How to Make Application Server for High Availability

Eric Wei
3 Min To Read
01 Oct, 2021
- Linux

If you want to keep your services online without any downtime, you must pay attention to the availability of your server.

WebRTC, for example, you should carefully check the availability of TURN Server, because TURN Server plays an important role to help two parties to connect to each other with Video or Audio streaming in different NAT networks. In common practice, you can keep telnet TURN Server at PORT 3478 or 3479 to check it is alive or not in a cron job.

In this post, we will show how to automatically monitor your application server and restart it during the downtime by a cron job.

Install pexpect lib in Python

sudo pip install pexpect --upgrade

Edit MonitorStun.py

Telnet your application server at opening port to check your application is alive or not.
If it goes down, ssh to your server and restart it

# MonitorStun.py
#!/usr/bin/env python
import socket
import subprocess
import sys
from datetime import datetime
from pexpect import pxssh


# ssh to Application server and restart it
def connect_turn_server():
  s = pxssh.pxssh()

  if not s.login ('Application Server IP', 'SERVER PORT', 'ACCOUNT', 'PASSWORD'):
    print "SSH session failed on login."
    print str(s)

  else:
    print "SSH session login successful"
    s.sendline ('sudo <your app>')
    s.prompt()         # match the prompt
    print s.before     # print everything before the prompt.
    s.logout()


# If your application server is online at port 8081, 8082
# Clear the screen
subprocess.call('clear', shell=True)

# Ask for input
remoteServer    = 'SERVER IP'
remoteServerIP  = socket.gethostbyname(remoteServer)

# Print a nice banner with information on which host we are about to scan
print "-" * 60
print "Please wait, scanning remote host", remoteServerIP
print "-" * 60

# Check what time the scan started
t1 = datetime.now()

# Using the range function to specify ports (here it will scans all ports between 1 and 1024)

# We also put in some error handling for catching errors

try:
    for port in range(8081, 8082):
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        result = sock.connect_ex((remoteServerIP, port))
        if result == 0:
            print "Port {}:      Open".format(port)
        else:
             print "Server is down"
             connect_server()
             print "restart Server OK"
        sock.close()


except KeyboardInterrupt:
 print "You pressed Ctrl+C"
    sys.exit()

except socket.error:
    print "Couldn't connect to server"
    sys.exit()

Add MonitorStun.py to cron job to check TURN Server in every 1 min.

*/1 * * * * /your_path/monitorStun.py

Conclusion

You have learned this technique to monitor any application at specific port by telnet, so as you can ensure high availability of your application server.

Of course, this technique can be applied to any services such as WebRTC TURN Server with port 3478, SIP Proxy with port 5060, Apache with port 80, or Tomcat with port 8080.