Carnegie Mellon University
Software Engineering Institute
Pittsburgh, PA 15213-3890
Software Architecture
in Practice
Paul C. Clements
Software Engineering Institute
Carnegie Mellon University
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PA 15213-3890
Software Engineering Institute to attendees.
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theDepartment
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Sponsored by by
the U.S.
of Defense of Defense
2002 byby
Carnegie
Mellon University
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Carnegie
Mellon University
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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.
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The SEI: Who we are - 2
First objective:
• Accelerate the introduction and widespread use of highpayoff 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 is
Software architecture
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The Ascendance of Software
Architecture
Over 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
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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.
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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
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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
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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.
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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
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Implications of this Definition – 2
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.
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Implications of this Definition – 3
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.
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Example of Multiple Views
Software 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
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Extended
computer
module
Function
driver
module
Behavior-Hiding Module
Hardware-Hiding Module
Device
interface
module
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Shared
services
module
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Software -Decision-Hiding Module
Module View (2 Levels)
Data banker
module
Physical
models module
Application
data types mod.
Filter
behavior module
Software
utilities module
System
generation mod.
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Data Flow View
Pilot, external world
Device interfaces
sensor inputs
values
to display
Data banker
computed values
stored
values
calculated
real-world
values
Physical models
stored values
Shared services
sensor
values
computed values
Function drivers
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filtered
values
Filter behaviors
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Layers View
Function drivers
Shared services
Data
banker
Physical
models
Filter
behaviors
Software
utilities
Device interfaces
Application data types
Extended computer
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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.
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Views -2
A view is a representation
of a set of architectural
elements and the
relations associated
with them.
Not all architectural
elements -- some of them.
All information
Some information
A view binds element
types and relation types
of interest, and shows
those.
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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.
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Architecture As Communication
Vehicle
Architecture 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
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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.
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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
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Architecture Permits/Precludes
Achievement of Quality Attributes
For example
If you desire
performance
modifiability
security
scalability
ability to subset
reusability
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Examine
inter-component communication
component responsibilities
inter-component communication,
specialized components (e. g., kernels)
localization of resources
inter-component usage
inter-component coupling
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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.
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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.
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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.
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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
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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.
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Software Product Lines
Market strategy/
Application domain
pertain to
is satisfied by
share an
Architecture
Products
used to structure
are built from
Components
CORE
ASSETS
Product lines
• take economic advantage of commonality
• bound variability
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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
earlier lifecycle
reuse
more
benefit
• components
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The Key Concepts
Use of a
common
asset base
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in
production
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of a related
set of
products
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Economics of Product Lines
Current
Practice
With Product Line Approach
Cumulative
Cost
Number of Products
Derived from data supplied by
Lucent Technologies
Bell Laboratories Innovations
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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
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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.
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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
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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
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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 personmonths. One prototype was built over a weekend.
Software quality is at an all-time high, which Cummins
attributes to product line approach.
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Quantitative Results - 2
• 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.
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Quantitative Results - 3
Supported
Components
Electronic control
modules (ECMs)
1992 1993 1994 1995 1996 1997 1998
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
Achieving this flexibility without the product line approach
would have required 3.6 times the current staff.
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Quantitative Results - 4
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.
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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ATAMSM Steps
1. Present the ATAMSM
2. Present business drivers
3. Present architectural views
4. Identify architectural approaches
Phase 1:
Evaluation team,
key decision-makers
5. Generate quality attribute utility tree
6. Analyze architectural approaches
7. Brainstorm and prioritize scenarios
8. Analyze architectural approaches
9. Present results
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Phase 2:
Evaluation team,
stakeholders
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Conceptual Flow of ATAMSM
Business
Drivers
Quality
Attributes
Scenarios
Analysis
Software
Architecture
Architectural
Approaches
Architectural
Decisions
Tradeoffs
impacts
Sensitivity Points
Risk Themes
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distilled
into
Non-Risks
Risks
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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
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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.
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“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.
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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.
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Which views are relevant? -2
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
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Which views are relevant? -3
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
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Which views are relevant? -4
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
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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 choose
the useful ones.
One thing that would help is to organize
the views into broad categories.
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What views are available? -2
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 runtime behavior and interactions?
• How does it relate to non-software structures in its
environment?
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What views are available? -3
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 runtime 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)
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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 set
requires understanding
the needs of the
stakeholders, and
the resources available.
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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.)
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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!
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Documenting a view -2
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.
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Documenting a view -3
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
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Documenting a view -4
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
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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
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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
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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.
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Source of material - 1
General overview
Software product lines
1998 (second edition 2003)
2001
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Source of material - 2
Architecture evaluation
Architecture documentation
2001
2002
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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 Clements
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213
clements@sei.cmu.edu
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