More on RESTful Communication

REST APIs use Uniform Resource Identifiers (URIs) to address resources. REST API designers should create URIs that convey a REST API's resource model to the potential clients of the API. When resources are named well, an API is intuitive and easy to use.

Mod Rewrite with POST requests

To partition API to POST and GET APIs, you need to modify Apache config to include some conditions. * Open your Apache config file, mine is located in /etc/apache2/sites-available/000-default.conf, and add the following with root permission

RewriteCond %{REQUEST_METHOD} ^POST$ [NC]
RewriteRule robot cgi/test.sh

The condition specifies how to redirect the address based on the request type. Check Apache site for more details about RewriteCond. * Restart your Apache server * To test POST requests, you cannot use your browser easily because the browser operates mainly on GET part of HTTP (unless you create a custom HTML form to submit data). Try curl command (sudo apt install curl if you don't have it on your system)

curl -X POST  '<address>/robot'
  • Check if you receive correct HTML response. You can use the same command for GET requests (replacing POST by GET). curl is quite simpler than nc as you don't need to specify HTTP headers. However, it is good to experience low-level interactions, particularly with HTTP protocol, a text-based protocol intended by design to help humans debug easily without sophisticated parsing tools.

Variables

Now, let's use the regex to have a POST URI that takes any number. Replace the RewriteRule by the following:

RewriteRule ^robot/([0-9]*\.?[0-9]*$) cgi/test.sh?data=$1

Let's parse it now: * ^ indicates the beginning of URI, where $ marks the end. So robot/ should be the first sequence of letters. () means a group of letters and is also used as a placeholder for variables in mod_rewrite. [0-9] means any number from 0 to 9, * means the previous character type (or block) is repeated once or multiple times. + means it is repeated 0 or many times. \ is a skip letter for special characters like .. ? says the previous char or block occurs zero or exactly one time. * Now, test the URI with a POST request

curl -X POST  '<address>/robot/123.23'
  • The output should be
<h1>Hello world from NSR 4.0!</h1>
<h2>My name is XXX </h2>
<h2> data=123.23 is selected</h2>
  • Try different addresses and see if the regex rule works as intended
curl -X POST  '<address>/robot/123.23'
curl -X POST  '<address>/robot/532'
curl -X POST  '<address>/robot/12a3.23'  # Error 404 Not found

Web communication in Python - Client side

It is quite easy to make HTTP requests in python using libraries like requests. Below is an example code. * Lets make a simple GET request to your previously designed API. Create a file, named req.py (or run interatively using ipython or Jupiter notebook )

import requests
url = '<ur server dns>/select/b'
x = requests.get(url)
print(x.text)
  • If you run print(x) you will get HTTP response status code, which follows a standard protocol: 1. Informational responses (100–199) 2. Successful responses (200–299) 3. Redirection messages (300–399) 4. Client error responses (400–499) 5. Server error responses (500–599) Check this link for more details.
  • You can replace x = requests.get(url) by x = requests.post(url) to run a POST request. POST allows attaching additional content to requests. One popular attachment is JSON data, a simple text-based data structure for key-value pairs (and more). So, you may modify the above code to attach content to your request.
myobj = {'somekey': 'somevalue'}
x = requests.post(url, json = myobj)

Web communication in Python - Server side

Instead of relying on bash script (which in many cases is useful for simple application), we will use a fully fledged programming language Python. * Create a file <web root>/cgi/test.py. 

sudo touch <web root>/cgi/test.py
  • Give the the write permission and owner
cd <web root>/cgi
sudo chown www-data:www-data *
sudo chmod 755 *

* means all files in Bash. * Now, you may modify to return a JSON response, which is more convenient response for an API (since apps calling the API can easily parse it, unlike html). We want the output to look like

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: <decided automatically>

{"number":"<received number> * 10"}
  • You may produce this output using bash script with cgi/test.sh, but I'd personally go with a programming language such as Python. Below is an example code, which is self-explanatory
#! /usr/bin/python3
import os

# Parsing QUERY_STRING
q = os.getenv('QUERY_STRING')
num_str = q.split('=')[1]
num = int(num_str) * 10

# Generating HTTP  response with JSON
response = "{\"number\": \"%d\"} "%num
print("Content-Type: application/json")
print("Content-Length: %d\n\n"% len(response))
print(response);