Style Guide

Event handlers in the state

The section about Changing Behavior described how commands and events can be handled differently depending on the state. One can take that one step further and define the event handler inside the state classes. In the next section the command handlers are also defined in the state.

The state can be seen as your domain object and it should contain the core business logic. Then it’s a matter of taste if event handlers and command handlers should be defined in the state or be kept outside of it.

Here we are using a bank account as the example domain. It has 3 state classes that are representing the lifecycle of the account; EmptyAccount, OpenedAccount, and ClosedAccount.

Scala

copysourceobject AccountEntity {
  // Command
  sealed trait Command extends CborSerializable
  final case class CreateAccount(replyTo: ActorRef[StatusReply[Done]]) extends Command
  final case class Deposit(amount: BigDecimal, replyTo: ActorRef[StatusReply[Done]]) extends Command
  final case class Withdraw(amount: BigDecimal, replyTo: ActorRef[StatusReply[Done]]) extends Command
  final case class GetBalance(replyTo: ActorRef[CurrentBalance]) extends Command
  final case class CloseAccount(replyTo: ActorRef[StatusReply[Done]]) extends Command

  // Reply
  final case class CurrentBalance(balance: BigDecimal) extends CborSerializable

  // Event
  sealed trait Event extends CborSerializable
  case object AccountCreated extends Event
  case class Deposited(amount: BigDecimal) extends Event
  case class Withdrawn(amount: BigDecimal) extends Event
  case object AccountClosed extends Event

  val Zero = BigDecimal(0)

  // State
  sealed trait Account extends CborSerializable {
    def applyEvent(event: Event): Account
  }
  case object EmptyAccount extends Account {
    override def applyEvent(event: Event): Account = event match {
      case AccountCreated => OpenedAccount(Zero)
      case _              => throw new IllegalStateException(s"unexpected event [$event] in state [EmptyAccount]")
    }
  }
  case class OpenedAccount(balance: BigDecimal) extends Account {
    require(balance >= Zero, "Account balance can't be negative")

    override def applyEvent(event: Event): Account =
      event match {
        case Deposited(amount) => copy(balance = balance + amount)
        case Withdrawn(amount) => copy(balance = balance - amount)
        case AccountClosed     => ClosedAccount
        case AccountCreated    => throw new IllegalStateException(s"unexpected event [$event] in state [OpenedAccount]")
      }

    def canWithdraw(amount: BigDecimal): Boolean = {
      balance - amount >= Zero
    }

  }
  case object ClosedAccount extends Account {
    override def applyEvent(event: Event): Account =
      throw new IllegalStateException(s"unexpected event [$event] in state [ClosedAccount]")
  }

  // when used with sharding, this TypeKey can be used in `sharding.init` and `sharding.entityRefFor`:
  val TypeKey: EntityTypeKey[Command] =
    EntityTypeKey[Command]("Account")

  // Note that after defining command, event and state classes you would probably start here when writing this.
  // When filling in the parameters of EventSourcedBehavior.apply you can use IntelliJ alt+Enter > createValue
  // to generate the stub with types for the command and event handlers.

  def apply(accountNumber: String, persistenceId: PersistenceId): Behavior[Command] = {
    EventSourcedBehavior.withEnforcedReplies(persistenceId, EmptyAccount, commandHandler(accountNumber), eventHandler)
  }

  private def commandHandler(accountNumber: String): (Account, Command) => ReplyEffect[Event, Account] = {
    (state, cmd) =>
      state match {
        case EmptyAccount =>
          cmd match {
            case c: CreateAccount => createAccount(c)
            case _                => Effect.unhandled.thenNoReply() // CreateAccount before handling any other commands
          }

        case acc @ OpenedAccount(_) =>
          cmd match {
            case c: Deposit      => deposit(c)
            case c: Withdraw     => withdraw(acc, c)
            case c: GetBalance   => getBalance(acc, c)
            case c: CloseAccount => closeAccount(acc, c)
            case c: CreateAccount =>
              Effect.reply(c.replyTo)(StatusReply.Error(s"Account $accountNumber is already created"))
          }

