Hanmer Software Patterns Bosc2009

Software Patterns for Reusable Design

Robert S. Hanmer

June, 2009

About me

MSCS, Northwestern University, 1987.
Working at Northwestern University Med School & computer center
benefited from shared software
While on AT&T 4ESS™ Switch project– learned about reusing design ideas and the
importance of architectural integrity
In Bell Labs – shared my knowledge with projects
Now in Alcatel-Lucent Operations – making sure we only use good 3rd party and free
software (FOSS)

Member of Alcatel-Lucent Technical Academy
Senior member of ACM

Writing patterns since 1994
President of Hillside Group
Author of Patterns for Fault Tolerant Software

2 | Software Patterns for Reusable Design | June 2009 All Rights Reserved © Alcatel-Lucent 2009

Agenda

1. Design, Reusable & Software Patterns

2. What is a Software Pattern?

3. Pattern Communities

4. Using Patterns

5. Writing and Collecting Patterns
6. Pattern Research

7. Patterns within your community

8. Wrapup

Design,
Reusable
& Software Patterns

Design
As a noun:

5 a: an underlying scheme that governs functioning, developing, or unfolding …
b: a plan or protocol for carrying out or accomplishing something (as a
scientific experiment) ; also : the process of preparing this

As a verb:

1: to conceive or execute a plan
2: to draw, lay out, or prepare a design
-- www.meriam-webster.com


Reusable
Adjective: capable of being used again or repeatedly
-- www.merriam-webster.com

Some adjectives that apply to things that are reusable:
flexible

Modular

understandable

-- Other fields have handbooks to capture their reusable information.


Reusable Design
Design and design elements that are

Modular

Flexible
Usable more than once

Can be communicated

Grady Booch: “Software development has been, is, and will likely remain
fundamentally hard.”


Software Patterns
“The pattern is, in short, at the same time a thing, which happens in the world,
and the rule which tells us how to create that thing, and when we must create
it. It is both a process and a thing; both a description of a thing which is alive,
and a description of the process which will generate that thing.”
-- Christopher Alexander,
The Timeless Way of Building

A software pattern is a description of modular proven solution to a design
problem with enough information so that the reader can flexibly adapt it to their
unique situations.


What is a Software Pattern?


Pattern: Leaky Bucket Counters

Context: A system that is intended to recognize and correct problems
automatically. In this environment the system needs to know if a problem is
something that comes and goes intermittently , or only happens once, i.e. is
"transient" or is a repeating “permanent” fault that needs to be corrected. Signal
crosstalk, static electric discharges, lightning strikes or intermittently broken
keys on a keyboard cause transient errors, which should be treated differently
than permanent faults.

Problem: How can you know if a fault is transient?

Forces: You want a module to exhibit a permanent error before taking drastic
action. The component that reports the error might be an innocent victim. It
shouldn’t be punished.

In many cases you are willing to allow a certain number of errors if they aren’t
too close together. If they come too close together you want the system to
trigger some sort of error processing action.


Leaky Bucket Counters (2)

Solution: Each failure group that is to be watched has a counter
(the leaky bucket counter) which is initialized to a
predetermined value when the system is first initialized. The
counter is incremented for each error or fault detected. The
counter is decremented on a periodic timed basis, but will
never be decremented beyond its initial value.

A counter incrementing past a pre-determined threshold
value indicates that the rate of error or fault events exceeds
the allowable rate (as defined by the empty or leak rate, rate
of errors and threshold value). This should be taken as an
indication that the fault is not transient.


Leaky Bucket Counters (3)

Resulting Context: A system in which errors are isolated and handled (by taking
devices out of service), but transient errors don't cause unnecessary loss of
service.

Author: Robert Hanmer, incorporating several versions by Robert Gamoke, James
Coplien and Gerard Meszaros. A revised version in a different format is in
Patterns for Fault Tolerant Software.


What is a Pattern?

