Introduction to Domain Driven Design - Entity and Value Object

Introduction to Domain Driven Design - Entity and Value Object

Slides from my talk at the Domain Driven Design Paris User Group in February 2013 hosted by Xebia
http://www.meetup.com/DDD-Paris/events/103367252/

6c7fb4b051926511027cec4116cfe8cf?s=128

Jérémie Grodziski

February 26, 2013
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Transcript

  1. DDD  User  Group  Paris   «  Introduc+on  to  Domain  Driven

     Design  »   «  En3ty  and  Value  Object  »   Février  2013  
  2. #whoami   Jérémie  Grodziski   So>ware  Designer  and  Programmer  

    @jgrodziski   jeremie@grodziski.com   blog.zenmodeler.com   www.redsen.fr/blog    
  3. DOMAIN  DRIVEN  DESIGN  -­‐   INTRODUCTION  

  4. Why  Domain  Driven  Design?   DDD’s  goal  is  to  create

     beEer  soFware  more   easily  by  focusing  on  the  domain  instead  of  the   technology   A  so>ware  is  good  if  its  capacity  to  process   problem  related  to  the  domain  is  good   – When  the  domain  is  complex  the  so>ware  is  a  lot   more  difficult  to  build  and  maintain.  DDD  brings   solu+ons  to  manage  the  domain  and  so>ware   complexity.  
  5. Domain?   That  knowledge  have  to  be  handled  and  automated

     by   the  so>ware  which  helps  solving  problem  related  to  the   domain   –  Examples  :  front-­‐office  finance,  tax,  e-­‐commerce,   accoun+ng,  so>ware  (IDE,  Modeler)  ,  electronics,  etc.   The  domain  represents  a  sphere  of  knowledge  
  6. DDD  in  a  nutshell   An  Approach   • Design  is

     driven  by   Models  located  in   par+cular  Contexts.   Models  are  explorated   in  a  crea+ve   collabora+on  of   domain  and  so>ware   prac++oners.   • An  Ubiquitous   Language  is  used  to   communicate   between  the  technical   and  the  business  side   Building  blocks   • En+ty   • Value  Object   • Service   • Aggregate   • Repository   • Factory   • Domain  Event   • Layered  Architecture   Design  principles   • For  design   suppleness  :  Supple   design   • To  preserve  the   model’s  integrity,   manage  the  design   effort  and  the   priority  :  Strategic   design  
  7. AN  APPROACH   An   Approach   Building   Blocks

      Supple   Design   Strategic   Design  
  8. Model-­‐Driven  Design?   There  must  be  a  straighborward  and  very

     literal  way  to   represent  the  domain  model  in  terms  of  so>ware   –  The  model  should  balance  these  2  requirements:  form   the  ubiquitous  language  of  the  development  team   and  be  representable  in  code   Tightly  rela3ng  the  code  to  an  underlying  model   gives   the   code   meaning   and   makes   the   model   relevant   Changing  the  code  means   changing  the  model   Refining  the  model  requires   a  change  to  the  code    
  9. Ubiquitous  Language?   A   language   structured   around

      the   domain   model   and   used   by   all   team   members   to   connect   the   ac3vi3es   of   the   team   with   the   soFware.  
  10. Design?   Design  is  both  an  ac3vity  and  an  artefact

      Design  ac3vity  is  made  of  every  decision  made  during  the  project   Business  analyst  are  designers   Developers  are  designers,  at  a  very  fine  grained  detail  level   Design  documenta3on  is  all  about  «  Why  ?  »  and  the  answers  leads  to  the  goals  and   constraints  stated  at  project  incep+on   Design  ac3vity  goals  is  to  deliver  the  best  solu3on  to  the  users  given  the  project   constraints  :  scope,  resources,  +me  and  the  most  important  one,  quality.   «  by  «  design  »  we  mean  a  way  of  organizing  the  domain  concepts,  which  in  turn  leads   to  the  way  in  which  we  organize  their  representa+on  in  code  »  Eric  Evans   (noun)  a  specifica+on  of  an  object,  manifested  by  an  agent,  intended  to   accomplish  goals,  in  a  par+cular  environment,  using  a  set  of  primi+ve   components,  sa+sfying  a  set  of  requirements,  subject  to  constraints   (verb,  transi5ve)  to  create  a  design,  in  an  environment  (where  the  designer   operates  
  11. Model?   «  All  models  are  wrong,  some  are  useful

