4 5 5 11 12

Fuel Systems

2 2 3 5 5 10 11

Engines

3 3 5 5 12 16 17

Features * ECM 60 80 180 370 1100 2200 2400


Today’s largest teams are smaller than yesterday’s smallest teams. Two-person teams are not unusual.

Cummins management has a history of embracing change, but carefully targeted change.

• They estimate that process improvement alone has brought a benefit/cost ratio of 2:1 to 3:1.

• They estimate that the product line approach has brought a benefit/cost ratio of 10:1.

Product line approach let them quickly enter and then dominate the industrial diesel engine market.



Lessons Learned

Cummins story echoes product line success themes seen on other successful efforts, namely• a compelling business case• deep domain expertise• a rich legacy base• a dedicated champion• organizational cohesion• courage to try new engineering approaches


Examples of Product Line Practice - 1

CelsiusTech - large shipboard command/control systems• hardware-to-software cost ratio switched from 35:65 to 80:20• staff decreased from over 200 to about 40

Motorola - FLEXworks Project (family of one-way pagers)• 4x cycle time improvement• 80% reuse

Hewlett Packard - printer systems• 2-7x cycle time improvement• Sample Project

–shipped 5x number of products–that were 4x as complex–and had 3x the number of features–with 4x products shipped/person


Examples of Product Line Practice - 2

Nokia - mobile phones• went from 4 different phones produced per year to 25-30

per year• 58 languages, multiple platforms, wide range of features

National Reconnaissance Office’s Control Channel Toolkit - ground-based satellite systems• first product using core assets requiring 1/10 the normal number of developers

Market Maker - website stock market software• each product uniquely configured for customer• three days to put up a customized system


Today’s talk






Architecture Evaluation

A software architecture is the earliest life-cycle artifact that

embodies significant design decisions. Analyzing for system qualities early in the life cycle allows for a comparison of architectural options.

With the advent of cost-effective, repeatable architecture evaluation methods, architecture evaluation should be a standard part of every architecture-based development methodology.


When and Why To Evaluate an Architecture

When building a system• Evaluation should be done when deciding on

architecture.

When acquiring a system• Architectural evaluation is useful if the system will

have a long lifetime within organization.• Evaluation provides a mechanism for understanding

how the system will evolve.• Evaluation can also provide insight into other visual

qualities.


Forces Preparation for Review

Documentation/specifications must be provided, hence they must exist or be created.

Some evaluations use standard questions, and the architect can prepare ahead to ensure that the architecture scores well.

Evaluations make the criteria for evaluation explicit by prioritizing requirements or quality goals.


Early Detection of Problems

The problems that can be found by an architecture evaluation include

• unreasonable requirements

• performance problems

• problems associated with potential future modifications

The earlier in the life cycle that problems are found, the easier it is to fix them.


Validation of Requirements

Evaluations put stakeholders in the same room with each other, often for the first time.

• uncovers conflicts and tradeoffs

• provides a forum for negotiated resolution of problems

It often results in the generation of new requirements or the clarification of existing requirements.


Improved Architectures

Development organizations anticipate types of questions raised at evaluations and

• design architectures with questions in mind

• prepare documentation of the type needed at evaluation

• give explicit consideration to qualities to be evaluated


The ATAMSM

The SEI has developed the Architecture Tradeoff Analysis MethodSM (ATAMSM).

The purpose of ATAMSM is: to assess the consequences of architectural decisions

in light of quality attribute requirements and business goals.


ATAMSM Steps

1. Present the ATAMSM

2. Present business drivers

3. Present architectural views

4. Identify architectural approaches

5. Generate quality attribute utility tree

6. Analyze architectural approaches

7. Brainstorm and prioritize scenarios

8. Analyze architectural approaches

9. Present results

Phase 1: Evaluation team,key decision-makers

Phase 2:Evaluation team, stakeholders


Conceptual Flow of ATAMSM

ArchitecturalDecisions

ScenariosQuality

Attributes

ArchitecturalApproaches

BusinessDrivers

Software Architecture

impacts

Risk Themes

distilledinto

Analysis

Risks

Sensitivity Points

Tradeoffs

Non-Risks


Today’s talk






Documenting an architecture

Architecture serves as the blueprint for the system, and the project that develops it.• It defines the work assignments.• It is the primary carrier of quality attributes.• It is the best artifact for early analysis.• It is the key to post-deployment maintenance and

mining.

Documenting the architecture is the crowning step to creating it.

Documentation speaks for the architect, today and 20 years from today.


“Views and Beyond” approach to documentation

Views give us our first principle of architecture documentation:

Document the relevant views,

and then add information

that applies to more than one view.


Which views are relevant? -1

Kruchten’s 4+1 views• Logical view: supports behavioral requirements. Key

abstractions, which are objects or object classes• Process view: addresses concurrency and distribution.

