PATTERNS. A Short Course. Juha Pärssinen, Senior Research Scientist VTT Technical Research Centre of Finland

Transcription

1 PATTERNS A Short Course Juha Pärssinen, Senior Research Scientist Juha.Parssinen@vtt.fi VTT Technical Research Centre of Finland 1 (43)

2 Presentation outline What is a pattern? - What are benefits of patterns? - What is a good pattern? - What is not a pattern? What is a pattern language? What kind of pattern forms there are? What is an essential pattern form? How to find patterns? How to read a pattern? How to write a pattern? How to improve a pattern? References Appendix: Pattern Example Appendix: Exercise for reader 2 (43)

3 What is a pattern? Some well-known quotes from original founder of patterns: Each pattern is a three-part rule, which express a relation between a certain context, a problem, and a solution. Christopher Alexander, Timeless Way of Building, 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, Timeless Way of Building, Each pattern describes a problem which 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 doing it the same way twice. Christopher Alexander, Sara Ishikawa, and Murray Silverstein, A Pattern Language: Towns, Buildings, Construction, (43)

4 What is a pattern (cont.)? Patterns are more like recipes than plans: I like to make the distinction between a plan and a recipe. A plan can be reverse engineered from a building but a recipe can t (easily) be reverse engineered from a cake. Our genome is a recipe, not a plan. Recipes seem to serve better as a schema of complex adaptive systems. Ward Cunningham, 20 February One of the first definitions of patterns in the area of computer science: A design pattern names, abstracts, and identifies the key aspects of a common design structure that make it useful for creating a reusable object-oriented design. The design pattern identifies the participating classes and instances, their roles and collaborations, and the distribution of responsibilities. Each design pattern focuses on a particular object-oriented design problem or issue. It describes when it applies, whether it can be applied in view of other design constrains, and the consequences and trade-offs its use. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides Design Patterns - Elements of Reusable Object-Oriented Software, (43)

5 What is a pattern? An example. Each pattern is a three-part rule, which express a relation between a certain context, a problem, and a solution. Christopher Alexander, Timeless Way of Building, Simple pattern example to clarify this rule [8]: Lecture s life Context Problem Solution A class of water, a cup of coffee, or a bottle of beer. 5 (43)

6 What is a pattern? An example (cont.). Lecture s life Context Problem Lecturer is thirsty. Solution A class of water, a cup of coffee, or a bottle of beer. 6 (43)

7 Context What is a pattern? An example (cont.). In a lecture hall at 10 a.m. In a cafe room at 2 p.m. After work on Friday at 6 p.m. Problem Lecturer is thirsty. Solution Lecture s life A class of water in a lecture hall, a cup of coffee in a cafe room, or a bottle of beer after work on Friday. 7 (43)

8 What is a pattern? An example (cont.). Actually this is a pattern language (or system) of three patterns: - on duty, at break and off duty. - Pattern languages are discussed in section "What is a pattern language?". Context In a lecture hall at 10 a.m. Problem Lecturer is thirsty. Solution Drink a class of water. Lecture s life: on duty 8 (43)

9 What is a pattern? Lecture s life: at break Context In a cafe room at 2 p.m. Problem Lecturer is thirsty. Solution Drink a cup of coffee. An example (cont.). Lecture s life: off duty Context After work on Friday at 6 p.m. Problem Lecturer is thirsty. Solution Drink a bottle of beer. 9 (43)

10 What are benefits of patterns? Some well-known benefits of patterns [1]: forms a common vocabulary in an application domain; make design-level reuse possible; provide a road map to the structure of existing system; helps designer to avoid wrong design decisions; can be used as a teaching aid for newcomers. 10 (43)

11 A good pattern [1]: What is a good pattern? must be a proven solution to a problem in a context; - Must be mature (rule of three). - Unproven patterns are sometimes called as proto-patterns or patlets. must be able to tell the problem solver what to do to solve the problem; should have a dense set of interacting forces that are independent of the forces in other patterns; tend to have geometric overtones; must contribute to human comfort or quality of life: - this is possible even in software, the quality without name [7]. 11 (43)

12 What is a good pattern (cont)? Any good pattern has the quality without name [7]. Words to describe this quality (and to ultimately confuse): alive, whole, comfortable, free, exact, egoless, and eternal. Some things about software that possesses this quality [3]: - It is not written to an unrealistic deadline. - Its modules and abstractions are not too big. - Any bad parts were repaired during maintenance, or are being repaired now. - If I look at any small part of it, I can se what is going on. I don t need to look other parts to understand. - If I look at any large part in overview, I can see what is going on. I don t need to know details. - Everything about it seems familiar. - I can imagine changing it, adding some functionality. 12 (43)