     »  G.  Box,  sta5s5cian   A  system  of  abstrac3ons  that  describes  selected   aspects  of  a  domain   A  model  describes  a  material  or  immaterial  reality  with  a  par3cular  filter  by   using  some  concepts  that  allows  classifying  the  observed  reality  :  these   filtering  concepts  goes  into  a  paradigm  and  are  formalized  through  a   metamodel   A  model  is  a  limited  and  oriented  descrip+on  of  reality   A  model  forms  the  basis  of  a  language  
  12. Good  or  Bad  Model?   Hence  Behavior-­‐Driven  Development  (BDD)  is

     the  perfect   complement  to  DDD  through  scenarios  that  describe  the   expected  usage  !   A  model  is  not  good  or  bad,  a  model  is  adapted   to  a  usage  or  not  
  13. Model  example   This  model  describes  the  outside   world.

     What  are  its  usage  and   the  objec3ve  guiding  its   design  ?  
  14. Abstrac+on  Level   The  act  of  abstrac3on  is  to  forget

     the  non   significant  details  and  focus  on  the  essen3als   Abstrac3on  is  the  act  of  isola3ng  some  objects  shared  characteris3cs   In  IT,  it’s  also  the  act  of  hiding  a  component  implementa3ons  details  (object,   system)  that  also  leads  to  the  encapsula+on  and  interface  concept.   Why  abstrac+on  ?  because  a  human  brain  can  only  manage  at  most  5   elements  at  a  3me  (3  actually  according  to  some  psychological  studies...)   It’s  the  modeler’s  fundamental  intellectual  tool   It  separates  the  responsability  on  a  ver3cal  axis  
  15. Abstrac+on  Level  in  real  life   Would  you  prefer  dealing

     with  this     or  that  ?   package dk.tigerteam.mdsd.demo.model.internal; @javax.persistence.Entity @javax.persistence.Table(name = "Customer") public class Customer extends dk.tigerteam.mdsd.demo.model.AbstractEnti ty { private static final long serialVersionUID = 2098912667L; @javax.persistence.Basic @javax.persistence.Column(name = "name", nullable = false) private String name; @javax.persistence.OneToOne(cascade = { javax.persistence.CascadeType.MERGE, javax.persistence.CascadeType.PERSIST, javax.persistence.CascadeType.REFRESH} , fetch = javax.persistence.FetchType.LAZY) @javax.persistence.JoinColumn(name = "addressId") @org.hibernate.validator.NotNull private dk.tigerteam.mdsd.demo.model.internal.Add ress address; @javax.persistence.OneToMany(cascade = { javax.persistence.CascadeType.MERGE, javax.persistence.CascadeType.PERSIST, javax.persistence.CascadeType.REFRESH} @javax.persistence.Basic @javax.persistence.Column(name = "comment", nullable = false) private String comment; @javax.persistence.Basic @javax.persistence.Temporal(javax.persist ence.TemporalType.TIMESTAMP) @javax.persistence.Column(name = "time", nullable = false) private java.util.Date time; @javax.persistence.Basic @javax.persistence.Column(name = "timeslot", nullable = false) private int timeslot; @javax.persistence.ManyToOne(cascade = { javax.persistence.CascadeType.MERGE, javax.persistence.CascadeType.PERSIST, javax.persistence.CascadeType.REFRESH} , fetch = javax.persistence.FetchType.LAZY) @javax.persistence.JoinColumn(nullable = false, name = "customerId") private dk.tigerteam.mdsd.demo.model.internal.Cus tomer customer; public String getComment() { return comment; public void setComment(String parameter) { } this.comment = parameter; public java.util.Date getTime() { package dk.tigerteam.mdsd.demo.model.internal; @javax.persistence.Entity @javax.persistence.Table(name = "Address") public class Address extends dk.tigerteam.mdsd.demo.model.AbstractEnti ty { private static final long serialVersionUID = 1697028161L; @javax.persistence.Basic @javax.persistence.Column(name = "street", nullable = false) private String street; @javax.persistence.Basic @javax.persistence.Column(name = "zipCode", nullable = false) private String zipCode; @javax.persistence.Basic @javax.persistence.Column(name = "city", nullable = false) private String city; public String getStreet() { return street; } public void setStreet(String parameter) { this.street = parameter; } public String getZipCode() { return zipCode; public void setZipCode(String parameter) { this.zipCode = parameter; } public String getCity() { return city; } public void setCity(String parameter) { this.city = parameter; } public dk.tigerteam.mdsd.demo.model.internal.Add ress withStreet( String street) { setStreet(street); return (dk.tigerteam.mdsd.demo.model.internal.Ad dress) this; public dk.tigerteam.mdsd.demo.model.internal.Add ress withZipCode( String zipCode) { setZipCode(zipCode); return (dk.tigerteam.mdsd.demo.model.internal.Ad dress) this; public dk.tigerteam.mdsd.demo.model.internal.Add ress withCity(String city) { setCity(city); return (dk.tigerteam.mdsd.demo.model.internal.Ad dress) this; } public dk.tigerteam.foundation.bean.mdsd.runtime .RuntimeMetaClazz getMetaType() { return metaType; } } , targetEntity = dk.tigerteam.mdsd.demo.model.internal.Boo king.class, mappedBy = "customer", fetch = javax.persistence.FetchType.LAZY) private java.util.Set<dk.tigerteam.mdsd.demo.mode l.internal.Booking> bookingCollection = new java.util.HashSet<dk.tigerteam.mdsd.demo. model.internal.Booking>(); public String getName() { } return name; public void setName(String parameter) { this.name = parameter; } public dk.tigerteam.mdsd.demo.model.internal.Add ress getAddress() { if (address instanceof org.hibernate.proxy.HibernateProxy) { return time; public void setTime(java.util.Date parameter) { this.time = parameter
  16. BUILDING  BLOCKS   An   Approach   Building   Blocks

