93 lines
4 KiB
Scala
93 lines
4 KiB
Scala
package lu.foyer
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import zio.*
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import zio.schema.Schema
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final case class Entity[T](entityId: String, data: T, version: Long)
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final case class Event[T](entityId: String, data: T, eventId: String)
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trait StateRepository[Data] extends Repository[Entity[Data], String]
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trait EventRepository[Data] extends Repository[Event[Data], String]:
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def fetchOne(entityId: String, eventId: String): Task[Option[Event[Data]]]
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def fetchMany(entityId: String, page: Page): Task[Paged[Event[Data]]]
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trait Reducer[Event, State]:
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def fromEmpty: PartialFunction[Event, State]
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def fromState: PartialFunction[(State, Event), State]
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def reduce(event: Event): Option[State] =
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fromEmpty.lift(event)
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def reduce(state: State, event: Event): Option[State] =
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fromState.lift((state, event))
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trait CommandHandler[+Command, +Event, +State]:
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def name: String
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def isCreate: Boolean
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inline def isUpdate: Boolean = !isCreate
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def commandSchema: Schema[?]
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trait CommandHandlerCreate[Command: Schema, Event] extends CommandHandler[Command, Event, Nothing]:
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def onCommand(entityId: String, command: Command): IO[JsonApiError | Throwable, Event]
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val isCreate = true
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val commandSchema = summon[Schema[Command]]
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trait CommandHandlerUpdate[Command: Schema, Event, State]
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extends CommandHandler[Command, Event, State]:
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def onCommand(entityId: String, state: State, command: Command) : IO[JsonApiError | Throwable, Event]
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val isCreate = false
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val commandSchema = summon[Schema[Command]]
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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: String)
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: IO[JsonApiError | Throwable, (lu.foyer.Event[Event], lu.foyer.Entity[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 = Entity(entityId, newState, entityOption.map(_.version).getOrElse(1))
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_ <-
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if entityOption.isEmpty then stateRepo.insert(newEntity.entityId, newEntity)
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else stateRepo.update(newEntity.entityId, newEntity)
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eventEntity <- Random.nextUUID.map(id => Event(newEntity.entityId, event, id.toString))
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_ <- eventRepo.insert(eventEntity.eventId, eventEntity)
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yield (eventEntity, newEntity)
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private def transition(
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command: Command,
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name: String,
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entityId: String,
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entityOption: Option[Entity[State]],
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handler: CommandHandler[Command, Event, State]
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): IO[JsonApiError | Throwable, (Event, Option[State])] = (entityOption, handler) match
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case (None, h) if !h.isUpdate =>
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h.asInstanceOf[CommandHandlerCreate[Command, Event]]
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.onCommand(entityId, command)
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.map(event => (event, reducer.reduce(event)))
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case (Some(entity), h) if h.isUpdate =>
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h.asInstanceOf[CommandHandlerUpdate[Command, Event, State]]
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.onCommand(entityId, entity.data, command)
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.map(event => (event, reducer.reduce(entity.data, event)))
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case (Some(_), h) if !h.isUpdate =>
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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, h) if h.isUpdate =>
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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: Tag, Event: Tag, State: Tag] =
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ZLayer.fromFunction(CommandEngine[Command, Event, State](_, _, _, _))
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