1. Ed Blackburn
Freelance Developer
@ejblackburn
http://ejb.name

2. Oyster Card
July 2003
© “Oyster Card" by Frank Murmann is licensed under CC BY3.0

3. Contactless
2014

4. “Prove it”

5. Components
• Identity Management
• Customer Services
• Customer Portal
• Payment and Risk Services
• Canonical, versioned reference data
• Path finding fare generation for routes
• Journey Construction, apportionment, capping and fares

6. “Behaviour”
© “Relativity" by M. C. Escheris, 1953

7. Evolving the MVP
• Revenue Protection
• Disruptions
• Travel Products (Weekly, Monthly, Annual…)

8. Philosophy
• Object-Orientation; an object has both state (data)
and behaviour (logic)
• Domain model focused
• Tell don’t ask
• SOLID

9. “Tell Don’t Ask”
https://pragprog.com/articles/tell-dont-ask

10. Tell Don’t Ask
public class Journey
{
…
public void AssociateRevenueInspectionTap(Tap tap) {..};
public void CounterZeroRateDueToRevenueProtection() {..};
…
}

11. Tell Don’t Ask?
public class Journey : Entity<Journey, JourneyKey>, IComparable<Journey>, IComparable<Tap>,
ITapSequence, ICanProtectRevenue, IJourneyAffectedByRevenueInspection, IMayNotExist,
IHaveTravelDayKey, IRateable, IDisruptable
{
…
public void AssociateRevenueInspectionTap(Tap tap) {..};
public void CounterZeroRateDueToRevenueProtection() {..};
public bool IsCounteredBy(Tap candidate) {..};
public bool Exists { get; }
public bool ContainsRevenueInspectionTap { get; }
…
}

13. internal IRule<Tap> CreateRule(TravelDay travelDay)
{
// Create rules
return Rule.Create(..);
}

14. private IRule<Tap> CreateRule(TravelDay travelDay)
{
// Create rules
return Rule.Create(specification: new IsRITWithinPreviousJourney(travelDay),
ifSatisfied: ScenarioT01,
ifNotSatisfied: Rule.Create(..));
}

15. private IRule<Tap> CreateRule(TravelDay travelDay)
{
// Create rules
return Rule.Create(specification: new IsRITWithinPreviousJourney(travelDay),
ifSatisfied: ScenarioT01,
ifNotSatisfied: Rule.Create(
specification: new
IsPreviousJourneyDisrupted(travelDay),
ifSatisfied: ScenarioTDIS01,
ifNotSatisfied: ScenarioT02
));
}

16. public sealed class IsTapTypeRailInspection : Specification<Tap>
{
private static readonly ISpecification<Tap> _compositeSpecification =
new IsTapModeRail().And(new IsTapTypeRevenueInspection());
protected override bool IsSatisfiedBy(Tap candidate)
{
return _compositeSpecification.IsSatisfiedBy(candidate);
}
}

17. “Small Is Beautiful”
• E. F. Schumacher
• To counter Bigger is Better
• Empowers developers to build complex features
through small lego pieces
• Easy to read

18. Aggregate
“Nothing outside the Aggregate boundary can
hold a reference to anything inside, except to
the root entity…”
- Eric Evans, Domain Driven Design, 2004

19. Aggregates & Continuations?
public class Card : Entity<Card, TravelTokenId> {
…
public void PerformCommand(IRuleCommand<Journey> command)
{
PerformCommand(command, () => _journeys.Select(item => … )
}
private void PerformCommand<TEntity>(IRuleCommand<TEntity> command,
Func<IEnumerable<TEntity>> getEntities)
{
Rule.Create(command).Process(getEntities());
}
…
}

20. Pipelines and decorators
public interface ICardOperation
{
Card Process(Card card);
}
public interface IDecoratedCardOperation
{
ICardOperation InnerCardOperation { get; private set; }
}
public class RevenueProtection : ICardOperation, IDecoratedCardOperation
{
public RevenueProtection (...)
{
...
}
public Card Process (Card card)
{
card = InnerCardOperation.Process (card);
...
return card;
}
public ICardOperation InnerCardOperation { get;set;}
}

21. Summary
• Abstractions rot code
• Small interfaces
• If you have a DTO reconsider your approach
• Even anaemic models can be encapsulated
• Immutability is your friend

22. Future?
• Token based software system
• Anything can create a token
• Vehicle registration plate
• Phone / wearable
• APIs (dog fooding with mobile)
• Data, data, data…