      Supple   Design   Strategic   Design  
  17. Building  blocks   encapsulate with MODEL-DRIVEN DESIGN express model with

    isolate domain with encapsulate with ENTITIES VALUE OBJECTS LAYERED ARCHITECTURE AGGREGATES REPOSITORIES act as root of SMART UI X FACTORIES encapsulate with express model with encapsulate with mutually exclusive choices access with maintain integrity with access with SERVICES express model with
  18. Domain   En+ty,  Value   Object,   Service,   Modules

      Applica3on   Services   User   Interfaces   Tests   Infrastructure   WS  and   Messaging   Databases   Programma3c     Interfaces   •  An  independent  object   model  serves  as  the   founda+on  of  the  applica+on.   •  Interfaces  are  defined  by   inner  layers,  and   implemented  by  outer   layers.   •  All  couplings  are  directed   towards  the  center  of  the   “onion”   •  It  is  possible  to  compile  and   run  the  core  code  of  an   applica3on  separately  from   the  infrastructure.   •  It  uses  the  Dependency   Inversion  Principle     So>ware  Architecture  –  from  Layers  to  «  Onion  »  
  19. SUPPLE  DESIGN   An   Approach   Building   Blocks

      Supple   Design   Strategic   Design  
  20. Supple  Design  Principles   Inten3on   Revealing   Interfaces  

    Name  classes  and  opera+ons  to  describe  their  effect  and  purpose,   without  reference  to  the  means  by  which  they  do  what  they  promise     Side-­‐Effect-­‐Free   func3ons   opera+ons  that  return  results  with  no  observable  side  effects     Asser3ons   State  post-­‐condi+ons  of  opera+ons  and  invariants  of  classes  and   aggregates   Standalone   classes   Self-­‐contained  class  with  no  external  dependencies   Closure  of   opera3ons     An  opera+on  whose  return  type  is  the  same  as  the  type  of  its   arguments  in  order  to  keep  away  dependencies  against  another  types  
  21. STRATEGIC  DESIGN   An   Approach   Building   Blocks

      Supple   Design   Strategic   Design  
  22. Maintaining  Model  Integrity  

  23. Strategic  Design  Principles   Context     team,  users  groups,

     a  par+cular  so>ware,  a  company’s  department,   etc.   Context   integra3on   Use  the  Upstream/Downstream  metaphor.  Different  «  levels  »  :   An+corrup+on  layer,  conformist,  shared  kernel,  Customer/Supplier   rela+onship,  separate  ways,  open  host   Design  effort   prioriza3on   Dis+lla+ons  :  core,  support  and  generic  domains   Large  scale   structure   Evolving  Order,  System  Metaphor,  Responsibility  Layers,  Knowledge   Level,  Pluggable  Component  Framework   Core   idea   :   each   model   and   its   concepts   are   located  in  a  par3cular  context    
  24. Context?   The   seang   in   which  

    a   word   or   statement   appears  that  determines  its  meaning.   CONTEXT   There  is  no  one  model  to  rule  them  all  :  every  model  is  +ed  to  a  context  
  25. Context  strategy   Separate   ways   An+Corrup+on   Layer

      Conformist   Customer/Supplier     teams   OpenHost   service   Shared  Kernel   Single     Bounded  Context   Control  of  all  associated  systems   Communica+ons  commitment/capability  of  teams  
  26. DDD  DISTILLATION     Not  all  of  a  large  system

     will  be     well  designed.  How  to  maximize  the  impact  of   DDD  ?   bit.ly/ddd-­‐distillation  
  27. Is  DDD  worth  it  ?   Does  your  project...  