13 A pattern is not [1]: What is not a pattern? a simple rule; - Obvious problem/solution pair is usually just a rule of thumb. a prescriptive example; an algorithm; a data structure; an isolated solution to a problem in a context. 13 (43)

14 What is not a pattern? (cont.) One possible test for "pattern-ness": - However, this is not a definitive test. A pattern-like entity that exhibits these characteristics is not definitely a pattern [9]. Nine essential characteristics of a pattern [9]: - a pattern implies an artifact; - a pattern bridges many levels of abstraction; - a pattern is both functional and nonfunctional; - a pattern is manifest in a solution; - a pattern captures system hot spots; - a pattern is part of a language; - a pattern is validated by use; - a pattern is grounded in a domain; - a pattern captures a big idea. Any entity that does not exhibit any one or more of the essential characteristics is not a pattern [9]. 14 (43)

15 What is a pattern language? No pattern is an isolated entity. Each pattern can exist in the world, only to the extent that it is supported by other patterns: the larger patterns in which it is embedded, the patterns of the same size that surround it, and the smaller patterns which are embedded in it. Christopher Alexander, Sara Ishikawa, and Murray Silverstein, A Pattern Language: Towns, Buildings, Construction, A pattern language is a collection of patterns that build on each other to generate a system. A pattern in isolation solves an isolated design problem: a pattern language builds a system. A pattern language has structure, but not the same level of formal structure that one finds in programming languages. A pattern language is typically designed for well-constrained application domain. 15 (43)

16 What kind of pattern forms there are? The form of an individual pattern is typically based at least one of these: Alexandrian Form [6] - No separate sections. - Only syntactic structure: Therefore GOF Form [10] - Contains 13 separated sections, pattern description tend to be very long. - Object-oriented software. The Portland Form - Ward Cunningham s on-line repository: the Portland Pattern Repository. The Coplien Form [1] - This is close to essential form presented next. 16 (43)

17 What is an essential pattern form? Name Context Problem Forces Solution Resulting Context 17 (43)

18 What is an essential pattern form (cont.)? Name Name a pattern with the same care as for a firstborn child. Names are often noun phrases. Pattern names tend to be based on the solution or problem. Short, concise, semantically rich, revealing or suggestive. Clever names are memorable - but don t get too clever. The pattern names should become part of the domain vocabulary. 18 (43)

19 What is an essential pattern form (cont.)? Context Defines when a pattern is applicable. Location of the pattern in the pattern language. Context includes a history of patterns that have been applied before the current pattern was considered. A good context is typically difficult to define. 19 (43)

20 What is an essential pattern form (cont.)? Problem Usually stated as a question, or as a problem statement. Usually the first thing a solution-seeker looks at. An understanding of the problem comes from an analysis of the forces. Some pattern forms reduce the problem to a single question or a summary formulation of the problem. In other forms, the problem statement is a small essay that motivates or illustrates a need. 20 (43)

21 What is an essential pattern form (cont.)? Forces The heart of a pattern. A whole is field of forces, or trade-offs. Individual patterns encapsulated related forces. Forces answer questions: What makes this a hard problem? What factors make the obvious solution the wrong one? Contradiction is O.K - conflict is great! 21 (43)

22 What is an essential pattern form (cont.)? Solution... Ken Auer writes that the solution statement should have the feeling that the reader has just been hit in the face, "Boom!"... Norm Kerth [5] The solution should solve the problem stated in the problem section. Solution balance forces. Some patterns provide only partial solutions but open a path to other patterns that balance unresolved forces. A good solution has enough detail so the designer knows what to do, but it is general enough to address a broad context. 22 (43)

23 What is an essential pattern form (cont.)? Sketch: Solution (cont.) If you can t draw a diagram of it, it isn t a pattern. The sketch usually conveys structure. Christopher Alexander, Timeless Way of Building, Class and object diagrams are typically used to present example solution structures in object-oriented design. Interaction diagrams that illustrate event ordering or other dynamics are equally useful. 23 (43)

24 What is an essential pattern form (cont.)? Resulting Context The resulting context is the wrap-up of the pattern. It tells: which forces were solved; which new problems may arise because of this pattern; what related patterns may come next. Ties patterns together into a pattern language. 24 (43)

