Add example for dealing with side effects

code-samples/src/main/scala/event_sourcing/BankAccount.scala

@@ -1,7 +1,6 @@
 package event_sourcing
 
 case class BankAccount(owner:String, private var postings :Seq[MonetaryAmount] = Seq.empty) {
-
   def balance = {
     var balance = MonetaryAmount(0)
     for (posting <- postings){

code-samples/src/main/scala/event_sourcing/BankAccountRepository.scala

@@ -0,0 +1,8 @@
+package event_sourcing
+
+import java.util.UUID
+
+trait BankAccountRepository {
+  def getAccount(id:UUID) : BankAccount
+  def saveAccount(account:BankAccount)
+}

code-samples/src/main/scala/event_sourcing/ChangeOwner.scala

@@ -1,9 +1,15 @@
 package event_sourcing
 
+import java.util.UUID
+
 // changeOwnerCommand
-class ChangeOwner(newOwner:String) extends Command[BankAccount] {
-  def execute(bankAccount:BankAccount): BankAccount = {
-    bankAccount.changeOwner(newOwner)
+class BankAccountCommandHandler(bankAccountRepo:BankAccountRepository) {
+  def handle(changeOwner:ChangeOwner) {
+    val account = bankAccountRepo.getAccount(changeOwner.accountID)
+    val modifiedAccount = account.changeOwner(changeOwner.newOwner)
+    bankAccountRepo.saveAccount(modifiedAccount)
   }
 }
+
+case class ChangeOwner(accountID: UUID, newOwner:String) extends Command
 // changeOwnerCommand

code-samples/src/main/scala/event_sourcing/Command.scala

@@ -1,5 +1,3 @@
 package event_sourcing
 
-trait Command[T] {
-  def execute(someObject:T) : T
-}
+trait Command

code-samples/src/main/scala/event_sourcing/EventBus.scala

@@ -0,0 +1,12 @@
+package event_sourcing
+
+import event_sourcing.events.DomainEvent
+
+object EventBus {
+  var eventStream = Seq.empty[DomainEvent]
+
+  def publishEvent(event:DomainEvent) : Unit = {
+    eventStream :+= event
+    print(event + " published")
+  }
+}

code-samples/src/main/scala/event_sourcing/OrderExample.scala

@@ -0,0 +1,54 @@
+package event_sourcing
+
+import java.util.UUID
+import event_sourcing.events.DomainEvent
+
+abstract class Order
+case class BuyOrder(amount:MonetaryAmount, isin:String) extends Order
+case class PerformBuyOrder(orderBookId:UUID, order:BuyOrder) extends Command
+case class OrderConfirmation(orderId:UUID, order:BuyOrder)
+case class OrderSentToBank(orderId:UUID, order:BuyOrder) extends DomainEvent
+object OrderSentToBank {
+  def from(orderConfirmation:OrderConfirmation) = {
+    OrderSentToBank(orderConfirmation.orderId, orderConfirmation.order)
+  }
+}
+
+// dealingWithSideEffects
+
+class BrokerCommandHandler (bankInterface:BankInterface,
+                            orderBookRepository:OrderBookRepository) {
+  def handle(buyCommand:PerformBuyOrder) = {
+    var orderBook = orderBookRepository.load(buyCommand.orderBookId)
+    val validationResult = bankInterface.validate(buyCommand.order)
+    if (!validationResult.succeeded)
+      throw new InvalidOrderException(validationResult)
+    val orderConfirmation = bankInterface.perform(buyCommand.order)
+    val orderPerformedEvent = OrderSentToBank.from(orderConfirmation)
+    orderBook = orderBook.handle(orderPerformedEvent)
+    orderBookRepository.save(orderBook)
+  }
+}
+
+case class OrderBook(orders:Seq[Order] = Seq.empty) {
+  def handle(orderSentToBank:OrderSentToBank) : OrderBook = {
+    this.copy(orders :+ orderSentToBank.order)
+  }
+}
+
+// dealingWithSideEffects
+
+trait BankInterface {
+  def validate(order:BuyOrder) : ValidationResult
+  def perform(order:BuyOrder) : OrderConfirmation
+}
+
+class InvalidOrderException(validationResult:ValidationResult) extends RuntimeException(validationResult.toString)
+
+case class ValidationResult(succeeded:Boolean)
+case class OrderFailedEvent(validationResult:ValidationResult) extends DomainEvent
+
+trait OrderBookRepository {
+  def load(orderBookID:UUID) : OrderBook
+  def save(orderBook:OrderBook)
+}

code-samples/src/main/scala/event_sourcing/DepositPerformed.scala → code-samples/src/main/scala/event_sourcing/events/DepositPerformed.scala

@@ -1,7 +1,8 @@
-package event_sourcing
+package event_sourcing.events
 
 import java.util.UUID
+import event_sourcing.MonetaryAmount
 
 // depositEvent
-case class DepositPerformed(account:UUID, amount:MonetaryAmount)
+case class DepositPerformed(account:UUID, amount:MonetaryAmount) extends DomainEvent
 // depositEvent

code-samples/src/main/scala/event_sourcing/events/DomainEvent.scala

@@ -0,0 +1,3 @@
+package event_sourcing.events
+
+trait DomainEvent

pages/slides/06-how-eventsourcing-works/09-command-sourcing-advantages.md

@@ -7,4 +7,4 @@
 
 ### Contra {.cons}
 
-- 
+- Replaying may inflict unexpected side effects

pages/slides/06-how-eventsourcing-works/10-side-effects.md

@@ -0,0 +1,8 @@
+## Dealing with side effects
+
+How do we deal with side effects?
+
+- Leave side effect to the command handler 
+{.slide}
+- Record result as domain event, if necessary 
+{.slide}

pages/slides/06-how-eventsourcing-works/11-side-effects-example.md

@@ -0,0 +1 @@
+@(dealingWithSideEffects)[../../../code-samples/src/main/scala/event_sourcing/OrderExample.scala]

static/custom.css

@@ -0,0 +1,3 @@
+div.codehilite {
+	line-height: 1;
+}

templates/base.html

@@ -10,10 +10,11 @@
 
   <!-- Required stylesheet -->
   <link rel="stylesheet" href="{{ STATIC_URL }}/core/deck.core.css">
+  <link rel="stylesheet" href="{{ STATIC_URL }}/custom.css">
 
   <!-- Extension CSS files go here. Remove or add as needed. -->
-    <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/960.css">
-      <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/pygments-colorful.css">
+  <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/960.css">
+  <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/pygments-colorful.css">
   <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/extensions/goto/deck.goto.css">
   <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/extensions/menu/deck.menu.css">
   <link rel="stylesheet" media="screen" href="{{ STATIC_URL }}/extensions/navigation/deck.navigation.css">