A proven solution

Rule of 3

More than an algorithm

A pattern explains the context when its applicable

A pattern explains the trade-offs and alternatives

“At the same time, a thing that happens in the world … and the rules to make
that thing …”


Patterns Are:

A resolution of forces

A form of architectural and design documentation

Relationships that cuts across system parts

“Solution to a Problem in a Context”

A way of explaining non-traditional solutions

A family of solutions that abstractly address related problems in a specific
context
A literary form


Patterns are useful for:

Learning from what has worked in the past

Not re-inventing the wheel

Not repeating past errors

Providing a vocabulary for architects/designers

Passing on architectural knowledge

Providing a window into the minds of the experts

Transferring proven solutions from one industry segment to another

To structure OO code


Patterns Will Not:

… make you an instant expert

… provide a “turn the crank” approach to software

… eliminate the need for intelligence and taste

… make you rich and famous
unless you become a snake oil salesman
… generate code
– Paul S. R. Chisholm, AT&T 10/94


Depth of Patterns

Some patterns are at a very low-level:

An example solution: Use a shift to multiple or divide by a power of 2

Some are very general:

For example: Model View Controller


The Quality Without A Name

“There is a central quality which is the root criterion of life and spirit in a man, a
town, a building, or a wilderness [or software]. This quality is objective and
precise, but it cannot be named.”
– Alexander, The Timeless Way of Building

Who was Christopher Alexander?
Building architect and theorist, Professor at UC Berkeley www.patternlanguage.com
Many books on architecture and art:
Patterns: A Pattern Language, The Timeless Way of Building, The Oregon Experiment, The
Production of Houses
Art: A Foreshadowing of 21st Century Art -- The Color and Geometry of Very Early Turkish
Carpets
Beyond patterns: The Nature of Order


Patterns Handbook

Patterns build the handbook of software design and construction

Patterns available in reference book easily accessible to designers

No need to memorize all the details -- the reference book is available


The Pattern Almanac

Published 3/2000

Summarizes all the published,
widely available patterns
Indices based upon pattern
intent

Planned to be updated regularly
and to be available on the internet


What is a Pattern Language?

A collection of patterns that work together

A collection of patterns that build upon each other to build something bigger
than any of the individual patterns can build

“All acts of building are governed by a pattern language of some sort.”
– TTWOB, p 193


An Example Pattern Language

Telecom IO

A specialized set of patterns for defining the human-machine interface has
come into use with the world of embedded telecommunications products.
These patterns provide an essential interface between a system and its human
masters.


Telecom IO Pattern Intents
Alarm Grid Group alarms into grids to help the workers identify problems.
Audible Alarms Sound audible alarms to alert office personnel of problems.
Beltline Terminal Allow workers to take their terminals with them.
Don't Let Them Forget Reassert alarms when necessary, purposely forgetting requests to retire the
alarms.
Five Minutes of No Escalation Don't confuse craft with too frequent messages.
Messages
George Washington is Still Dead Issue state change messages only when the state changes, not to remind about
the current state.
IO Gatekeeper Put one process in charge of IO for the system.
IO Triage Add a priority tag to each output message and sort the output using them.
Mind Your Own Business Only send output to concerned terminal groups (logical channels).
MML Use a standardized IO language.
Office Alarms Allow the alarm system to be customizable with site specific alarms.
Pseudo-IO Provide for internal subsystems to add IO to the stream.
Raw I/O Provide a way for recovery systems to bypass the IO Gatekeeper.
Shut Up And Listen Give human input/output messages a high priority.
String A Wire Provide a system to system emergency information channel.
The Bottom Line Issue messages to summarize a number of events rather than for each of
many events.
Timestamp Add a timestamp and/or a sequence number to each output message.
Who Asked? for Reusable Design | June 2009
24 | Software Patterns Return output only toAlcatel-Lucent 2009channel/terminal that requested it.
All Rights Reserved © the logical

Telecom IO

PLoPD-4


What is a Software Pattern?

A description of a reusable proven design element

with enough information to recreate the design, customized for the situation

An element of a pattern language

A collection of patterns that work together to solve larger problems

The building block for a software handbook


Pattern Communities


Pattern Communities

The Hillside Group

A worldwide organization to make the life of people touching software better
and to sponsor pattern conferences. Hillside.net

Hillside Europe
A European group centered around Germany that sponsors conferences and SE
Radio. hillside.net/europlop/HillsideEurope, se-radio.net

HCI pattern community

www.hcipatterns.org – Jan Borchers

www.welie.com – Martijn van Welie


Patterns Community

Pattern movement origins
1989-1992
Advanced C++ Programming Styles & Idioms by James Coplien
OOPSLA
Hillside Group
Pattern Conferences
PLoP
hillside.net/plop
EuroPLoP
hillside.net/europlop
SugarLoaf PLoP
VikingPLoP
ChiliPLoP
KoalaPLoP
Using Patterns (UP)

“Culture”

“5a: the integrated pattern of human knowledge, belief, and behavior that
depends upon man's capacity for learning and transmitting knowledge to
succeeding generations

“5b : the customary beliefs, social forms, and material traits of a religious, or
social group

“5c : the set of shared attitudes, values, goals, and practices that characterizes
a company or corporation”
– Merriam Webster Collegiate Dictionary on the web http://www.m-
w.com/dictionary.htm

Shared experiences
Shared rituals
Writers’ Workshop
Shared values

Writers’ Workshops

Good patterns are highly reviewed and refined.

