Webserver Programming I

Info 253a: Web Architecture
Kay Ashaolu

What is a server?

  • A computer program or a device that provides functionality for other programs or devices, called "clients"
  • Servers can provide various functionalities, often called "services", such as sharing data or resources among multiple clients, or performing computation for a client
  • A single server can serve multiple clients, and a single client can use multiple servers

- Wikipedia

Server = "computer program"

  • A server is technically not the computer that runs this computer program.
  • A server is a program that is hosted by a computer (called a host)
  • We will learn how to write and run these programs to service "clients"
  • Example of clients include desktop browsers, mobile browsers, curl (via the terminal)

The Request-Response model

  • Client sends a request to the Server, Server responds back with data and typcially an acknolwedgement of receipt
  • The Server has declared specific paths (called routes) that when accessed would execute specific functions
  • Functions would return a response, which is sent to the Client

Review

Let's build our own server

  • We will be using NodeJS, a runtime environment for building server-side applications
  • NodeJS uses JavaScript as its programming language

NodeJS is an application like everything else

  • Need to install it on your host (e.g. your computer)
  • Once installed, you can pass it a JavaScript file with the instructions on how to run your server

Let's get started

					 

 

And that's it for installation!

sudo apt-get install nodejs

Well not quite

  • One of the strengths of NodeJS is the sheer number of open source libraries that exist
  • These libraries add functionality to the core NodeJS modules
  • For this week we will be using the library express to demonstrate server programming
  • Afterwards we will be introducing another framework NextJS that is built around React

NPM

sudo apt-get install npm
  • And you are done!

    • Let's create our first server

    • Open a temrinal and navigate to a blank folder
    • npm init -y
    • Now you have a NodeJS project

    What did that do?

    • Created a single file named package.json
    • Think of this file as the manifest for your server application
    • It indicates the name of your application and where to find it
    • It also contains all of the information to install and run your application

    Package.json

    {
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"

}

    Let's install external libraries

    • Here we are going to install express, a NodeJS library for writing web servers
    • Do it by typing the following: 
      npm install express --save
    • The "npm install express" part downloads the express module to a folder called "node_modules"
    • The "--save" part saves a reference to the dependency (library) in that "package.json" file

    Updated package.json

    {
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.14.0"
  }
}

    Now let's run our own webserver

    app.js

    var express = require('express'); // Adding the express library 

var app = express(); // initializing applicaiton

// Define your routes here
app.get('/', function (req, res) {
  res.send("<h1>Hello World</h1>");
});

// Start up server on port 3000 on host localhost
var server = app.listen(3000, function () {
  console.log('Server on localhost listening on port 3000');
});

    Run your webserver

    • nodejs app.js
    • And then go to "http://localhost:3000/" on a browser on your computer

    What's going on?

    • We first created a variable that points to a function that contains the express module
    • Then we ran the express function and it returned a variable that we assigned to "app"
    • We then set up some routes
    • And at the end we started our server by telling our app to listen to port 3000
    • Note the console.log statement: in the server runtime environment console.log logs to the terminal

    Note about routes

    					 
    • Routes associate a path to a function that sends a response back to the client
    • So if a client asks for the root path: http://localhost:3000/, it will run the above function
    • The response body will be "<h1>Hello World</h1>"
    app.get('/', function (req, res) {
  res.send("<h1>Hello World</h1>");
});

    Questions?