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

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

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.” 7 | Software Patterns for Reusable Design | June 2009 All Rights Reserved © Alcatel-Lucent 2009

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

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

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

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

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 …” 14 | Software Patterns for Reusable Design | June 2009 All Rights Reserved © Alcatel-Lucent 2009

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Handbook of Software Architecture Grady Booch is collecting The Handbook of Software Architecture. http://www.handbookofsoftwarearchitecture.com 47 | Software Patterns for Reusable Design | June 2009 All Rights Reserved © Alcatel-Lucent 2009

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

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

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

Wrapup

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

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

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

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

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