Building an event-sourced web application with Akka - Part 1

service-01.scala

class InvoiceService {
  val invoicesRef = Ref(Map.empty[String, Invoice]) // naive approach

  // ...
}

service-02.scala

import akka.stm._

class InvoiceService {
  val invoicesRef = Ref(Map.empty[String, Invoice]) // naive approach

  def addInvoiceItem(invoiceId: String, invoiceItem: InvoiceItem) = atomic {
    invoicesRef().get(invoiceId) match {
      case None          => DomainError("no invoice with id %s").fail
      case Some(invoice) => invoice.addItem(invoiceItem).result { (events, updated) =>
        invoicesRef alter { invoices => invoices + (invoiceId -> updated) }
        // TODO: log events
      }
    }
  }

  // ...
}

service-03.scala

trait EventLog[E <: Event] {
  private val eventsRef = Ref[List[E]](Nil)

  def log(events: List[E]) {
    eventsRef alter { current => events ::: current }
  }

  def store()
  def storeAsync()
}

service-04.scala

class InvoiceService {
  val invoicesRef = Ref(Map.empty[String, Invoice])
  val eventLog: EventLog[InvoiceEvent] = ...

  def addInvoiceItem(invoiceId: String, invoiceItem: InvoiceItem) = atomic {
    invoicesRef().get(invoiceId) match {
      case None          => DomainError("no invoice with id %s").fail
      case Some(invoice) => invoice.addItem(invoiceItem).result { (events, updated) =>
        invoicesRef alter { invoices => invoices + (invoiceId -> updated) }
        // write events to transactional event log
        eventLog log events
        // on commit, transfer events to persistent event log
        deferred { eventLog.storeAsync() }
      }
    }
  }

  // ...
}

service-05.scala

class InvoiceService {
  val invoicesRef = Ref(Map.empty[String, Invoice])

  def getInvoice(invoiceId: String): Option[Invoice] = invoicesRef().get(invoiceId)
  def getInvoices: Iterable[Invoice] = invoicesRef().values
}

service-06.scala

class InvoiceService {
  val invoicesRef = Ref(Map.empty[String, Ref[Invoice]])
  val eventLog: EventLog[InvoiceEvent] = ...

  // ...
}

service-07.scala

class InvoiceService {
  val invoicesRef = Ref(Map.empty[String, (Ref[Invoice], EventLog[InvoiceEvent])])

  // ...
}