On Being Explicit - Speaker Deck

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

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

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

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A healthy obsession with language

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

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

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

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

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Constructor enforces decimals. Operations protect against wrong currencies.

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

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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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Mixing currencies & precisions becomes impossible

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Also see: Time zone type safety

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

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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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Lots of incentives for fraud

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

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

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

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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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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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Event Storming

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

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Missing concept: Matching Data => ?

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Missing concept: Matching Data => Earning Trust

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

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Trust as a Value Object

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Trust can be passed around

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Trust attracts behaviour

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"Trust" adopted into Ubiquitous Language

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The business started getting interested and involved

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Polymorphic strategies for changing Trust Event-based workﬂows

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0 <-> 5 <-> 10

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A Trust Arithmetic Invented by the business 0 <-> 5 <-> 10 Group (closure, associativity, identity, invertibility)

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Metaphors as objects

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

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

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Example: Phone call bill rating

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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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"It's just an algorithm"

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Missing Metaphor: Buckets

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Mental Model ~ Visual Model ~ Implementation Model

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

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«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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Example: Bulb Management System Context Mapping

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Blub Management System

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

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

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

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

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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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«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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Heuristic: Follow the Money

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