Writers’ Workshop from the literature/poetry community are used to review
patterns.

Structured discussion of the merits and suggestions for improvement.

Author is present for note taking but does not participate except at very
specific times.

A strong moderator role to guide discussion.

Authors are expected to revise and improve their patterns.


After writing, workshopping and revising: publish

Patterns are meant to be read!

Conference proceedings
hillside.net/conferences points to proceedings of PLoP and EuroPLoP
EuroPLoP, SugarLoaf PLoP and VikingPLoP publish hard copy proceedings
PLoP and EuroPLoP publish proceedings in ACM Digital Library.
Transactions on Pattern Languages of Programming
(hillside.net/tplop)

Books
hillside.net/patterns/books/index.htm for a partial index


Pattern Ethics

Buschmann’s rule: Never capture your own ideas in a pattern

- Focus on broad, lasting, positive patterns

Intellectual currency paradox: Ideas are worth more if given away

• The author should retain ownership and copyright

“Aggressive Disregard for Originality” (Brian Foote)

Let’s encourage people to be secure in telling their secrets

Let’s reward people who created these techniques or who first took the trouble
to commit them to writing

Patterns are solutions that have withstood the test of time

Don’t Hype!


Using Patterns


“Patterns Generate Architectures”

Johnson, OOPSLA ’92

“Documenting Frameworks using Patterns”

Documented a framework using patterns

Beck and Johnson ECOOP ’94

“Patterns Generate Architectures”

Derived an architecture for a HotDraw
interactive graphics system using patterns


Documenting Architecture

Patterns are useful to document built systems.

Allows maintenance engineers a snapshot of the designer’s thinking.

Points out to the future maintainers/enhancers where the “load bearing walls”
are.

Parnas & Clements: “A Rational Design Process: How and Why to Fake It”.

Example:

Architectural training for the 4ESS Switch project in Lucent Technologies
4ESS Switch design begun early 1970’s. First office application January 1976.
New enhancements are still being added.
Training uses patterns document key principles.


Assisting Design

If you were designing an alarm component for a system, the Telecom IO Alarm
patterns might be helpful.

Alarm Grid

Audible Alarm

Office Alarms

Don’t Let Them Forget

By examining these patterns you will see how to build such a component, and the
order in which you should think about using them.


Designing with Relevant Patterns

When starting a project that patterns and pattern languages might be useful for:

Create a list of the relevant patterns from the handbook (i.e. all available resources)

Example: to build a garden, Alexander suggests these patterns:

Half-Hidden Garden Building Edge
Terraced Slope Sunny Place
Fruit Trees Outdoor Room
Tree Places Six-Foot Balcony
Garden Growing Wild Connection to the Earth
Entrance Transition Greenhouse
Courtyards Which Live Garden Seat
Roof Garden TTWOB, p 310


Using a Pattern language … “Never the same way twice.”

Applying a pattern language is not a “turn the crank” exercise.

Every application of patterns or pattern languages must be customized for the
situation at hand.

Every situation poses unique problems.

Every situation requires a unique solution.

The pattern maps described earlier represent one way that the patterns can be
combined to create a solution.


Where do you find the patterns that you need?

Pattern Almanac

Lists all the patterns published up to mid-2000.

Web

Many websites contain links to patterns exist now.
hillside.net
www.welie.com
PLOP proceedings, TPLoP and PLOPD books.

Special topic books.

Basically: be familiar with the Handbook.


Writing and Collecting Patterns

Pattern mining

“… don’t count on being able to ‘mine’ a pattern language. A pattern language
comes together like a jigsaw puzzle from patterns that have been collected over
months, years or decades. At some point it becomes publishable, but its
evolution doesn’t cease at that same point.”
– Coplien, C++ Report: Pattern Languages


Digging deeper

What is required for this pattern to work?