    •  Deals  with  Core  Domain  ?   •  Or  has  intricacy  in  the  domain/ problem  area  you  are  trying  to   address  ?   •  Or  needs  to  bring  clarity  ?   •  Or  has  business  exploring  ?  not   so>ware  exploring.  If  they  know  what   their  goal  is,  but  they  don't  know  quite   how  to  accomplish  it.  Need  so>ware   development  that  can  explore  with   them   Do  you  have  what  it  takes  to   make  it  work?  Can  you  make  it   work?   •  Clean  bounded  context   •  Access  to  domain  experts.  You  must   obtain  access  to  domain  experts,  and   then  establish  a  real  rela+onship,  a   crea+ve  collabora+on   •  Skilled  team   •  You  want  your  internal  team  focused   on  the  Core  Domain  because  that  is   where  the  most  valuable,  specialized   knowledge  is.  You  want  that  core  team   accumula+ng  knowledge  and  skills  in   the  core  domain.  If  you  outsource,  you   may  get  a  good  piece  of  so>ware  but   not  that  accumulated  knowledge  
  28. BUILDING  BLOCKS   Focus  on  the  En+ty  and  Value  Object

     
  29. DDD  Building  blocks   encapsulate with MODEL-DRIVEN DESIGN express model

    with isolate domain with encapsulate with ENTITIES VALUE OBJECTS LAYERED ARCHITECTURE AGGREGATES REPOSITORIES act as root of SMART UI X FACTORIES encapsulate with express model with encapsulate with mutually exclusive choices access with maintain integrity with access with SERVICES express model with
  30. DDD  Building  blocks   encapsulate with MODEL-DRIVEN DESIGN express model

    with isolate domain with encapsulate with ENTITIES VALUE OBJECTS LAYERED ARCHITECTURE AGGREGATES REPOSITORIES act as root of SMART UI X FACTORIES encapsulate with express model with encapsulate with mutually exclusive choices access with maintain integrity with access with SERVICES express model with
  31. VALUE  OBJECTS  

  32. Value?   Examples  :     –  The  number  13

      –  A  color  (Yellow)  :  determined  by  the  propor+on  of  Red  (0),  Green  (255)  and   Blue  (255)   –  A  bank  note  of  10  €.  I  can  exchange  my  10  €  bank  note  with  yours,  it  doesn’t   make  a  difference  to  us  about  its  usage.   A  value  is  something  intangible  and  immutable   Intangible  =  that  exists  outside  +me  and  space   Immutable  =  that  doesn't  change  
  33. Values  Characteris+cs   Value  types  differ  in  the  generality  and

     focus  of  their  domain     –  Some  are  mathema+cal:  Integers,  Double   –  Some  are  programma+c:  strings  (but  with  no  seman+cs  at  all...)   –  Some  are  real  world:  ISBNs,  Money,  Dura+on,  Phone  Number,   etc.   Value  types  reflect  constraints   –  ISBNs  have  a  well-­‐formedness  rule   –  Int  (32  bits)  is  a  bounded  subset  of  integers     Some  value  types  have  an  intrinsic  or  natural  ordering     –  Ordering  may  be  based  on  magnitude,  lexicographical  or  ordinal   criteria   –  Be  careful  about  imposing  an  ordering  on  unordered  types.   Ordering  is  not  an  essen+al  feature  for  all  value  types        
  34. Opera3ons,  Rela3onships  and   Constraints  form  systems  of  values  

      •  Degree   •  Posi+on  (Long./degree  +  Lat./degree)  :     •  Distance  +  Dura+on  =  Speed  (10  km/h)   •  Speed  +  Direc+on  =  Velocity  (10  km/h  north)   •  Accelera+on  =  Deriva+ve  of  Velocity  or  Speed   with  respect  to  +me  (3  m/s2)  
  35. «  Time  »  System  of  Values   Dura+on  :  eg.