        case ClosedAccount =>
          cmd match {
            case c: Deposit =>
              replyClosed(accountNumber, c.replyTo)
            case c: Withdraw =>
              replyClosed(accountNumber, c.replyTo)
            case GetBalance(replyTo) =>
              Effect.reply(replyTo)(CurrentBalance(Zero))
            case CloseAccount(replyTo) =>
              replyClosed(accountNumber, replyTo)
            case CreateAccount(replyTo) =>
              replyClosed(accountNumber, replyTo)
          }
      }
  }

  private def replyClosed(
      accountNumber: String,
      replyTo: ActorRef[StatusReply[Done]]): ReplyEffect[Event, Account] = {
    Effect.reply(replyTo)(StatusReply.Error(s"Account $accountNumber is closed"))
  }

  private val eventHandler: (Account, Event) => Account = { (state, event) =>
    state.applyEvent(event)
  }

  private def createAccount(cmd: CreateAccount): ReplyEffect[Event, Account] = {
    Effect.persist(AccountCreated).thenReply(cmd.replyTo)(_ => StatusReply.Ack)
  }

  private def deposit(cmd: Deposit): ReplyEffect[Event, Account] = {
    Effect.persist(Deposited(cmd.amount)).thenReply(cmd.replyTo)(_ => StatusReply.Ack)
  }

  private def withdraw(acc: OpenedAccount, cmd: Withdraw): ReplyEffect[Event, Account] = {
    if (acc.canWithdraw(cmd.amount))
      Effect.persist(Withdrawn(cmd.amount)).thenReply(cmd.replyTo)(_ => StatusReply.Ack)
    else
      Effect.reply(cmd.replyTo)(
        StatusReply.Error(s"Insufficient balance ${acc.balance} to be able to withdraw ${cmd.amount}"))
  }

  private def getBalance(acc: OpenedAccount, cmd: GetBalance): ReplyEffect[Event, Account] = {
    Effect.reply(cmd.replyTo)(CurrentBalance(acc.balance))
  }

  private def closeAccount(acc: OpenedAccount, cmd: CloseAccount): ReplyEffect[Event, Account] = {
    if (acc.balance == Zero)
      Effect.persist(AccountClosed).thenReply(cmd.replyTo)(_ => StatusReply.Ack)
    else
      Effect.reply(cmd.replyTo)(StatusReply.Error("Can't close account with non-zero balance"))
  }

}

Java

copysourcepublic class AccountEntity
    extends EventSourcedBehaviorWithEnforcedReplies<
        AccountEntity.Command, AccountEntity.Event, AccountEntity.Account> {

  public static final EntityTypeKey<Command> ENTITY_TYPE_KEY =
      EntityTypeKey.create(Command.class, "Account");

  // Command
  interface Command extends CborSerializable {}

  public static class CreateAccount implements Command {
    public final ActorRef<StatusReply<Done>> replyTo;

    @JsonCreator
    public CreateAccount(ActorRef<StatusReply<Done>> replyTo) {
      this.replyTo = replyTo;
    }
  }

  public static class Deposit implements Command {
    public final BigDecimal amount;
    public final ActorRef<StatusReply<Done>> replyTo;

    public Deposit(BigDecimal amount, ActorRef<StatusReply<Done>> replyTo) {
      this.replyTo = replyTo;
      this.amount = amount;
    }
  }

  public static class Withdraw implements Command {
    public final BigDecimal amount;
    public final ActorRef<StatusReply<Done>> replyTo;

    public Withdraw(BigDecimal amount, ActorRef<StatusReply<Done>> replyTo) {
      this.amount = amount;
      this.replyTo = replyTo;
    }
  }

  public static class GetBalance implements Command {
    public final ActorRef<CurrentBalance> replyTo;

    @JsonCreator
    public GetBalance(ActorRef<CurrentBalance> replyTo) {
      this.replyTo = replyTo;
    }
  }

  public static class CloseAccount implements Command {
    public final ActorRef<StatusReply<Done>> replyTo;

    @JsonCreator
    public CloseAccount(ActorRef<StatusReply<Done>> replyTo) {
      this.replyTo = replyTo;
    }
  }

  // Reply
  public static class CurrentBalance implements CborSerializable {
    public final BigDecimal balance;

    @JsonCreator
    public CurrentBalance(BigDecimal balance) {
      this.balance = balance;
    }
  }

  // Event
  interface Event extends CborSerializable {}

  public enum AccountCreated implements Event {
    INSTANCE
  }

  public static class Deposited implements Event {
    public final BigDecimal amount;

