Towards Modelling Processes

Processes are at the heart of how many businesses operate. The processes are governed by policies, agreed upon with contracts, and guided by documents. In software systems on the other hand, we often bury the processes. There’s a domain model for sure, but it’s all about “things” and “actions”. Let’s explore how Event Sourcing allows us to explicitly model the effects of changes over time.

http://verraes.net/2015/05/towards-modelling-processes/

Transcript

1. Towards Modelling Processes
   @mathiasverraes
   

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2. Mathias Verraes
   verraes.net
   @mathiasverraes
   Independent Software Consultant
   Student of Systems
   Meddler of Models
   Labourer of Legacy
   

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3. Domain-Driven Design workshops
   verraes.net/workshops
   

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5. Domain-Driven Design
   has evolved a lot
   in +10 years
   

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6. this talk is about
   things processes
   the grand dichotomy
   

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7. Great models are
   ╳ an accurate representation of reality
   ✓ useful abstractions
   ✓ solve problems
   

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8. Use heuristics
   

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9. Being
   Behaving
   Becoming
   Inspired by ”Rethinking Systems Analysis and Design” — Gerald M. Weinberg
   

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10. Being
    How it is structured
    Morphology
    

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11. Behaving
    How it reacts to inputs
    Repeatable, reversible
    

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12. Becoming
    How it changes into something else
    Evolution
    

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13. structural modelling
    focus on artefacts
    

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14. “As a shopper, in order to buy stuff
    Given I have a product X with price EUR100
    When I …”
    

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15. “As a shop owner, in order to sell stuff
    Given I have a product X,
    And that product is priced EUR 100 in the pricing table
    When I …”
    

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16. pseudo-behavioural
    

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17. imposes a structural model
    

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18. “As a shopper, in order to buy stuff
    Given the shop owner has priced a product X at EUR100
    When I …”
    

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19. “Teachers have multiple courses, each with
    multiple modules.
    Students have multiple courses.”
    

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20. “Courses are only visible when their status is
    Approved”
    

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21. What changes?
    Why does it change?
    Under what circumstances?
    Who changes it?
    How often does it change?
    What are the consequences of that change?
    

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22. Behaving:
    “Only committee members can approve courses”
    

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23. Becoming:
    “Modules are added and removed”
    

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24. What happens to students who already completed
    some of the modules that are now obsolete?
    

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25. time
    event sourcing
    E E E E E E
    A
    E = domain event, A = artefact
    

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26. event storming
    

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27. event storming
    E
    Invoice was paid
    time
    

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28. E
    C
    C = command
    Pay for invoice Invoice was paid
    time
    

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29. E
    C
    B = business rule
    B
    Pay for invoice Invoice was paid
    

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30. E
    C B
    Invoice was paid
    Invoice was partially paid
    Pay for invoice
    Paid amount < Invoice amount
    Paid amount = Invoice amount
    

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31. E
    C B
    Invoice was paid
    Invoice was partially paid
    Paid amount < Invoice amount
    Paid amount = Invoice amount
    Paid amount > Invoice amount
    Invoice was overpaid
    

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32. E
    C B
    = end of lifecycle
    Invoice was paid
    

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33. E
    C B
    = depends on
    payment amount
    Pay for invoice
    

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34. C B
    E
    E
    invoice amount
    Invoice was
    created
    

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35. C B
    E
    C
    E
    Create invoice
    

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36. C B
    E
    C
    E C B
    E
    

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37. C B
    E
    C
    E C B
    E
    All paid amounts < Invoice amount
    All paid amounts = Invoice amount
    All paid amounts > Invoice amount
    

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38. C B
    E
    C
    E C B
    E
    E
    Invoice was paid
    Invoice was partially paid
    

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39. the primitives
    of our model
    

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40. artefacts
    relations
    + foo() behaviour
    

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41. time
    knowledge
    constraints
    history
    intentions
    branches
    processes
    And more: actors, queries, …
    

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42. going further…
    actors
    queries
    immediate consistency
    eventual consistency
    aggregates
    

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43. “Neither a static view
    nor a dynamic view
    can be the whole view.”
    Gerald M. Weinberg — Rethinking Systems Analysis and Design
    

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44. E E E E
    A
    E E E E
    process process
    

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45. E E E E E E E E
    collaborative
    construction
    execution
    artefact
    process process
    

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46. E E E E E E E E
    becoming being behaving
    

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47. invoice drafting
    invoice document
    invoicing process
    

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48. invoice drafting
    invoice document
    invoicing process
    the artefact prescribes the execution process
    

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49. ordering
    receipt
    receiving & putaway
    

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50. negotiation
    insurance contract
    claim
    

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51. collaborative
    construction
    execution
    artefact
    

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52. collaborative
    construction
    execution
    artefact
    analysis
    tracking
    feedback
    

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53. invoice drafting
    invoice document
    invoicing process
    implicit internal policy
    

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54. invoice drafting
    invoice document
    invoicing process
    explicit policy
    

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55. invoice drafting
    invoice document
    invoicing process
    evolving policy
    

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56. invoice drafting
    invoice document
    invoicing process
    evolving policy
    

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57. configurable policies per tenant
    ordering
    receipt
    receiving & putaway
    

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58. product changes
    negotiation
    insurance contract
    claim
    internal policy
    

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59. implementation
    suggestions
    

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60. Processes with:
    Immediately consistent rules
    => aggregates
    Eventually consistent rules
    => process managers
    

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61. 1 2 3 4
    policy changes
    Versioning
    2
    E E E
    B B
    1
    2
    3
    4
    2 2
    

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62. E E E
    1 2 3 4
    policy changes
    Specification
    B B
    

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63. E E E
    1 2 3 4
    policy changes
    Specification
    B B
    auditing
    

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64. E E E
    1 2 3 4
    policy changes
    Strategy
    B B
    

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65. E E E
    1 2 3 4
    policy changes
    Running process affected by policy changes
    B B
    

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66. complex business
    problems require a
    temporal model
    

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67. “The only reason
    for time
    is so that
    everything
    doesn’t happen
    at once.”
    Albert Einstein
    

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68. @mathiasverraes
    verraes.net
    dddeurope.com
    Brussels, January 2016
    

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