     1  Hour  or  60  seconds   Instant  =  seconds  since  1st  january  1970  00:00   |Instant  2  –  Instant  1  |  =  Dura+on   Instant  1  +  Dura+on  =  Instant  2   Interval  =  start  instant  +  end  instant   Timezone   Chronology   An  Instant  (point  in  +me)  is  a  value,  as  is  the  difference  between  two   Instant,  but  Instant,  Dura5on  and  Interval  are  not  equivalent  types.   See  Joda  Time  Library  for  an  implementa+on.   Instant  2   Instant  1   Dura+on   Time  
  36. Value  Objects  example  –  tradi+onal   code   public class

    CustForm extends ActionForm { String phone; public String getPhone() { return phone; } public void setPhone(String phone) { this.phone = phone; } } public class AddCustAction extends Action { CustomerService custserv = null; @Override public ActionForward execute( ActionMapping actionMapping, ActionForm actionForm, HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse) { CustForm form = (CustForm) actionForm; try { String phone = form.getPhone(); custserv.addCust(0, phone, "foo"); return actionMapping. findForward("success"); } catch (ValidationException e) { return actionMapping. findForward("invaliddata"); } } } public interface CustomerService { void addCust(int i, String phone, String s) throws ValidationException; } public class CustomerServiceImpl implements CustomerService { public void addCust(int i, String phone, String s) throws ValidationException { PreparedStatement dbstmt = “INSERT ...”; if (!justnumbers(phone)) throw new ValidationException(); try { dbstmt.setInt(1, i); dbstmt.setString(3, phone); dbstmt.setString(4, s); dbstmt.executeUpdate(); } catch (SQLException e) { throw new RuntimeException(); } static boolean justnumbers(String s) { return s.matches("[0-9]*"); } }
  37. Value  Objects  example  –  Phone   numbers  as  strings  

    class CustForm extends ActionForm private String phone class AddCustAction extends Action ... ... execute(...) String phone = form.getPhone(); custserv.addCust(..., phone, ...); class CustomerServiceBean ... void addCust(..., String phone, ...) throws ValidationException ... if (!justnumbers(phone)) ... ... throw new ValidationException(); dbstmt.setString(4, phone); static boolean justnumbers(String s) ... Form   Controller   Business  Logic   Persistence  
  38. Value  Objects  example   public class PhoneNumber { private final

    String number; public PhoneNumber(String number) { if(!isValid(number)) throw ... this.number = number; } public String getNumber() { return number; } static public boolean isValid(String number) { return number.matches("[0-9]*"); } public String getAreaCode() { String prefix = null; for (int i=0; i< number.length(); i++){ String begin = number.subString(0,i); if(isAreaCode(begin)) { prefix = begin; break; } return prefix; } private boolean isAreaCode(String prefix) { ... } } Make  implicit   concepts  explicit  !    
  39. Service  API  Clarity   FROM   void  addCustomer(String,  String,  String,

      int,  int,  String,  String,  boolean)     TO   void  addCustomer(Name,  PhoneNumber,  PhoneNumber,   CreditStatus,   SalesRepId,   Name,  PhoneNumber,  PartnerStatus)  
  40. Value  Objects  prac+cal  +ps   Construc3on  should  result  in  a

     meaningful  and  well-­‐formed  object   –  Par+al  ini+alisa+on  is  problema+c  for  objects  —  especially  for  value  objects     –  Consider  the  constructor  to  be  transac3onal:  it  returns  a  correct  object  or  an   excep+on   Respect  and  enforce  state  invariant   Comparison…   –  Equality  is  a  fundamental  concept   –  Total  ordering  may  or  may  not  apply     Classifica3on  and  conversion…   –  Avoid  inheritance  and  favor  composi+on   –  May  support  conversions  (Km  -­‐>  Miles),  (Hour  -­‐>  Second)   Value  Object  Smells  :   –  Anemic  Value  Object  :  Lizle  behaviour  beyond  field  access     –  Role  creep  :  Too  much  responsibility  and  external  dependencies     –  Non-­‐meaningful  constructor   –  Constructor  without  useful  enforcement  or  sufficient  quality  of  failure    
  41. DTO  vs  VO   DTO  –  Data  Transfer  Object  