    @JsonCreator
    Deposited(BigDecimal amount) {
      this.amount = amount;
    }
  }

  public static class Withdrawn implements Event {
    public final BigDecimal amount;

    @JsonCreator
    Withdrawn(BigDecimal amount) {
      this.amount = amount;
    }
  }

  public static class AccountClosed implements Event {}

  // State
  interface Account extends CborSerializable {}

  public static class EmptyAccount implements Account {
    OpenedAccount openedAccount() {
      return new OpenedAccount(BigDecimal.ZERO);
    }
  }

  public static class OpenedAccount implements Account {
    final BigDecimal balance;

    @JsonCreator
    public OpenedAccount(BigDecimal balance) {
      this.balance = balance;
    }

    OpenedAccount makeDeposit(BigDecimal amount) {
      return new OpenedAccount(balance.add(amount));
    }

    boolean canWithdraw(BigDecimal amount) {
      return (balance.subtract(amount).compareTo(BigDecimal.ZERO) >= 0);
    }

    OpenedAccount makeWithdraw(BigDecimal amount) {
      if (!canWithdraw(amount))
        throw new IllegalStateException("Account balance can't be negative");
      return new OpenedAccount(balance.subtract(amount));
    }

    ClosedAccount closedAccount() {
      return new ClosedAccount();
    }
  }

  public static class ClosedAccount implements Account {}

  public static AccountEntity create(String accountNumber, PersistenceId persistenceId) {
    return new AccountEntity(accountNumber, persistenceId);
  }

  private final String accountNumber;

  private AccountEntity(String accountNumber, PersistenceId persistenceId) {
    super(persistenceId);
    this.accountNumber = accountNumber;
  }

  @Override
  public Account emptyState() {
    return new EmptyAccount();
  }

  @Override
  public CommandHandlerWithReply<Command, Event, Account> commandHandler() {
    CommandHandlerWithReplyBuilder<Command, Event, Account> builder =
        newCommandHandlerWithReplyBuilder();

    builder.forStateType(EmptyAccount.class).onCommand(CreateAccount.class, this::createAccount);

    builder
        .forStateType(OpenedAccount.class)
        .onCommand(Deposit.class, this::deposit)
        .onCommand(Withdraw.class, this::withdraw)
        .onCommand(GetBalance.class, this::getBalance)
        .onCommand(CloseAccount.class, this::closeAccount);

    builder
        .forStateType(ClosedAccount.class)
        .onAnyCommand(() -> Effect().unhandled().thenNoReply());

    return builder.build();
  }

  private ReplyEffect<Event, Account> createAccount(EmptyAccount account, CreateAccount command) {
    return Effect()
        .persist(AccountCreated.INSTANCE)
        .thenReply(command.replyTo, account2 -> StatusReply.ack());
  }

  private ReplyEffect<Event, Account> deposit(OpenedAccount account, Deposit command) {
    return Effect()
        .persist(new Deposited(command.amount))
        .thenReply(command.replyTo, account2 -> StatusReply.ack());
  }

  private ReplyEffect<Event, Account> withdraw(OpenedAccount account, Withdraw command) {
    if (!account.canWithdraw(command.amount)) {
      return Effect()
          .reply(
              command.replyTo,
              StatusReply.error("not enough funds to withdraw " + command.amount));
    } else {
      return Effect()
          .persist(new Withdrawn(command.amount))
          .thenReply(command.replyTo, account2 -> StatusReply.ack());
    }
  }

  private ReplyEffect<Event, Account> getBalance(OpenedAccount account, GetBalance command) {
    return Effect().reply(command.replyTo, new CurrentBalance(account.balance));
  }

  private ReplyEffect<Event, Account> closeAccount(OpenedAccount account, CloseAccount command) {
    if (account.balance.equals(BigDecimal.ZERO)) {
      return Effect()
          .persist(new AccountClosed())
          .thenReply(command.replyTo, account2 -> StatusReply.ack());
    } else {
      return Effect()
          .reply(command.replyTo, StatusReply.error("balance must be zero for closing account"));
    }
  }

