(src: https://www.youtube.com/watch?v=9ZgSz6xAfL8)

Model: Clue Bucket (aka toolbox)

Model: Patterns vs Principles vs Heuristics

Model: distill your heuristics

Model: competing heuristics

Heuristic: things that change together should stay together

  • named: Single Responsibility

Model: Method for deliberate design choices

Exercise

./resources/video-design-heuristics-mathias-verraes-kandddinsk.resources/unknown_filename.png Model: Domain Event

Exercise: Which attributes should be in this domain event?

  • who placed it
  • when was it placed
    • Heuristic: when did something happen, is that relevant?
    • Heuristic: what are all the timestamps that matter
      • when it happened
      • when we learned about it
      • when we recorded it
      • pre-dated
  • who cares about this? what do they need
  • what was ordered

heuristics for deciding what to put in there

  • always verify assumptions about time

    • consumer sees it immediately or later
    • data could change over time
    • what if a constraint / business rule / invariant is not met immediately but some time goes by

  • redundancy

    • minimal events
    • Model: Fat events
    • Heuristic: consumer might not listen to all events
    • Heuristic: immutable attributes we should probably add in there
      • mutable: name
      • immutable: customer_id

  • consumer

    • Heuristic: serve the consumers needs

    • Heuristic: ignore individual consumer’s needs

    • Heuristic: 80-20 the consumers needs

    • DDD context mapping patterns

    • Pattern: internal to our bounded context, easily changeable

    • Pattern: conformist: leak into my domain

    • Pattern: partnership: their success relies on our success

    • Pattern: open host service (eg. Google Maps)

    • how many consumers are there

      • 1
      • 5-10 consumers -> 80-20 rule?

  • Temporal Coupling

    • are the consumers gonna query us to get more data
    • Distinction: Notification vs Domain Event
    • temporal coupling:
      • “This system needs to be online at the time that the consumer wants to know what the order was like”
      • reduces our autonomy
      • if not online, I can’t do anything

  • Heuristic: sensitive information

    • Datasparsamheid
    • GDPR compliance
    • sensitive information

  • Heuristic: how would it look in a paper world?

  • Heuristic: can we do multiple events for the same event, specific per consumer

Heuristic: if you can imagine it as a 3rd party system, then you can have it as a bounded context

Distinction: Notification vs Domain Event - forces a lookup Model: Domain Event - completeness guarantee Model: Fat events - redundant information

  • safe, immutable, stable
  • evolve over time, mutable
  • sensitive information, supposed to throw away after a certain amount of time