On Being Explicit

DDD London
November 9 2016

On Being Explicit

“Make the implicit explicit” must be one of the most valuable advices I ever got about software modelling and design. Gather around for some tales from the trenches: stories from software projects where identifying a missing concept, and bringing it front and centre, turned the model inside out. Our tools: metaphors, pedantry, type systems, the age old heuristic of “Follow the money”, visual models, and a healthy obsession with language.

Mathias Verraes
@mathiasverraes
http://verraes.net

Mathias Verraes advises companies on building software for complex environments. This involves primarily analysis and modelling, but also code, architecture, testing, and refactoring “unmaintainable” systems. He has worked with clients in Finance, Automotive, Government, Logistics, E-Commerce, and more. He teaches Domain-Driven Design courses and co-organises the DDD Europe conference. When he’s not working, he’s at home in Kortrijk, Belgium, helping his two sons build crazy Lego train tracks.

Transcript

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2. On Being Explicit
   @mathiasverraes
   verraes.net
   Software consulting
   for complex environments
   

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3. View Slide

4. Two day conference ticket
   €599 instead of €699
   bit.ly/bddx-november
   

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5. Logger
   {
   log(...)
   {
   ...
   }
   }
   

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6. A healthy obsession
   with language
   

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7. View Slide

8. Log
   {
   append(...)
   {
   ...
   }
   }
   

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9. Make the implicit explicit
   What are we really doing?
   What missing concepts can we identify?
   How can we bring them forward?
   

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10. Example:1
    Money
    1 http://verraes.net/2016/02/type-safety-and-money/
    

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11. Support multiple currencies
    8 decimal precision for calculations
    Use currency's official division in UIs and APIs
    Compliance reporting in select markets
    Convert to € for internal reporting
    

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12. Testable != idiot-proof2
    2 Typical idiots include future self
    

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13. * You may not want to use floats.
    "Patterns of Enterprise Application Architecture", Martin Fowler, 2002
    

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14. Constructor enforces decimals.
    Operations protect against wrong currencies.
    

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15. a = new Money(1.123456789, eur)
    // The constructor rounds it to 1.12345679
    b = a.round()
    // rounds it to 1.12 and
    // instantiates a new Money
    // The constructor rounds it to 1.12000000
    c = a.add(b)
    // icky
    

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16. PreciseMoney {
    round() : RoundedMoney {
    return new RoundedMoney(amount, currency)
    }
    ...
    }
    RoundedMoney {
    add(RoundedMoney other) : RoundedMoney {...}
    multiply(Float operand) : PreciseMoney {...}
    ...
    }
    

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18. RoundedMoney {
    ...
    switch(currency)
    case EUR: this.amount = round(amount, 2)
    case USD: this.amount = round(amount, 2)
    case BTC: this.amount = round(amount, 8)
    ...
    

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19. Mixing currencies & precisions
    becomes impossible
    

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20. Also see:
    Time zone type safety
    

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21. Example:
    Data Matching
    Paying out money based on
    usage of content
    Determine ownership by
    matching data from various
    sources
    

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22. Data sources:
    Own research by employees
    Off shore data cleaning
    Public wiki-style
    Public but curated
    Private, paid
    Direct submissions from individuals
    Agencies representing owners
    

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23. Lots of incentives for fraud
    

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24. Is this our Core Domain?
    ✗ It's just technical data imports
    ⬜
    ⬜
    ⬜
    ⬜
    

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25. Is this our Core Domain?
    ✗ It's just technical data imports
    ✗ Business is not interested
    ⬜
    ⬜
    ⬜
    

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26. Is this our Core Domain?
    ✗ It's just technical data imports
    ✗ Business is not interested
    ✔ Money changing hands
    ⬜
    ⬜
    

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27. Is this our Core Domain?
    ✗ It's just technical data imports
    ✗ Business is not interested
    ✔ Money changing hands
    ✔ The code you keep secret
    ⬜
    

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28. Is this our Core Domain?
    ✗ It's just technical data imports
    ✗ Business is not interested
    ✔ Money changing hands
    ✔ The code you keep secret
    ✔ Oldest code in the system
    

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29. Event Storming
    

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30. Temporality
    The quality of data
    changes over time,
    based on who changes it
    

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31. Missing concept:
    Matching Data
    =>
    ?
    

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32. Missing concept:
    Matching Data
    =>
    Earning Trust
    

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33. What if Trust was a tangible thing
    in our domain model?
    

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34. Trust as a Value Object
    

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35. Trust can be passed around
    

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36. Trust attracts behaviour
    

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37. "Trust" adopted into Ubiquitous
    Language
    

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38. The business started getting
    interested and involved
    

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39. Polymorphic strategies for changing Trust
    Event-based workflows
    

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40. 0 5 10
    

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41. A Trust Arithmetic
    Invented by the business
    0 5 10
    Group
    (closure, associativity, identity, invertibility)
    

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42. Metaphors as objects
    

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43. «His people are completely literal.
    Metaphors go over his head.»
    

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44. «Nothing goes over my head.
    My reflexes are too fast,
    I would catch it.»
    

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45. Example:
    Phone call bill rating
    

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46. Price per minute, price after the 60th minute
    Bundles: Credit minutes, lower rates
    Subscriptions: recurring bundles
    Promotions and incentives
    End of month transfers, expiries
    Flexible rating strategies in different markets
    

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47. "It's just an algorithm"
    

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48. Missing Metaphor:
    Buckets
    

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51. Mental Model
    ~
    Visual Model
    ~
    Implementation Model
    

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52. New opportunity:
    Long lead time to change pricing
    =>
    Lively experimentation with pricing
    

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53. «Habitable software (...)
    enables people to understand
    its construction and intentions
    and to change it
    comfortably and confidently.»3
    Programmers are not the only
    ones inhabiting our systems
    3 "Patterns of Software", Richard P. Gabriel, 1996
    

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54. Example:
    Bulb Management
    System
    Context Mapping
    

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55. Blub
    Management
    System
    

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56. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    

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57. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    changes
    

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58. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    changes
    breaks
    

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59. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    changes
    breaks
    requests
    

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60. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    CEO
    Senior
    Manager
    Middle
    Manager
    reports to
    changes
    breaks
    requests
    

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61. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    CEO
    Senior
    Manager
    Middle
    Manager
    reports to
    changes
    breaks
    requests
    

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62. «Any organization
    that designs a system
    will produce a design
    whose structure is a copy
    of the organization's
    communication structure.»5
    5 "How Do Committees Invent?", Melvin E. Conway, 1968
    

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63. Heuristic:
    Follow the Money
    

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64. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    CEO
    Senior
    Manager
    Middle
    Manager
    revenue
    costs
    

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65. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    CEO
    Senior
    Manager
    Middle
    Manager
    revenue
    costs
    

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66. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    CEO
    Senior
    Manager
    Middle
    Manager
    Blub
    Algorithms
    revenue
    costs
    

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67. Blub
    Management
    System
    Germanic
    countries
    Benelux
    +10 more
    regions
    UK &
    Ireland
    France
    Customers Customers Customers
    Customers Customers
    CEO
    Senior
    Manager
    Middle
    Manager
    Blub
    Algorithms
    

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68. Observations on
    better software design
    

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69. Obsess over language
    

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70. Visualise different models
    structural
    temporal
    spatial
    organisational
    

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71. Grasp the underlying problem
    domain
    

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72. Use metaphor
    

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73. Follow the money
    

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74. Make unspoken concepts explicit
    

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75. Thank you —
    @mathiasverraes
    verraes.net
    dddeurope.com
    bit.ly/bddx-november
    

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