75 lines
3.2 KiB
Scala
75 lines
3.2 KiB
Scala
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package lu.foyer
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import zio.*
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import java.util.UUID
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final case class Entity[T](entityId: UUID, data: T, version: Long)
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final case class Event[T](entityId: UUID, data: T, eventId: UUID)
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trait StateRepository[State] extends Repository[Entity[State], UUID]
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trait EventRepository[State] extends Repository[Event[State], UUID]
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trait Reducer[Event, State]:
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def reduce(event: Event): Option[State]
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def reduce(state: State, event: Event): Option[State]
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trait CommandHandler[Command, Event, State]:
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def name: String
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trait CommandHandlerCreate[Command, Event, State] extends CommandHandler[Command, Event, State]:
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def onCommand(entityId: UUID, command: Command): Task[Event]
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trait CommandHandlerUpdate[Command, Event, State] extends CommandHandler[Command, Event, State]:
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def onCommand(entityId: UUID, state: State, command: Command): Task[Event]
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class CommandEngine[Command, Event, State](
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val handlers: List[CommandHandler[Command, Event, State]],
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reducer: Reducer[Event, State],
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val eventRepo: EventRepository[Event],
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val stateRepo: StateRepository[State]):
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def handleCommand(command: Command, name: String, entityId: UUID): Task[(Event, State)] =
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for
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handler <- ZIO
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.succeed(handlers.find(_.name == name))
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.someOrFail(new IllegalArgumentException(s"No handler found for command $name"))
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entityOption <- stateRepo.fetchOne(entityId)
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(event, newStateOption) <- transition(command, name, entityId, entityOption, handler)
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newState <-
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ZIO
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.succeed(newStateOption)
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.someOrFail(new IllegalArgumentException("Reducer cannot resolve state transition"))
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newEntity <-
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Random.nextUUID.map(Entity(_, newState, entityOption.map(_.version).getOrElse(1)))
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_ <- if entityOption.isEmpty then stateRepo.insert(newEntity)
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else stateRepo.update(newEntity.entityId, newEntity)
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eventEntity <- Random.nextUUID.map(Event(newEntity.entityId, event, _))
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_ <- eventRepo.insert(eventEntity)
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yield (event, newState)
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private def transition(
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command: Command,
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name: String,
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entityId: UUID,
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entityOption: Option[Entity[State]],
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handler: CommandHandler[Command, Event, State]
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): Task[(Event, Option[State])] = (entityOption, handler) match
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case (None, h: CommandHandlerCreate[Command, Event, State]) =>
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h.onCommand(entityId, command)
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.map(event => (event, reducer.reduce(event)))
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case (Some(entity), h: CommandHandlerUpdate[Command, Event, State]) =>
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h.onCommand(entityId, entity.data, command)
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.map(event => (event, reducer.reduce(entity.data, event)))
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case (Some(_), _: CommandHandlerCreate[Command, Event, State]) =>
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ZIO.fail(
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new IllegalArgumentException(s"State already exists when applying create command $name")
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)
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case (None, _: CommandHandlerUpdate[Command, Event, State]) =>
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ZIO.fail(new IllegalArgumentException(s"No state found to apply the update command $name"))
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case _ => ZIO.fail(new IllegalArgumentException("Impossible state"))
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end CommandEngine
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object CommandEngine:
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def layer[Command, Event, State](using Tag[Command], Tag[Event], Tag[State]) =
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ZLayer.fromFunction(CommandEngine[Command, Event, State](_, _, _, _))
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