What if absent would mean that this cannot work?

What’s missing?

“When every pattern has its principle components given by the smaller patterns
which lie immediately below it in the language, then the language is complete.”
– TTWOB, p 322.


Pattern Refactoring

Real Time and Resource Overload is an example of pattern refactoring.
It combines patterns from previous works into a language.

Meszaros, “A Pattern Language for Improving the Capacity of Reactive
Systems”, PLOPD-2, ch 35.
Hanmer and Wu, “Traffic Congestion”, PLoP 1999 Proceedings.

Real Time and Resource overload Workshopped at PLOP 2000 conference.

Refactored again and included in Patterns for Fault Tolerant Software.


Pattern Refactoring (2)

The pattern you are about to write to complete a language might already have
been written by someone else.

Can you cite it and move on?

Do you need to rephrase it (continuing to give the original authors credit of
course) to fit in your language?


Pattern Research
Current Efforts

The Open Problem


Handbook of Software Architecture

Grady Booch is collecting The Handbook of Software Architecture.
http://www.handbookofsoftwarearchitecture.com


HCI Repository
Martijn van Welie has a repository of HCI patterns that many say is the
cornerstone of the HCI community. http://www.welie.com


Open Problem

Finding appropriate patterns

Many sources: books, web, articles

Many patterns with the same name: Null object, Shed Load

No centralized coordination

Some attempts to resolve

Many student projects to build repositories

Repository sites that wither and die
?
2000 2010


Patterns in your Community


Bioinformatics Open Source Patterns

How can you make use of patterns to foster development of “better”
bioinformatics software?

Does that heading make sense?

What are the patterns that you have already mined?

What do you do with patterns?

Is there a repository?

Is there a “keeper” of the repository?


We’ve looked at

What pattern and pattern languages look like

Pattern communities

Some ways of using patterns

Some ideas about mining your patterns

The repository and indexing problem

Some thoughts about Bioinformatics patterns


Software Patterns for Reusable Design

A software pattern is a description of modular proven solution to a design
problem with enough information so that the reader can flexibly adapt it to their
unique situation.

A form of architectural documentation

An element of the pattern handbook

Is it part of the Handbook of Bioinformatics Open Source Software?

Questions?

References
Sources of more information


References (1)

Alexander, C. et. al. A Pattern Language. New York: Oxford University Press. 1977.
Alexander, C. The Timeless Way of Building. New York: Oxford University Press. 1979.
Beck, K. and R. Johnson, “Patterns Generate Architectures,” Object-Oriented
Programming 8th European Conference (ECOOP ’94), Springer-Verlag, 1994.
Coplien, J. O. Software Patterns. New York: SIGS Publications. 1996.
Gabriel, R. P. Patterns of Software. New York: Oxford University Press. 1996.
Gamma, E., R. Helm, R. Johnson and J. Vlissides. Design Patterns: Elements of Re-usable
object-Oriented Design. Addison-Wesley. 1994.

Hanmer, R. S. and G. Stymfal, “An Input and Output Pattern Language: Lessons from
Telecommunications”, in [14], pp 502-536.

Hanmer, R. S. Patterns for Fault Tolerant Software, Chichester: John Wiley & Sons. 2007.

Hanmer, R. S. “Using Patterns to Teach Software Architecture,” Proceedings of the 6th
Biennial World Conference on Integrated Design and Process Technology, Pasadena,
CA, June, 2002.


References (2)

Harrison, N., B. Foote, H. Rohnert, eds. Pattern Languages of Program Design-4.
Reading, MA: Addison-Wesley, 2000.
PLOPD – PLOPD5: Pattern Languages of Program Design, volumes 1-5. Various editors.
Reading, MA: Addison-Wesley, 1995-2006.
PLoP Conference Proceedings: http://hillside.net/plop/
POSA volumes: Pattern Oriented Software Architecture, volumes 1-5. Various authors.
Chichester, John Wiley & Sons, 1996-2007.
Rising, L. The Pattern Alamanac 2000. Reading, MA: Addison-Wesley, 2000.

Rising, L. The Pattern Handbook. Cambridge: Cambridge University Press, 1998.

For more books see: http://hillside.net/patterns/books/index.htm
For pattern mailing lists see: http://hillside.net/patterns/mailing.htm


www.alcatel-lucent.com
www.alcatel-lucent.com


Hanmer Software Patterns Bosc2009

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