    –  purpose:  data  transfer  –  technical  construct   –  bunch  of  data  –  not  necessarily  coherent   –  no/lizle  behaviour     VO  –  Value  Object   –  purpose:  domain  representa+on   –  high-­‐coherent  data   –  rich  on  behaviour     «  Data  Transfer  Object  is  an  oxymoron.  »  Dan  North,  Qcon   London  2011  Conference  
  42. Value  Objects   Descrip+on   •  Many  objects  have  no

     conceptual  iden5ty.   These  objects  describe  some  characteris5c   of  a  thing.   •  Tracking  the  iden+ty  of  en++es  is   essen+al,  but  azaching  iden+ty  to  other   objects  can  hurt  system  performance,   add  analy+cal  work,  and  muddle  the   model  by  making  all  objects  look  the   same.   •  So>ware  design  is  a  constant  bazle  with   complexity.  We  must  make  dis+nc+ons  so   that  special  handling  is  applied  only   where  necessary.  However,  if  we  think  of   this  category  of  object  as  just  the  absence   of  iden+ty,  we  haven’t  added  much  to   our  toolbox  or  vocabulary.  In  fact,  these   objects  have  characteris+cs  of  their  own,   and  their  own  significance  to  the  model.   These  are  the  objects  that  describe  things.     Therefore   •  When  you  care  only  about  the  aEributes   of  an  element  of  the  model,  classify  it  as   a  value  object.     •  Make  it  express  the  meaning  of  the   aEributes  it  conveys  and  give  it  related   func3onality.     •  Treat  the  value  object  as  immutable.     •  Don’t  give  it  any  iden3ty  and  avoid  the   design  complexi3es  necessary  to   maintain  en33es.  
  43. Value  Objects  benefits   Computa3on  complexity  moved  to  value  objects

        Adding  new  terminology  to  language   Compound  value  objects  can  swallow  lots  of   computa3onal  complexity   Provides  advanced  language     En3ty  relieved  of  complexity     Uses  advanced  language     Improved  extensibility     Especially  clarity,  testability  and  concurrency  issues  
  44. ENTITIES  

  45. En+ty?   Some  objects  have  a  con3nuity  and  a  fixed

     iden3ty  whatever   values  their  states  or  azributes  are   Iden33es  are  mental  tools  we  use  to  superimpose  con3nuity   on  a  world  which  is  constantly,  func3onally,  crea3ng  new   values  of  itself   By  iden+ty  we  mean  a  stable  logical  en+ty  associated  with  a  series   of  different  values  over  +me   An  En5ty  is  an  object  that  is  not  defined  by  its   azributes  but  rather  by  a  thread  of  con+nuity   and  its  iden+ty  
  46. En+ty   An  en3ty  is  an  object  that  has  an

     iden3ty  and  a   state,  so:   What  does  it  means  to  be  the  same  thing  ?   How  dis+nguish  between  two  different  objects  ?   Its  naming  uses  a  nominal  group  that  refers  to   something  or  a  role   Examples  :   A  credit  card  :  has  an  iden+ty  (number)  and  a  state,  that’s  why   there  is  a  chip  in  it  in  Europe   A  person  in  a  plane  is  an  En5ty  because  a  seat  is  associated  with   her.  A  person  in  a  bus  is  a  Value  Object  because  her  iden+ty   doesn’t  mazers  (or  with  low-­‐cost  air  carrier).  It  depends  of  the   context…    
  47. En++es   Descrip+on   •  Many  objects  are  not  fundamentally

     defined   by  their  aOributes,  but  rather  by  a  thread  of   con5nuity  and  iden5ty.   •  Some  objects  are  not  defined  primarily  by   their  azributes.  They  represent  a  thread  of   iden+ty  that  runs  through  +me  and  o>en   across  dis+nct  representa+ons.  Some+mes   such  an  object  must  be  matched  with   another  object  even  though  azributes  differ.   An  object  must  be  dis+nguished  from  other   objects  even  though  they  might  have  the   same  azributes.  Mistaken  iden+ty  can  lead   to  data  corrup+on.   Therefore   •  When  an  object  is  dis3nguished  by  its   iden3ty,  rather  than  its  aEributes,  make   this  primary  to  its  defini3on  in  the  model.     •  Keep  the  class  defini3on  simple  and   focused  on  life  cycle  con3nuity  and   iden3ty.     •  Define  a  means  of  dis3nguishing  each   object  regardless  of  its  form  or  history.     •  Be  alert  to  requirements  that  call  for   matching  objects  by  aEributes.     •  Define  an  opera3on  that  is  guaranteed  to   produce  a  unique  result  for  each  object,   possibly  by  aEaching  a  symbol  that  is   guaranteed  unique.  This  means  of   iden3fica3on  may  come  from  the  outside,   or  it  may  be  an  arbitrary  iden3fier  created   by  and  for  the  system,  but  it  must   correspond  to  the  iden3ty  dis3nc3ons  in   the  model.     •  The  model  must  define  what  it  means  to  be   the  same  thing.  
  48. The  Iden+ty  Opera+on   Each  ENTITY  must  have  an  opera+onal

