We can write a client that will wait for a particular network service forever or for a timeout. In this example, by default, we would like to check when a web server is up in localhost. If you specified some other remote host or port, that information will be used instead.

Listing 3.3 shows waiting for a remote network service, as follows:

#!/usr/bin/env python 
# Python Network Programming Cookbook, Second Edition -- Chapter - 3 
# 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 argparse 
import socket 
import errno 
from time import time as now 
 
DEFAULT_TIMEOUT = 120 
DEFAULT_SERVER_HOST = 'localhost' 
DEFAULT_SERVER_PORT = 80 
 
class NetServiceChecker(object): 
    """ Wait for a network service to come online""" 
    def __init__(self, host, port, timeout=DEFAULT_TIMEOUT): 
        self.host = host 
        self.port = port 
        self.timeout = timeout 
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) 
     
    def end_wait(self): 
        self.sock.close() 
 
    def check(self): 
        """ Check the service """ 
        if self.timeout: 
            end_time = now() + self.timeout 
     
        while True: 
            try: 
                if self.timeout: 
                    next_timeout = end_time - now() 
                    if next_timeout < 0: 
                        return False 
                    else: 
                        print ("setting socket next timeout %ss"
                                 %round(next_timeout)) 
                        self.sock.settimeout(next_timeout) 
                self.sock.connect((self.host, self.port)) 
            # handle exceptions 
            except socket.timeout as err: 
                if self.timeout: 
                    return False 
            except socket.error as err: 
                print ("Exception: %s" %err) 
            else: # if all goes well 
                self.end_wait() 
                return True 
 
if __name__ == '__main__': 
    parser = argparse.ArgumentParser(description='Wait for
              Network Service') 
    parser.add_argument('--host', action="store", dest="host",  
    default=DEFAULT_SERVER_HOST) 
    parser.add_argument('--port', action="store", dest="port", 
    type=int, default=DEFAULT_SERVER_PORT) 
    parser.add_argument('--timeout', action="store", dest="timeout", 
    type=int, default=DEFAULT_TIMEOUT) 
    given_args = parser.parse_args()  
    host, port, timeout = given_args.host, given_args.port, 
                          given_args.timeout 
    service_checker = NetServiceChecker(host, port, timeout=timeout) 
    print ("Checking for network service %s:%s ..." %(host, port)) 
    if service_checker.check(): 
        print ("Service is available again!") 

If a web server is running on your machine, this script will show the following output:

$ python 3_3_wait_for_remote_service.py 
Waiting for network service localhost:80 ... 
setting socket next timeout 120.0s 
Service is available again!

If you do not have a web server already running in your computer, make sure to install one such as Apache 2 web server:

$ sudo apt install apache2
  

Now, stop the Apache process:

$ sudo /etc/init.d/apache2 stop
  

It will print the below message while stopping the service.

[ ok ] Stopping apache2 (via systemctl): apache2.service.
  

Run this script, and start Apache again.

$ sudo /etc/init.d/apache2 start
[ ok ] Starting apache2 (via systemctl): apache2.service.
  

The output pattern will be different for a different machine. On my machine, the following output pattern was found:

Exception: [Errno 103] Software caused connection abort
setting socket next timeout 119.0s
Exception: [Errno 111] Connection refused
setting socket next timeout 119.0s
Exception: [Errno 103] Software caused connection abort
setting socket next timeout 119.0s
Exception: [Errno 111] Connection refused
setting socket next timeout 119.0s
    
And finally when Apache2 is up again, the following log is printed:
    
Service is available again!
  

The following screenshot shows the waiting for an active Apache web server process: