01.md

01. The bank interface

We will first be building a banking server and a teller client. The banking server represents the bank vault, and the client is what tellers use to deposit and withdraw funds from the vault. But we're already getting ahead of ourselves. Let's start with the general bank scaffold.

Server

To keep everything simple we will be building a simple TCP server that understands JSON objects, using the core net module and duplex-json-stream:

// 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 can be used to send a reply
  })
})

server.listen(3876)

Client

To interact with our bank, we will need a client that talks the same protocol. The server and client are asymmetric, just like in HTTP, so our clients will be ephemeral, while our server is where state lives.

// 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 can be used to send a request and close the socket

Problem

First extend the client to send {cmd: 'balance'} to the bank, and have the bank reply with {cmd: 'balance', balance: 0}

Testing

To test your application, open two terminals and run node bank.js in one session and run node teller.js in the other. The teller should print out Teller received: {cmd: 'balance', balance: 0} and exit, while the bank should print Bank received: {cmd: 'balance'} and keep running.

Well done, continue to problem 02