  @Override
  public EventHandler<Account, Event> eventHandler() {
    EventHandlerBuilder<Account, Event> builder = newEventHandlerBuilder();

    builder
        .forStateType(EmptyAccount.class)
        .onEvent(AccountCreated.class, (account, created) -> account.openedAccount());

    builder
        .forStateType(OpenedAccount.class)
        .onEvent(Deposited.class, (account, deposited) -> account.makeDeposit(deposited.amount))
        .onEvent(Withdrawn.class, (account, withdrawn) -> account.makeWithdraw(withdrawn.amount))
        .onEvent(AccountClosed.class, (account, closed) -> account.closedAccount());

    return builder.build();
  }
}

Notice how the eventHandler delegates to the applyEvent in the Account (state), which is implemented in the concrete EmptyAccount, OpenedAccount, and ClosedAccount. Notice how the eventHandler delegates to methods in the concrete Account (state) classes; EmptyAccount, OpenedAccount, and ClosedAccount.

Optional initial state

Sometimes it’s not desirable to use a separate state class for the empty initial state, but rather treat that as there is no state yet. null can then be used as the emptyState, but be aware of that the state parameter will then be null for the first commands and events until the first event has be persisted to create the non-null state. It’s possible to use Optional instead of null but that results in rather much boilerplate to unwrap the Optional state parameter and therefore null is probably preferred. The following example illustrates using null as the emptyState. Option[State] can be used as the state type and None as the emptyState. Pattern matching is then used in command and event handlers at the outer layer before delegating to the state or other methods.

Scala

copysourceobject AccountEntity {
  // Command
  sealed trait Command extends CborSerializable
  final case class CreateAccount(replyTo: ActorRef[StatusReply[Done]]) extends Command
  final case class Deposit(amount: BigDecimal, replyTo: ActorRef[StatusReply[Done]]) extends Command
  final case class Withdraw(amount: BigDecimal, replyTo: ActorRef[StatusReply[Done]]) extends Command
  final case class GetBalance(replyTo: ActorRef[CurrentBalance]) extends Command
  final case class CloseAccount(replyTo: ActorRef[StatusReply[Done]]) extends Command

  // Reply
  final case class CurrentBalance(balance: BigDecimal) extends CborSerializable

  // Event
  sealed trait Event extends CborSerializable
  case object AccountCreated extends Event
  case class Deposited(amount: BigDecimal) extends Event
  case class Withdrawn(amount: BigDecimal) extends Event
  case object AccountClosed extends Event

  val Zero = BigDecimal(0)

  // type alias to reduce boilerplate
  type ReplyEffect = akka.persistence.typed.scaladsl.ReplyEffect[Event, Option[Account]]

  // State
  sealed trait Account extends CborSerializable {
    def applyCommand(cmd: Command): ReplyEffect
    def applyEvent(event: Event): Account
  }
  case class OpenedAccount(balance: BigDecimal) extends Account {
    require(balance >= Zero, "Account balance can't be negative")

    override def applyCommand(cmd: Command): ReplyEffect =
      cmd match {
        case Deposit(amount, replyTo) =>
          Effect.persist(Deposited(amount)).thenReply(replyTo)(_ => StatusReply.Ack)

        case Withdraw(amount, replyTo) =>
          if (canWithdraw(amount))
            Effect.persist(Withdrawn(amount)).thenReply(replyTo)(_ => StatusReply.Ack)
          else
            Effect.reply(replyTo)(StatusReply.Error(s"Insufficient balance $balance to be able to withdraw $amount"))

        case GetBalance(replyTo) =>
          Effect.reply(replyTo)(CurrentBalance(balance))

        case CloseAccount(replyTo) =>
          if (balance == Zero)
            Effect.persist(AccountClosed).thenReply(replyTo)(_ => StatusReply.Ack)
          else
            Effect.reply(replyTo)(StatusReply.Error("Can't close account with non-zero balance"))

        case CreateAccount(replyTo) =>
          Effect.reply(replyTo)(StatusReply.Error("Account is already created"))

      }

    override def applyEvent(event: Event): Account =
      event match {
        case Deposited(amount) => copy(balance = balance + amount)
        case Withdrawn(amount) => copy(balance = balance - amount)
        case AccountClosed     => ClosedAccount
        case AccountCreated    => throw new IllegalStateException(s"unexpected event [$event] in state [OpenedAccount]")
      }

    def canWithdraw(amount: BigDecimal): Boolean = {
      balance - amount >= Zero
    }