     way  of  establishing  its   iden+ty  with  another  object—dis+nguishable  even  from   another  object  with  the  same  descrip+ve  azributes   An  iden+fying  azribute  must  be  guaranteed  to  be  unique   within  the  system  however  that  system  is  defined—even  if   distributed,  even  when  objects  are  archived   See  UUID,  Master  Data  Management  Mechanisms,  etc.  
  49. Is  it  an  En+ty  or  a  Value  Object?   It

     depends...   If  the  object  has  a  lifecycle  then  are  you  interested   in  it?  
  50. Value  Objects  rela+on  with  En5ty   Value  objects  capture  the

     state  of  other  (en3ty)  objects.   What  is  interes+ng  about  them  is  not  their  iden+ty  but   the  value  that  they  represent   From  a  programming  perspec+ve,  we  can  model  values   as  objects     –  Hence  value  objects  and  value  types   –  Value  objects  have  significant  state  but  insignificant   iden+ty   –  A  value  object  is  a  kind  or  style  of  object  that  realises  a   value      
  51. En+ty  vs  Value  Object   Characteris3c     Value  Object

      En3ty   Mutability   Should  be  immutable   Usually  mutable     Equality     Compare  by  value,  as  in  a.equals(b)   Compare  by  iden+ty,  as  in   a==b   Scale   Typically  small  in  scale     Of  any  size   Serializability     Should  be  serializable;  may  be   parseable   Need  not  be  serializable     Defaults     O>en  have  a  natural  default:  the   number  0,  or  “today  »   No  natural  default   Comparability   O>en  have  a  natural  order:  1,  2,   3.  .  .   Typically  no  natural  sort   order   Closure  (or  Algebra)   O>en  have  a  closed  set  of   opera+ons:  2  +  2  =  4   Unlikely  to  have  any  closed   set  of  opera+ons     Iden3ty     May  represent  an  iden+fier,  such  as   an  SSN   Something  that  is  iden+-­‐   fied,  such  as  a  Person     User  Interface     Have  dedicated  widgets:  checkbox,   calendar.  .  .     Coded  by  hand  
  52. OBJECT-­‐ORIENTED  PARADIGM   Let’s  get  back  to  the  roots  

  53. Object  paradigm  -­‐  roots   Alan  Cur3s  Kay  (born  May

     17,  1940)  is   an  American  computer  scien+st,  known  for  his  early   pioneering  work  on  object-­‐oriented  programming  (with   Smalltalk)  and  windowing  graphical  user  interface  design,   and  for  coining  the  phrase,  "The  best  way  to  predict  the   future  is  to  invent  it.”   "OOP  to  me  means  only  messaging,  local  reten6on  and   protec6on  and  hiding  of  state-­‐process,  and  extreme  late-­‐ binding  of  all  things.”  Alan  Kay   http://www.purl.org/stefan_ram/pub/doc_kay_oop_en  
  54. Is  state  bad  ?   Stateful   Stateless  

  55. State?   State  is  o>en  «  summarized  »  with  a

     past  par3ciple  that  have   a  sta3c  and  stable  meaning  (paid  order,  loggued  user,  etc.)   State  influences  the  object’s  behavior   State,  in  its  defini+on,  imply  that  between  several  different   stable  states,  there  are  en+ty’s  transforma3ons  occuring   during  a  state  transi6on  triggered  by  an  event   A  state  is  associated  with  an  iden6ty  in  order  to  track  the   different  state  transi5ons   A  state  is  a  rela+on  among  individual  en++es  and   values  at  a  par+cular  instant;  it  can  change  over   +me  
  56. State  –  example   High  visibility  and  low  mutability  

    data  in  the  Informa+on  System   Immutable,  con+nuous   Immutable,  con+nuous   Mutable,  con+nuous   Mutable,  discrete   Immutable,  con+nuous   Immutable,  discrete   Events  that  triggers  state  transi+on  
  57. State  Transi+ons  -­‐  example   State-­‐transi+on  Diagram  example  for  a

     Person   object  
  58. Temporal  aspect  of  State  management   and  Iden5ty   Iden3ty