25 Books: Pattern Almanac [17]; How to find patterns? PLoPD 1-4 [13][14][15][16]; GOF [10], POSA [11], and POSA2 [12]. On-line resources: URL: - On-line pattern catalog; - PLoP conferences on-line proceedings; - Pattern mailing lists. News groups: - comp.software.patterns (started at 11th October 2001) - comp.object 25 (43)

26 How to read a pattern? Abstract/Context oriented approach Browse patterns reading their name and abstract (if there is one), or read context, if there is no abstract. Read problem and solution together. Read other parts. Problem oriented approach Browse patterns reading their name and problem briefly. Read context, and then problem and solution together. Read other parts. 26 (43)

27 Approach vary with forms. How to write a pattern? Linear approach is rarely successful. Often good to start with forces, or maybe with solution. Name and problem can become later. Next five slides from page 28 to page 32 present a set of questions which can be used to find sections of patterns descriptions. 27 (43)

28 How to write a pattern (cont.)[4]? During writing process of your pattern, answer following questions: When is the reader ready to consider this piece of wisdom? What decisions have they had to make, in order for this pattern to be useful to them? What other conditions must be present for this to be a valid pattern for the reader to consider? This leads to the CONTEXT. - When is this pattern applicable. 28 (43)

29 How to write a pattern (cont.)[4]? During writing process of your pattern, answer following questions: To wisely decide on a particular design decision, what are all the elements that need to be considered? What are the various problems that all need to be considered in order for your reader to choose wisely? This leads to the FORCES. - What to consider when focusing one s attention to this design decision, i.e., discuss all the facets of the design decision from the perspective of understanding the problem. 29 (43)

30 How to write a pattern (cont.)[4]? During writing process of your pattern, answer following questions: What solution do you recommend? What are the various facets? What are the variations? What are the tests your reader can use to affirm they have chosen a wise design? This leads to the SOLUTION. - Discuss the solution and solution options given the problem highlighted in the FORCES section. - Refer to the items in the FORCES section as needed. - Make sure that you have addressed each of the forces in your SOLUTION section. 30 (43)

31 How to write a pattern (cont.)[4]? During writing process of your pattern, answer following questions: Given the answers to the questions 1, 2, and 3, ask: How do we name this information in such a way that our wisdom will be memorable, in the sea of all the other wisdom out there? This leads to the NAME. 31 (43)

32 How to write a pattern (cont.)[4]? During writing process of your pattern, answer following questions: With these design issues resolved, what are the next important design issues to consider? This gives the place of your pattern in the pattern language. 32 (43)

33 How to improve a pattern? Shepherding process of PLoP conferences Goals [18]: - Obvious: Improve the work. - Not Obvious: Instill the patterns culture. - Giving to the community - Protecting the dignity of the author Three Facets of Shepherding [18]: the logistics of shepherding; - Three Iterations pattern interpersonal relationships; - The Shepherd Knows the Sheep pattern the content of the patterns. - Big Picture pattern 33 (43)

34 How to improve a pattern (cont.)? This is an example pattern from The Language of the Shepherds by Neil Harrison [18]. Three Iterations The amount of work required to shepherd any work is an unknown quantity a prior. Therefore: Get started early, and plan on about three iterations. Pace yourself to be able to complete three iterations. 34 (43)

35 How to improve a pattern (cont.)? This is an example pattern from The Language of the Shepherds by Neil Harrison [18]. The Shepherd Knows the Sheep There is a danger that the author will not accept - or understand - your comments. Therefore: Establish a personal rapport with the author up front; get to know the author, and tell the author about yourself. Give both positive reinforcement and suggestions for improvement. 35 (43)

36 How to improve a pattern (cont.)? This is an example pattern from The Language of the Shepherds by Neil Harrison [18]. BigPicture It s hard to know where to start with a pattern. Therefore: Get the big picture by reading the problem and solution together first. They should pretty much stand on their own. 36 (43)

37 How to improve a pattern (cont.)? Pattern writers workshops The pattern is discussed by a group of people including its author, a moderator, and a group of reviewers - who are familiar with paper. All participants are authors by themselves. Circle organization. Structure of a workshop (one paper / ,5 hours) [19]: - Welcome the Author - Author reads selection - Summarization - Positive feedback first - Suggestion for improvement - Author asks for clarification - Thank the author 37 (43)