  }
  case object ClosedAccount extends Account {
    override def applyCommand(cmd: Command): ReplyEffect =
      cmd match {
        case c: Deposit =>
          replyClosed(c.replyTo)
        case c: Withdraw =>
          replyClosed(c.replyTo)
        case GetBalance(replyTo) =>
          Effect.reply(replyTo)(CurrentBalance(Zero))
        case CloseAccount(replyTo) =>
          replyClosed(replyTo)
        case CreateAccount(replyTo) =>
          replyClosed(replyTo)
      }

    private def replyClosed(replyTo: ActorRef[StatusReply[Done]]): ReplyEffect =
      Effect.reply(replyTo)(StatusReply.Error(s"Account is closed"))

    override def applyEvent(event: Event): Account =
      throw new IllegalStateException(s"unexpected event [$event] in state [ClosedAccount]")
  }

  // when used with sharding, this TypeKey can be used in `sharding.init` and `sharding.entityRefFor`:
  val TypeKey: EntityTypeKey[Command] =
    EntityTypeKey[Command]("Account")

  def apply(persistenceId: PersistenceId): Behavior[Command] = {
    EventSourcedBehavior.withEnforcedReplies[Command, Event, Option[Account]](
      persistenceId,
      None,
      (state, cmd) =>
        state match {
          case None          => onFirstCommand(cmd)
          case Some(account) => account.applyCommand(cmd)
        },
      (state, event) =>
        state match {
          case None          => Some(onFirstEvent(event))
          case Some(account) => Some(account.applyEvent(event))
        })
  }

  def onFirstCommand(cmd: Command): ReplyEffect = {
    cmd match {
      case CreateAccount(replyTo) =>
        Effect.persist(AccountCreated).thenReply(replyTo)(_ => StatusReply.Ack)
      case _ =>
        // CreateAccount before handling any other commands
        Effect.unhandled.thenNoReply()
    }
  }

  def onFirstEvent(event: Event): Account = {
    event match {
      case AccountCreated => OpenedAccount(Zero)
      case _              => throw new IllegalStateException(s"unexpected event [$event] in state [EmptyAccount]")
    }
  }

}

Java

copysourcepublic class AccountEntity
    extends EventSourcedBehaviorWithEnforcedReplies<
        AccountEntity.Command, AccountEntity.Event, AccountEntity.Account> {

  public static final EntityTypeKey<Command> ENTITY_TYPE_KEY =
      EntityTypeKey.create(Command.class, "Account");

  // Command
  interface Command extends CborSerializable {}

  public static class CreateAccount implements Command {
    public final ActorRef<StatusReply<Done>> replyTo;

    @JsonCreator
    public CreateAccount(ActorRef<StatusReply<Done>> replyTo) {
      this.replyTo = replyTo;
    }
  }

  public static class Deposit implements Command {
    public final BigDecimal amount;
    public final ActorRef<StatusReply<Done>> replyTo;

    public Deposit(BigDecimal amount, ActorRef<StatusReply<Done>> replyTo) {
      this.replyTo = replyTo;
      this.amount = amount;
    }
  }

  public static class Withdraw implements Command {
    public final BigDecimal amount;
    public final ActorRef<StatusReply<Done>> replyTo;

    public Withdraw(BigDecimal amount, ActorRef<StatusReply<Done>> replyTo) {
      this.amount = amount;
      this.replyTo = replyTo;
    }
  }

  public static class GetBalance implements Command {
    public final ActorRef<CurrentBalance> replyTo;

    @JsonCreator
    public GetBalance(ActorRef<CurrentBalance> replyTo) {
      this.replyTo = replyTo;
    }
  }

  public static class CloseAccount implements Command {
    public final ActorRef<StatusReply<Done>> replyTo;

    @JsonCreator
    public CloseAccount(ActorRef<StatusReply<Done>> replyTo) {
      this.replyTo = replyTo;
    }
  }

  // Reply
  public static class CurrentBalance implements CborSerializable {
    public final BigDecimal balance;

    @JsonCreator
    public CurrentBalance(BigDecimal balance) {
      this.balance = balance;
    }
  }

  // Event
  interface Event extends CborSerializable {}

  public enum AccountCreated implements Event {
    INSTANCE
  }

  public static class Deposited implements Event {
    public final BigDecimal amount;

    @JsonCreator
    Deposited(BigDecimal amount) {
      this.amount = amount;
    }
  }

  public static class Withdrawn implements Event {
    public final BigDecimal amount;

