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

2. On Being Explicit @mathiasverraes verraes.net Software consulting for complex environments
4. Two day conference ticket €599 instead of €699 bit.ly/bddx-november
5. Logger { log(...) { ... } }
6. A healthy obsession with language
8. Log { append(...) { ... } }
9. Make the implicit explicit What are we really doing? What missing concepts can we identify? How can we bring them forward?
10. Example:1 Money 1 http://verraes.net/2016/02/type-safety-and-money/
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
12. Testable != idiot-proof2 2 Typical idiots include future self
13. * You may not want to use floats. "Patterns of Enterprise Application Architecture", Martin Fowler, 2002
14. Constructor enforces decimals. Operations protect against wrong currencies.
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
16. PreciseMoney { round() : RoundedMoney { return new RoundedMoney(amount, currency) } ... } RoundedMoney { add(RoundedMoney other) : RoundedMoney {...} multiply(Float operand) : PreciseMoney {...} ... }
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) ...
19. Mixing currencies & precisions becomes impossible
20. Also see: Time zone type safety
21. Example: Data Matching Paying out money based on usage of content Determine ownership by matching data from various sources
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
23. Lots of incentives for fraud
24. Is this our Core Domain? ✗ It's just technical data imports ⬜ ⬜ ⬜ ⬜
25. Is this our Core Domain? ✗ It's just technical data imports ✗ Business is not interested ⬜ ⬜ ⬜
26. Is this our Core Domain? ✗ It's just technical data imports ✗ Business is not interested ✔ Money changing hands ⬜ ⬜
27. Is this our Core Domain? ✗ It's just technical data imports ✗ Business is not interested ✔ Money changing hands ✔ The code you keep secret ⬜
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
29. Event Storming
30. Temporality The quality of data changes over time, based on who changes it
31. Missing concept: Matching Data => ?
32. Missing concept: Matching Data => Earning Trust
33. What if Trust was a tangible thing in our domain model?
34. Trust as a Value Object
35. Trust can be passed around
36. Trust attracts behaviour
37. "Trust" adopted into Ubiquitous Language
38. The business started getting interested and involved
39. Polymorphic strategies for changing Trust Event-based workflows
40. 0 <-> 5 <-> 10
41. A Trust Arithmetic Invented by the business 0 <-> 5 <-> 10 Group (closure, associativity, identity, invertibility)
42. Metaphors as objects
43. «His people are completely literal. Metaphors go over his head.»
44. «Nothing goes over my head. My reflexes are too fast, I would catch it.»
45. Example: Phone call bill rating
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
47. "It's just an algorithm"
48. Missing Metaphor: Buckets
51. Mental Model ~ Visual Model ~ Implementation Model
52. New opportunity: Long lead time to change pricing => Lively experimentation with pricing
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
54. Example: Bulb Management System Context Mapping
55. Blub Management System
56. Blub Management System Germanic countries Benelux +10 more regions UK & Ireland France
57. Blub Management System Germanic countries Benelux +10 more regions UK & Ireland France changes
58. Blub Management System Germanic countries Benelux +10 more regions UK & Ireland France changes breaks
59. Blub Management System Germanic countries Benelux +10 more regions UK & Ireland France Customers Customers Customers Customers Customers changes breaks requests
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
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
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
63. Heuristic: Follow the Money
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
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
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
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
68. Observations on better software design
69. Obsess over language
70. Visualise different models structural temporal spatial organisational
71. Grasp the underlying problem domain
72. Use metaphor
73. Follow the money
74. Make unspoken concepts explicit
75. Thank you — @mathiasverraes verraes.net dddeurope.com bit.ly/bddx-november