38 References [1] J. Coplien, Software Patterns, Lucent Technologies, [2] J. Coplien, N. Gaerther, Mastery of Pattern Concept thought Pattern Writing, course material, Lucent Technologies, [3] R. Gabriel, Patterns of Software, Oxford University Press, [4] N. Kerth, personal communication, 17th April [5] N. Kerth, personal communication, 7th May [6] C. Alexander, et al. A Pattern Language, Oxford University Press, [7] C. Alexander, The Timeless Way of Building, Oxford University Press, [8] D. Deugo, an example in the workshop at PLoP2001, Monticello, 13th September (43)

39 References (cont.) [9] T. Winn, P. Calder, Is This a Pattern?, IEEE Software, January/ February [10]E. Gamma, R. Helm, R. Johnson, J. Vlissides, Design Patterns - Elements of Reusable Object-Oriented Software, Addison-Wesley, [11]F. Buschmann, R. Meunier, H. Rohnert, P. Sommerlad, M. Stal, Pattern-Oriented Software Architecture: A System of Patterns, Wiley, 1996 [12]D. Schmidt, M. Stal, H. Rohnert, M. Stal, F. Buschmann, Pattern- Oriented Software Architecture Volume 2: Patterns for Concurrent and Networked Objects, Wiley, 2000 [13]J. Coplien, D. C. Schmidt (eds), Pattern Languages of Program Design, Addison-Wesley, (43)

40 References (cont.) [14]J. Vlissides, J. Coplien, N. Kerth (eds), Pattern Languages of Program Design 2, Addison-Wesley, [15]R. Martin, D. Riehle, F. Buschmann (eds), Pattern Languages of Program Design 3, Addison-Wesley Longman, [16]N. Harrison, B. Foote, H. Rohnert (eds), Pattern Languages of Program Design 4, Addison-Wesley Longman, [17]L. Rising, The Pattern Almanac 2000, Addison-Wesley, [18]N. Harrison, The Language of the Shepherds, PLoP 99 proceedings, [19]J. Coplien, B. Woolf, A Pattern Language for Writer s Workshops, Pattern Languages of Program Design 4, pp , Addison-Wesley Longman, (43)

41 Name. Hands In View [1] Appendix: Pattern Example Problem. The skier fails to commit downhill on steeps and bumps, resulting in slides, backward falls,andyardsales. Context. In order to explore the entire mountain environment, a skier must be comfortable and adaptable to any terrain and rapid terrain change.to take advantage of this pattern the skier should be skiing at a level at which parallel turns can be linked consistently. Forces. Fear of falling is the most basic of all responses. Reliance on equipment is essential. Continuous movement is essential. Fatigue can be a factor in long descents. Commitment downhill over skis is essential for skis to function as designed Solution. Concentrate on keeping the hands in view. Bring then into sight immediately after each pole plant and turn. Resulting Context. Keeping the hands in view changes the alignment of the body from sitting timidly back and allowing the edges to skid out from under the skier. Thus, keeping the hands in view pulls the body forward and thus downhill, bringing the skier s weight over the downhill ski, forcing the edge to bite and turn. 41 (43)

42 Appendix: Pattern Example (cont.) Rationale. As steepness increases, the natural tendency of any sane person is to sit back against the hill and retain the perpendicularity the inner ear prefers. Unfortunately, skis must be weighted to perform as designed, the weight causing flex, which in turn pushes the edges into the snow in an arc, making a turn. Therefore it is essential to throw oneself down the mountain and over the skis, depending on them to catch the fall as they bite into the snow to turn underneath the perpetually falling skier. Intellectually this can be clearly understood but fear prevents execution. Concentrating on something as simple and indirect as look an your hands causes the desired behavior without directly confronting the fear. This is directly analogous to what occurs when an individual walks: the weight is thrown forward in a fall, with the consequent forward thrust of the leg to catch this fall, repeated for left and right sides in a continuous tension and release of yielding to gravity in order to defy it. Author. Don Olson 95/07/07 Originator. Anonymous ski instructor somewhere in Utah. Wherever you are, thanks for providing the breakthrough to better skiing for the author. 42 (43)

43 Appendix: Exercise for reader Write one or more patterns about something you know well (1 hour). Subject can be practically anything. You are free to use any pattern format you like. - However, Coplien or Alexandrian are recommend... Use questions from How to write a pattern? on page 27 [4]. Present your pattern briefly ( min.), or if possible, organize a patterns writer s workshop [19]. 43 (43)