event-sourcing-example.js

// without event sourcing
function transferMoneyBetweenAccounts(amount, fromAccount, toAccount) {
  BankAccount.where({ id: fromAccount.id })
    .decrement({ amount });
  BankAccount.where({ id: toAccount.id })
    .increment({ amount });
}

function makeOnlinePayment(account, amount) {
  BankAccount.where({ id: account.id })
    .decrement({ amount });
}

// with event sourcing
function transferMoneyBetweenAccounts(amount, fromAccount, toAccount) {
  dispatchEvent(new TransferFrom(fromAccount, amount, toAccount));
  dispatchEvent(new TransferTo(toAccount, amount, fromAccount));
}

function makeOnlinePayment(account, amount) {
  dispatchEvent(new OnlinePaymentFrom(account, amount));
}

class TransferFrom extends Event {
  constructor(account, amount, toAccount) {
    this.account = account;
    this.amount = amount;
    this.toAccount = toAccount;
  }
  
  handle() {
    // store new OutwardTransfer event in database
    OutwardTransfer.create({ from: this.account, to: this.toAccount, amount: this.amount, date: Date.now() });
    
    // also update the current state of the account
    BankAccount.where({ id: this.account.id })
      .decrement({ amount: this.amount });
  }
}

class TransferTo extends Event {
  constructor(account, amount, fromAccount) {
    this.account = account;
    this.amount = amount;
    this.fromAccount = fromAccount;
  }
  
  handle() {
    // store new InwardTransfer event in database
    InwardTransfer.create({ from: this.fromAccount, to: this.account, amount: this.amount, date: Date.now() });
    
    // also update the current state of the account
    BankAccount.where({ id: this.account.id })
      .increment({ amount: this.amount });
  }
}

class OnlinePaymentFrom extends Event {
  constructor(account, amount) {
    this.account = account;
    this.amount = amount;
  }
  
  handle() {
    // store new OnlinePayment event in database
    OnlinePayment.create({ from: this.account, amount: this.amount, date: Date.now() });
    
    // also update the current state of the account
    BankAccount.where({ id: this.account.id })
      .decrement({ amount: this.amount });
  }
}