Let's write a simple web server that can return a single line of text to any connected web browser.

The core idea is during the initialization of this web server, we should make a call to select.epoll() and register our server's file descriptor for event notifications. In the web server's executive code, the socket event is monitored as follows:

Listing 2.4 Simple web server using select.epoll 
#!/usr/bin/env python 
# Python Network Programming Cookbook, Second Edition -- Chapter - 2 
# This program is optimized for Python 2.7.12 and Python 3.5.2. 
# It may run on any other version with/without modifications. 
 
 
import socket 
import select 
import argparse 
 
SERVER_HOST = 'localhost' 
 
EOL1 = b'

' 
EOL2 = b'

' 
SERVER_RESPONSE  = b"""HTTP/1.1 200 OK
Date: Mon, 1 Apr 2013 01:01:01 GMT
Content-Type: text/plain
Content-Length: 25

 
Hello from Epoll Server!""" 
 
 
class EpollServer(object): 
    """ A socket server using Epoll""" 
     
    def __init__(self, host=SERVER_HOST, port=0): 
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) 
        self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) 
        self.sock.bind((host, port)) 
        self.sock.listen(1) 
        self.sock.setblocking(0) 
        self.sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1) 
        print ("Started Epoll Server") 
        self.epoll = select.epoll() 
        self.epoll.register(self.sock.fileno(), select.EPOLLIN) 
     
    def run(self): 
        """Executes epoll server operation""" 
        try: 
            connections = {}; requests = {}; responses = {} 
            while True: 
                events = self.epoll.poll(1) 
                for fileno, event in events: 
                    if fileno == self.sock.fileno(): 
                        connection, address = self.sock.accept() 
                        connection.setblocking(0) 
                        self.epoll.register(connection.fileno(),
                        select.EPOLLIN) 
                        connections[connection.fileno()] = connection 
                        requests[connection.fileno()] = b'' 
                        responses[connection.fileno()] = SERVER_RESPONSE 
                    elif event & select.EPOLLIN: 
                        requests[fileno] += connections[fileno].recv(1024) 
                        if EOL1 in requests[fileno] or EOL2 
                        in requests[fileno]: 
                            self.epoll.modify(fileno, select.EPOLLOUT) 
                            print('-'*40 + '
' + requests[fileno].decode()
                                   [:-2]) 
                    elif event & select.EPOLLOUT: 
                        byteswritten = connections[fileno].
                                       send(responses[fileno]) 
                        responses[fileno] = responses[fileno]
                                            [byteswritten:] 
                        if len(responses[fileno]) == 0: 
                            self.epoll.modify(fileno, 0) 
                            connections[fileno].shutdown(socket.SHUT_RDWR) 
                    elif event & select.EPOLLHUP: 
                        self.epoll.unregister(fileno) 
                        connections[fileno].close() 
                        del connections[fileno] 
        finally: 
            self.epoll.unregister(self.sock.fileno()) 
            self.epoll.close() 
            self.sock.close() 
 
 
if __name__ == '__main__': 
    parser = argparse.ArgumentParser(description='Socket Server 
                                                  Example with Epoll') 
    parser.add_argument('--port', action="store", dest="port",
                                              type=int, required=True) 
    given_args = parser.parse_args()  
    port = given_args.port 
    server = EpollServer(host=SERVER_HOST, port=port) 
    server.run()     
 

If you run this script and access the web server from your browsers, such as Google Chrome or Mozilla Firefox, by entering http://localhost:8800/, the following output will be shown in the console:

$ python 2_4_simple_web_server_with_epoll.py --port=8800
Started Epoll Server
----------------------------------------
GET / HTTP/1.1
Host: localhost:8800
Connection: keep-alive
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
DNT: 1
Accept-Encoding: gzip, deflate, sdch, br
Accept-Language: en-US,en;q=0.8
    
----------------------------------------
GET /favicon.ico HTTP/1.1
Host: localhost:8800
Connection: keep-alive
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36
Accept: image/webp,image/*,*/*;q=0.8
DNT: 1
Referer: http://localhost:8800/
Accept-Encoding: gzip, deflate, sdch, br
Accept-Language: en-US,en;q=0.8  

You will also be able to see the following line in your browser:

Hello from Epoll Server!  

The following screenshot shows the scenario: