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Christopher Alexander Et Al, a Pattern Language- Towns.Buildings PLoP® Pattern Writing Bootcamp 2012 Natal - Brazil Joseph (Joe) Yoder [email protected] Mary Lynn Manns [email protected] Rosana Braga [email protected] September 21st, 2012 - 1 PLoP is a registered Trademark of The Hillside Group. Copyright Joseph Yoder & Refactory Inc. Objectives • At the end of this “bootcamp” you will: – Have knowledge about pattern writing – Have a better understanding of the “Quality Without A Name” – Have helped write a pattern – Be the newest members of the pattern community! Thanks Rebecca Thanks Linda & Bob September 21st, 2012 - 2 Agenda • Introductions • Why Patterns? • Discussion of What’s a pattern? • Parts of a pattern • Patterns criticism and categorization • Choosing a topic • Group Pattern Writing • Group Pattern Writing Continued • Sample Writers’ Workshop • Pattern Languages • Pattern Ethics September 21st, 2012 - 3 Introduction to Patterns September 21st, 2012 - 4 The Reason for Patterns • Experts at work often use their knowledge of how they solved similar problems to solve the current problem. • What if they could share their knowledge with others? • Pattern descriptions record experts’ wisdom so others can reuse it. September 21st, 2012 - 5 An Example of a Pattern* Window place *Christopher Alexander et al, A Pattern Language- Towns.Buildings. Construction, 1977 September 21st, 2012 - 6 The Window Place Pattern • Everybody loves window seats, bay windows, and big windows with low sills and comfortable chairs drawn up to them…A room without a place like this seldom allows you to feel comfortable • If the room contains no window which is a “place”, a person will be drawn between: 1. Wanting to sit down and be comfortable 2. Being drawn toward the light • If comfortable places are away from the windows, there is no way of overcoming this conflict… • Therefore: In every room where you spend any length of time during the day, make at least one window into a “window place” September 21st, 2012 - 7 Part 1: Pattern Basics September 21st, 2012 - 8 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 September 21st, 2012 - 9 Christopher Alexander • Building architect and theorist • Many books on architecture and art: A Pattern Language The Timeless Way of Building The Oregon Experiment The Production of Houses A Foreshadowing of 21st Century Art -- The Color and Geometry of Very Early Turkish Carpets The Nature of Order (4 volumes) • http://www.livingneighborhoods.org September 21st, 2012 - 10 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. This quality is objective and precise, but it cannot be named.” – Alexander, The Timeless Way of Building • (“but with an acronym” -- Joe Davison) September 21st, 2012 - 11 What is a Pattern Quite often you hear: “A pattern is a proven solution to a problem in a context.” Alexander writes: “Each pattern is a three-part rule, which expresses a relation between a certain context, a problem, and a solution.” Are these definitions the same? September 21st, 2012 - 12 What is a Pattern It must be a solution to a problem in a context You must be able to tell the problem solver what to do, how to solve the problem It must be a mature, proven solution (Rules of 3) It must contribute to human comfort or quality of life It must be something you didn’t invent yourself (Buschmann’s Rule) The solution should build on the insight of the problem solver, and can be implemented a million times without being the same twice It cannot be formalized or automated (if it can, do that instead of writing a pattern) It should have a dense set of interacting forces that are independent of the forces in other patterns September 21st, 2012 - 13 Patterns Are: • A form of architectural documentation • An architectural relationship 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 • A resolution of forces September 21st, 2012 - 14 A Patterns Should • Provide Facts (ref manual) about solving problem • Be prescriptive about the solution • Tell a good story which captures the experience of the experts • Help to comprehend existing systems • Help to construct new systems September 21st, 2012 - 15 What is not a Pattern Just a simple solution to a problem in a context • A simple rule • A prescriptive recipe • An algorithm • A data structure September 21st, 2012 - 16 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 September 21st, 2012 - 17 Part 2: A Pattern’s Parts September 21st, 2012 - 18 Alexander’s Pattern Definition Each pattern describes a problem that occurs over and over again in our environment and then describes the core of the solution to that problem in such a way that you can use this solution a million times over without ever doing it the same way twice. Alexander - building architect and author – The Timeless Way of Building – A Pattern Language September 21st, 2012 - 19 Parts of a Pattern (Alexander) Problem - when to use the pattern Solution - what to do to solve problem Context - when to consider the pattern Forces - pattern is a balance of forces Consequences, positive and negative Examples: – Teach both problem and solution – Are the best teacher – Are proof of pattern-hood September 21st, 2012 - 20 Small Meeting Rooms September 21st, 2012 - 21 Small Meeting Rooms The larger meetings are, the less people get out of them. But institutions often put their money and attention into large meeting rooms and lecture halls. It has been shown that the number of people in a group influences both the number who never talk, and the number who feel they have ideas which they have not been able to express. There is no particularly natural threshold for group size; but it is clear that the number who never talk climbs very rapidly [with group size]. In a group of 12, one person never talks. In a group of 24, there are 6 people who never talk. Make at least 70% of all meeting rooms really small -- for 12 people or less. Locate them in the most public parts of the building, evenly scattered among the workplaces. See: Light on Two Sides of Every Room, Sitting Circle, Different Chairs, Pools of Light, The Shape of Indoor Space September 21st, 2012 - 22 Small Meeting Rooms September 21st, 2012 - 23 Design Patterns “Design Patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.” —Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software September 21st, 2012 - 24 Parts of a Pattern in Design Patterns (Also known as GOF for Gang of Four; Gamma et. al.) Intent - brief description of problem and solution Also Known As Motivation - prototypical example Applicability - problem, forces, context Structure/Participants/Collaborations - solution Consequences - forces Implementation/Sample Code - solution Known Uses Related Patterns September 21st, 2012 - 25 Adapter Intent – Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. Target Adapter Adaptee – Use Interfaces in Java and C# to get the equivalence look of multiple inheritance September 21st, 2012 - 26 Patterns-Oriented Software Architecture (POSA) Pattern Form Name, Aliases Brief Description Example Implementation Context Example Resolved Problem Variants Forces Known Uses Solution Consequences Structure Related Patterns Dynamics Credits September 21st, 2012 - 27 POSA Example: Pipes and Filters Brief Description: “The Pipes and Filters architectural pattern provides a structure for systems that process a stream of data. Each processing step is encapsulated in a filter component. Data is passed through pipes between adjacent filters. Recompiling filters allows you to build families of related systems.” September 21st, 2012 - 28 POSA Example Pipes: and Filters Structure Filter Pipe Class or component •Gets input data Pipe •Transfers data Data •Performs a •Buffers data Source function on the •Synchronizes Data Sink data active neighbors Filter •Supplies output Responsibilities Collaborators data Data Source Data Sink •Consumes output Pipe •Delivers input to Pipe processing pipeline September 21st, 2012 - 29 POSA Example: Pipes and Filters Dynamics Filter 1 Buffering Data Source Filter 2 Data Sink Pipe read read data f1 write filtered data read write synch data f1 write filtered data write synch data f2 write filtered data September 21st, 2012 - 30 Example Pattern Form Name, Aliases Context Forces Problem Solution Resulting Context (Consequences) Rationale Related Patterns Known Uses Sketch Author References Examples September 21st, 2012 - 31 Pattern Example How long should pattern sections be? As short as possible but no shorter! Albert Einstein and David Parnas September 21st, 2012 - 32 Name Word or short phrase—the essence of the pattern, some say noun phrase. Naming is not trivial and very important. Good names enhance communication—especially when you can guess the intent from the name. Patterns “build” something. The name should say what the pattern builds. Iterator, Adaptor, Do Food, … September 21st, 2012 - 33 Aliases The same pattern may exist elsewhere, at another company, in a publication.
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