02. Depositing schmeckels

Congratulations on passing problem 01. As you probably noticed the bank so far doesn't do much, so let's try and expand the functionality a bit. Pinky promise we'll get to the crypto soon!

// bank.js
var jsonStream = require('duplex-json-stream')
var net = require('net')

var server = net.createServer(function (socket) {
  socket = jsonStream(socket)

  socket.on('data', function (msg) {
    console.log('Bank received:', msg)
    socket.write({cmd: 'balance', balance: 0})
  })
})

server.listen(3876)

// teller.js
var jsonStream = require('duplex-json-stream')
var net = require('net')

var client = jsonStream(net.connect(3876))

client.on('data', function (msg) {
  console.log('Teller received:', msg)
})

client.end({cmd: 'balance'})

Tracking transactions

Next you should add a deposit command, which takes an amount and adds this to the bank vault balance. But to keep an audit trail all deposits should be kept in a transaction log. This will prove to have lots of benefits later. A transaction log is just another way of saying that all operations that modify the state are kept in an array, and the current state is derived by .reduceing this array.

For example, here's a transaction log that should reduce to balance of 250 schmeckels:

var log = [
  {cmd: 'deposit', amount: 130},
  {cmd: 'deposit', amount: 0},
  {cmd: 'deposit', amount: 120}
]

Side quest

As an optional exercise, try writing a .reduce function that takes the above log, and arrives at the correct balance. To test it out, try adding more entries to the log.

Problem

You job now is to expand the teller.js and bank.js with the deposit command, that is stored in a transaction log and updates the bank state (ie. it's balance). When the bank gets a deposit command, it should reply with the current balance like this: {cmd: 'balance', balance: someNumber}. A good idea is to make teller.js a very simple CLI tool, reading commands and arguments from process.argv.

switch (command) {
  case 'balance':
    // ...
    break

  case 'deposit':
    // ...
    break

  default:
    // Unknown command
    break
}

Testing

Spin up your new bank.js to make a couple of deposit up to 250 schmeckels in your bank, using node teller.js deposit 123, and checking your balance.

Continue to problem 03