Server Programming I

Info 253: 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 typically an acknowledgement 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 Flask, a Python microframework that makes buliding web servers and API's easier
• This will mean that you may have to learn yet another programming language

One step back: What's Python?

• Python is a high-level programming language that focuses on readability
• Python syntax is easier to understand (than even JavaScript)
• Python is a general programming language that can be used for a host of applications from webservers to back end applications

Learn Python through JavaScript

function helloWorld(first_name, last_name) {
  var message = "Hello World " + first_name + " " + last_name;
  return message;
}
					

def hello_world(first_name, last_name):
  message = "Hello World " + first_name + " " + last_name
  return message
					

Learn Python through JavaScript

function countToX(x) {
  var message = "";
  for(var i = 0; i <= x; i = i+1) {
    message = message + i + " ";
  }
  return message;
}
					

def count_to_x(x):
  message = ""
  for i in range(x):
    message = message + str(i) + " "
  return message
					

Learn Python through JavaScript

JavaScript

var student = {};
student.name = prompt("Enter your name");
student.attempts = [];

answer = false
while(answer != true) {
	value = prompt("What is 8+8?");
	student.attempts.push(value);

	if(value == "16") {
		answer = true;
	}
	else {
		answer = false;
	}
}

alert(student.name + " answers: " + student.attempts);
					

Learn Python through JavaScript

Python

student = {}

student["name"] = input("Enter your name ")
student["attempts"] = []

answer = False
while answer != True:
  value = input("What is 8+8? ")
  student["attempts"].append(value)

  if value == "16":
    answer = True
  else:
    answer = False

print(student["name"] + " answers: " + str(student["attempts"]))
					

Back to Flask: an application like everything else

• Need to install it as a module of your Python installation
• Once installed, you can use it in your Python program

Let's get started

Quick note on pip

• Pip is a Python package manager manager that makes it easy to install new python modules
• There is a list of packages to install in the requirements.txt file of the assignment 2 repo
• We use pip to install Flask and Requests

Let's install the Python packages we need

• pip install flask

• pip install requests

Let's write our Python webserver application

webserver.py

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
  return "<h1>Hello World!</h1>"
						

Let's create our first server

• Open a terminal and navigate to a blank folder

• export FLASK_APP=webserver.py

• flask run --host=0.0.0.0

What did that do?

• You just created your own web server!
• Go to localhost:5000 on your browser to see it work

What's going on?

• We first imported the Flask class from the flask module that we just installed
• Next we created a variable that points to a new flask application
• We then set up a route (the root "/" route)
• We defined a function that if someone goes to the root of our webserver, then the server should send the string "Hello World"

Note about routes

@app.route('/')
def hello_world():
  return "<h1>Hello World!</h1>"
					

• 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:5000/, it will run the above function
• The response body will be "<h1>Hello World</h1>"

Questions?