Maps threads to objects. • Development view: organization of software modules,

libraries, subsystems, units of development.• Physical view: maps other elements onto processing

and communication nodes.• “Plus one” view: Maps the other views onto important

use cases to show how they work.



Siemens Four-Views (Hofmeister, Nord, Soni, Applied Software Architecture, 2000):• Conceptual view• Module interconnection view• Execution view• Code view

Herzum & Sims (Business Component Factory, 1999):• Technical architecture• Application architecture• Project management architecture• Functional architecture



Software Cost Reduction method (Parnas, et al., 1980s)• Module view: shows modules as units of

encapsulation; used to isolate changes and achieve modifiability

• Process view: shows processes and how they synchronize and communicate at run-time; used to achieve performance

• Uses view: shows programs and how they depend on each other; used to achieve incremental development and the ability to quickly field subsets



Which views are relevant? It depends on:

• who the stakeholders are

• how they will use the documentation.

Three primary uses for architecture documentation:

• Education -- introducing people to the project

• Communication -- among stakeholders

• Analysis -- assuring quality attributes


What views are available? -1

Plenty! Too many!

Already, we’ve seen 15 different views, and many more are available, based on examining the literature.

An architect needs a way to choosethe useful ones.

One thing that would help is to organize the views into broad categories.



An architect must consider the system in three ways:

• How is it structured as a set of code units?

• How is it structured as a set of elements that have run-time behavior and interactions?

• How does it relate to non-software structures in its environment?



This suggests looking for three kinds of views that help:

• How is it structured as a set of code units?Module views (module viewtype)

• How is it structured as a set of elements that have run-time behavior and interactions?Component-and-connector views (C&C viewtype)

• How does it relate to non-software structures in its environment?Allocation views (allocation viewtype)


How many views do we need in our documentation package?Each view comes with a cost.

Each view comes with a benefit.

Planning a view setrequires understandingthe needs of the

stakeholders, andthe resources available.


How to proceed?

1. Build a table.

• ROWS: Enumerate the stakeholders

• COLUMNS: Enumerate the set of styles that could apply to the architecture being documented. This is our potential set of views.

• Check box (x,y) if stakeholder x would like view y.

2. Combine views appropriately to reduce number.

3. Prioritize views based on need. (Some stakeholders may have extra weight.)


Documenting a view -1

1. A primary presentation

• Usually graphical (we call this a cartoon)• May be textual -- e.g., a table• If graphical, includes a key explaining the notation (or

pointing to explanation)• Shows elements and relationships among them• Shows information you wish to convey about the view

(view packet) first

Many times, the primary presentation is all you get. It’s not enough!



2. An element catalog• Explains the elements depicted in the primary

presentation• Lists elements and their properties (as defined by the

relevant style guide)• Explains relations, and any exceptions or additions to

the relations shown in the primary presentatio• Interfaces of elements

3. A context diagram• Shows how system (or portion shown in this view

packet) relates to its environment.



4. A variability guide• Shows the architectural mechanisms available to

change the element

5. Architecture background• Rationale for design decisions that apply to the entire

view (or to that portion of the view being shown), including rejected alternatives and factors that constrained the design

• Analysis results validating the design decisions• Assumptions about the environment and about the

need that the system is fulfilling



6. Other information

• System- and project-specific.

• CM information, ownership information

• Mapping to requirements

• Not architectural, strictly speaking. But useful to capture alongside the architecture anyway.

7. Related view packets

• Pointers to sibling, child, and parent view packets


Documentation beyond views - 1

1. Documentation roadmap

• How the documentation is organized to serve a stakeholder

• List of views, with the elements/relations of each, and a statement of what the view is for

• Scenarios for using the documentation, showing which parts should be consulted

2. View template

• Explanation of how each view is documented

• The standard organization for each view


Documentation beyond views -2

3. System overview

• An informal, prose description of the system and its purpose and functionality

• Goal is to provide context for new member

• Perfectly OK to point to overview elsewhere if one exists in overall system documentation

4. Mapping between views

• Establishes useful/insightful correspondence between various views

• Tabular


Documentation beyond views -3

5. Directory

• An index showing where every element, relation, and property is defined and used.

6. Architecture glossary and acronym list

• May be subset of overall system glossary and acronym list. OK to point to larger document if so.

7. Background, design constraints, and rationale

• As in views, but applied to cross-view design decisions.


Source of material - 1

General overview

1998 (second edition 2003)

Software product lines

2001


Source of material - 2

Architecture evaluation

2001

Architecture documentation

2002


For more information

SEI’s software architecture work:

• www.sei.cmu.edu/architecture

SEI’s software product line work:

• www.sei.cmu.edu/plp/plp_init

Contact information:Paul ClementsSoftware Engineering InstituteCarnegie Mellon UniversityPittsburgh, PA [email protected]

this material is approved for public release. distribution is limited by the software engineering...

Documents

software architecture

software elements

benefit of architecture

software product lines

single elements

resource usage slide

weapons delivery slide

visible properties of