    @JsonCreator
    Withdrawn(BigDecimal amount) {
      this.amount = amount;
    }
  }

  public static class AccountClosed implements Event {}

  // State
  interface Account extends CborSerializable {}

  public static class OpenedAccount implements Account {
    public final BigDecimal balance;

    public OpenedAccount() {
      this.balance = BigDecimal.ZERO;
    }

    @JsonCreator
    public OpenedAccount(BigDecimal balance) {
      this.balance = balance;
    }

    OpenedAccount makeDeposit(BigDecimal amount) {
      return new OpenedAccount(balance.add(amount));
    }

    boolean canWithdraw(BigDecimal amount) {
      return (balance.subtract(amount).compareTo(BigDecimal.ZERO) >= 0);
    }

    OpenedAccount makeWithdraw(BigDecimal amount) {
      if (!canWithdraw(amount))
        throw new IllegalStateException("Account balance can't be negative");
      return new OpenedAccount(balance.subtract(amount));
    }

    ClosedAccount closedAccount() {
      return new ClosedAccount();
    }
  }

  public static class ClosedAccount implements Account {}

  public static AccountEntity create(String accountNumber, PersistenceId persistenceId) {
    return new AccountEntity(accountNumber, persistenceId);
  }

  private final String accountNumber;

  private AccountEntity(String accountNumber, PersistenceId persistenceId) {
    super(persistenceId);
    this.accountNumber = accountNumber;
  }

  @Override
  public Account emptyState() {
    return null;
  }

  @Override
  public CommandHandlerWithReply<Command, Event, Account> commandHandler() {
    CommandHandlerWithReplyBuilder<Command, Event, Account> builder =
        newCommandHandlerWithReplyBuilder();

    builder.forNullState().onCommand(CreateAccount.class, this::createAccount);

    builder
        .forStateType(OpenedAccount.class)
        .onCommand(Deposit.class, this::deposit)
        .onCommand(Withdraw.class, this::withdraw)
        .onCommand(GetBalance.class, this::getBalance)
        .onCommand(CloseAccount.class, this::closeAccount);

    builder
        .forStateType(ClosedAccount.class)
        .onAnyCommand(() -> Effect().unhandled().thenNoReply());

    return builder.build();
  }

  private ReplyEffect<Event, Account> createAccount(CreateAccount command) {
    return Effect()
        .persist(AccountCreated.INSTANCE)
        .thenReply(command.replyTo, account2 -> StatusReply.ack());
  }

  private ReplyEffect<Event, Account> deposit(OpenedAccount account, Deposit command) {
    return Effect()
        .persist(new Deposited(command.amount))
        .thenReply(command.replyTo, account2 -> StatusReply.ack());
  }

  private ReplyEffect<Event, Account> withdraw(OpenedAccount account, Withdraw command) {
    if (!account.canWithdraw(command.amount)) {
      return Effect()
          .reply(
              command.replyTo,
              StatusReply.error("not enough funds to withdraw " + command.amount));
    } else {
      return Effect()
          .persist(new Withdrawn(command.amount))
          .thenReply(command.replyTo, account2 -> StatusReply.ack());
    }
  }

  private ReplyEffect<Event, Account> getBalance(OpenedAccount account, GetBalance command) {
    return Effect().reply(command.replyTo, new CurrentBalance(account.balance));
  }

  private ReplyEffect<Event, Account> closeAccount(OpenedAccount account, CloseAccount command) {
    if (account.balance.equals(BigDecimal.ZERO)) {
      return Effect()
          .persist(new AccountClosed())
          .thenReply(command.replyTo, account2 -> StatusReply.ack());
    } else {
      return Effect()
          .reply(command.replyTo, StatusReply.error("balance must be zero for closing account"));
    }
  }

  @Override
  public EventHandler<Account, Event> eventHandler() {
    EventHandlerBuilder<Account, Event> builder = newEventHandlerBuilder();

    builder.forNullState().onEvent(AccountCreated.class, () -> new OpenedAccount());

    builder
        .forStateType(OpenedAccount.class)
        .onEvent(Deposited.class, (account, deposited) -> account.makeDeposit(deposited.amount))
        .onEvent(Withdrawn.class, (account, withdrawn) -> account.makeWithdraw(withdrawn.amount))
        .onEvent(AccountClosed.class, (account, closed) -> account.closedAccount());

    return builder.build();
  }
}