      Single   Married   Adult   Minor   Time   Adult  state   Marital  state   Divorced   •  Business  Id  :  12345678   •  John   •  Doe   •  5  November  1976  in  Paris   •  …   En3ty   State   En5ty  Object  at  a  given   moment  in  +me   Iden3ty   State  
  59. State,  Iden+ty  and  Func+on  or  the   Saint-­‐Graal  of  mixing

     OO  and  FP   State   Iden3ty   State  1   State  2   State  3   Func+on   Func+on   Event  triggers   Event  triggers   •  value  1a   •  value  1b   •  value  1c   •  value  1a   •  value  1b   •  value  1c   •  value  2a   •  value  2b   •  value  1a   •  value  1b   •  value  1c   •  value  2a   •  value  2b   •  value  3a   Publish  Internal   Event   Publish  Internal   Event   ...   Time  
  60. Example  with  Order  en+ty   Iden3ty  :  Order  N°  123456

      Submized   Paid   Shipped   pay   ship   Payment  details   from  Payment   Gateway  received   Carrier  data   q  Customer  Data   q  Product  Lines   (product  +  qty)   q  Shipping  Data   States   submit   q  Customer  Data   q  Product  Lines   (product  +  qty)   q  Shipping  Data   q  Payment  Data   q  Customer  Data   q  Product  Lines   (product  +  qty)   q  Shipping  Data   q  Payment  Data   q  Carrier  Data   Front  website  form   submized   Order  submized   event   Order  paid  event   Order  shipped   event   Time  
  61. Essen+al  OO  concepts     Objects  have  a  state,  a

     behavior  and  an  iden3ty.   Encapsula3on  allows  to  focus  on  the  external   proper3es,  the  object  interface,  while  hiding   internal  mechanisms.     Objects  communicates  by  messages  through   rela3onships.     Class  defines  objects  set  that  share  the  same   proper+es.  Class  are  object  factory.     Generaliza3on  rela+onship  allows  to  factorize   common  proper+es  and  permits  polymorphism.     Object  lifecycle  is  described  by  a  finite  state  machine   that  react  to  events.  
  62. Essen+al  FP  concepts     Nested,  first-­‐class  func3ons   Means

     of  composing  func3ons  with   other  func+ons  to  solve  problems   Immutability  [and  no  side-­‐effects  but  to   a  certain  degree  according  to  the   interlocutor’s  religion,  Haskell  or  Lisp]   Few  Data  Structures,  Lot  of  Opera3ons   Recursion   Lazyness     First-­‐Class  Func3on   Immutability   Recursion/Lazyness  
  63. DDD’s  Essen+als   Crea3ve  collabora3on  of  domain  experts  and  soFware

      experts  :  what  is  the  value  the  domain  expert  acquired  during   the  modeling  process?  it  must  be  a  crea+ve  process   Explora3on  and  experimenta3on  :  exploring  model  is  really   cheap  (on  the  black  board),  we  do  not  finish  un+l  we  produce  3   bad  models,  if  you  only  produce  good  things  you  are  not   crea+ve.  We  need  to  discuss,  let's  celebrate  the  bad  ideas  !     Emerging  models  shaping  and  reshaping  the  ubiquitous   language   Explicit  context  boundaries   Focus  on  the  core  domain  
  64. Designer’s  key  sentences   It’s  all  about  the  language  

    Make  the  implicit  explicit   Get  real  and  concrete  usage  through  scenarios   There  is  no  one  model  to  rule  them  all:  every  model  is  +ed  to  a   context   Not  all  domains  are  equivalent,  some  are  more  important  than   other  :  the  core  domain   Collabora5on  between  domain  expert  and  so>ware  expert  is  the   key  :  if  you  want  to  go  fast,  go  alone,  if  you  want  to  go  far,  go   together...  
  65. Content’s  azribu+on   Some  of  this  presenta+on’s  content  comes  

    from  :   Domain  Driven  Design  –  PaEern  Summaries   Distributed  under  the  Crea+ve  Commons  Deed:   Azribu+on  2.0.  You  are  free:     •  to  copy,  distribute,  display,  and  perform  the  work     •  to  make  deriva+ve  works     •  to  make  commercial  use  of  the  work   AEribu3on  :  Eric  Evans,  DOMAIN-­‐DRIVEN   DESIGN,  Tackling  complexity  in  the  Hearth  of   So>ware  Addison-­‐Wesley,  ©  Eric  Evans,  2004.   Eric  Evans,   Domain  Driven   Design’s  author