6.005 18. Designing a Photo Organizer

MIT OpenCourseWarehttp://ocw.mit.edu 6.005 Elements of Software Construction Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 6.005elements ofsoftwareconstruction how to design a photo catalog Daniel Jackson topics for today a problem � conceptual design of a photo organizer a new paradigm � computation over relational structures � today, the abstract design level: object modelling � determines, in particular, model part of MVC (see last lecture) object modelling � snapshot semantics � basic notation: domain/range, multiplicity, classification � some classic patterns © Daniel Jackson 2008 2 the problem problem Screenshot of Adobe Photoshop Lightroom removed due to copyright restrictions. In the Library view, you can select images to add or remove. The left-hand sidebar includes Collections that you can define. design a photo cataloguing application � Lightroom, iView MediaPro, iPhoto, Aperture, Picasa, etc © Daniel Jackson 2008 4 what kind of problem is this? mostly about conceptual design � what are the key concepts? � how are they related to one another? � what kinds of structures? good conceptual design leads to � straightforward path to implementation � simplicity and flexibility in final product © Daniel Jackson 2008 5 why a new model? why not use datatype productions? � tree-like structures only: no sharing � immutable types only why not state machines? � our catalog is a state machine � but the problem lies in the structure of the state � our state machine notation assumed simple states a new approach: object models � structure is a labelled graph � put another way: sets of objects + relations © Daniel Jackson 2008 6 the relational paradigm computation is about � actions, states, transitions � functions, expressions, values � and now: updates and queries on relations why is this useful? � conceptual modeling � relational databases � object-oriented programming* � semantic web, document object models, etc *for proposals to make relations explicit in object-oriented programming, see this survey: James Noble, Roles and Relationships, ECOOP 2007 Workshop on Roles and Relationships in Object-Oriented Programming, Multiagent Systems, and Ontologies; http://iv.tu-berlin.de/TechnBerichte/2007/2007-09.pdfbasic OM notation snapshots a snapshot or object diagram � shows a single instance of a structure example for photo organizer � in this case, two sets Photo (shown in beige) Collection (in grey) � and two relations photos: Collection -> Photo subs: Collection -> Collection 9 a collection a relationship: C0 is subcollection of C1 a photo a relationship: P0 in collection C2 © Daniel Jackson 2008 more snapshots how can we summarize this infinite set? © Daniel Jackson 2008 10 an object model each box � denotes a (maybe empty) set of objects each arc � denotes a relation, ie. set of links between objects note � objects have no internal structure! � all structure is in the relations exercise � draw a snapshot that the OM rules out © Daniel Jackson 2008 11 Collection Photo subs photos enriching the notation what’s wrong with these snapshots? � how would we rule them out? key idea: multiplicity � measure the in-degree and out-degree of each relation © Daniel Jackson 2008 12 multiplicity A BR m n multiplicity markings � on ends of relation arc � show relative counts interpretation � R maps m A’s to each B � R maps each A to n B’s marking/meaning + one or more * zero or more ! exactly one ? at most one omitted marking equivalent to * © Daniel Jackson 2008 13 R kinds of function standard kinds of function � easily expressed with multiplicities ? A B A R BR R is a function ! R is a total function ? ! A B A R BR R is an injection + ! R is a surjection ! ! A B R is a bijection © Daniel Jackson 2008 14 multiplicity example we’ve added naming � always an important and subtle issue � is the multiplicity constraint desirable? necessary? Collection Photo subs photos Collection Name name ?! * * ? © Daniel Jackson 2008 15 classifying objects suppose we to classify photos � by file location: online, offline, missing � by selection: selected, focus Photo Photo Offline Missing Online Selected Focus oval means singleton set © Daniel Jackson 2008 16 classification syntax can build a taxonomy of objects � introduce subsets � indicate which are disjoint � and which exhaust the superset A B A B C abstract A B C B 㱪B 㱯 C = 㱵B 㱮 C = A © Daniel Jackson 2008 17 relations on subsets when placing a relation � can place on subset � loose multiplicity is a hint Photo Filepath path ! Offline Missing Online Volume Date vol since ! ! © Daniel Jackson 2008 18 composite a classic pattern � hierarchical containment � file systems, org charts, network domains, etc you’ve seen this with datatypes � technical differences though � OM allows cycles (but often rule out) � OM can say just one root Collection FolderFolio subs Photo photos © Daniel Jackson 2008 19 hotel locking example: hotel locking modelling physical, distributed state state in OM need not represent � a centralized store � data stored in a computer © Daniel Jackson 2008 21 hotel locking recodable locks (since 1980) � new guest gets a different key � lock is ‘recoded’ to new key � last guest can no longer enter how does it work? � locks are standalone, not wired a recodable locking schemecard has two keysif first matches lock,recode with secondif second matches,just openexercise draw an object model � showing the essential state of hotel locking � state includes front desk, locks, keys held by guests review � did you exploit multiplicities? keys are all about uniqueness � did you include only the sets and relations that are needed? � are your sets really sets, or are some of them ‘singleton placeholders’? � do all your sets and relations have a clear interpretation? � where are the various parts of the state stored physically? � which relations are modifiable? a solution Guest Card holds Room Key key occupies fst, snd ? ! ! Issued g->r in occupies: guest g has checked in for room r but has not yet checked out k in Issued: key k has already been issued by front desk on some card: used to ensure that locks are always recoded with fresh keys some subtleties � guest may occupy more than one room � family members may have identical cards common errors Desk Key issues Card Key has Hotel Guest guests Key Snd Fst be wary of top-level singleton � Desk and Hotel not needed relations represent state, not actions � so issues is suspect need enough information in state to support application � has is not enough: need to know which key is first, second scope of classification � classification of keys into first and second, is by card, not global � so need relation, not subsets to indicate the distinction colour palettes example: colour palettes modelling the state of an application � how colours are organized Screenshots of color schemes in the Keynote and PowerPoint presentation programs removed due to copyright restrictions. essential idea � elements are coloured � can assign colour from palette � gives consistent appearance © Daniel Jackson 2008 28 colour colour colour palette object models three subtly different approaches � think what happens when palette is modified � hard vs. soft links: as in Unix palette Doc Element elements Palette Swatch swatches palette Doc Element elements Palette Swatch swatches value key lcolour palette Colour value Name key Colour value lcocolour Doc elements Palette Swatch swatches Element Name keylcolour Name Colour “Every problem in computer science can be solved by introducing another level of indirection” --David Wheeler © Daniel Jackson 2008 29 completing the organizer issues to resolve can collections hold photos and subcollections? � decision: yes, so not Composite pattern how are “all photos” in catalog represented? � decision: introduce non-visible root collection unique collection names? � decision: file system style, so siblings have distinct names do parents hold children’s photos? � in logic: all c: Collection | c.subs.photos in c.photos ? � decision: use two relations instead c.inserted: the photos explicitly inserted into collection c c.photos: the photos in collection c implicitly and explicitly invariant relates these: c.photos = c.inserted + c.subs.photos © Daniel Jackson 2008 31 final object model Collection Visible subs Photo inserted, photos Root Selected Image image ! Collection Namename ! ! additional constraints � all collections reachable from root (implies acyclic) Collection in Root.*subs � implicit photos are inserted photos plus photos in subcollections all c: Collection | c.photos = c.inserted + c.subs.photos � names unique within parent all c: Collection | no c1, c2: c.subs | c1 != c2 and c1.name = c2.name © Daniel Jackson 2008 32 modeling hints hints how to pick sets � be as abstract as possible (thus Name, not String; SSN, not Number) � but values to be compared must have same type (so Date, not Birthday) � beware of singletons --often a sign of code thinking how to pick relations � represent state, not actions (so atFloor: Elevator->Floor, not arrives) � direction is semantic; doesn’t constrain ‘navigation’ choosing names � choose names that make interpretation clear � include a glossary explaining what relations and sets mean © Daniel Jackson 2008 34 summary principles data before function � before thinking about system function, think about data an object model is an invariant � meaning is set of structured states � declared sets + subset relationships + relations between sets + multiplicities � augment diagram with textual constraints (in Alloy, as above, or just English) model objects are immutable � all state kept in subsets and relations � model objects have no ‘contents’ � important to keep coding options open © Daniel Jackson 2008 36