Check instances are really up with Cloud-Init

server.py

#!/usr/bin/env python

"""
Boot the VMs with the following cloud-init, and then start this code:
#cloud-config
phone_home:
 url: http://x.x.x.x:8000/$INSTANCE_ID/
 post: [ hostname, fqdn ]
"""

import SocketServer
import BaseHTTPServer
import requests
import os
import urlparse

class MyHandler(BaseHTTPServer.BaseHTTPRequestHandler):
    def do_HEAD(s):
        s.send_response(200)
        s.send_header("Content-type", "text/html")
        s.end_headers()

    def do_GET(s):
        """Respond to a GET request."""
        global instances
        if s.path.startswith("/reset"):
            instances = []
            s.send_response(200)
            s.send_header("Content-type", "text/html")
            s.end_headers()
            return
        body = "%d instances registered\n" % len(set(instances))
        for instance in set(instances):
            body = body + instance + "\n"
        s.send_response(200)
        s.send_header("Content-type", "text/html")
        s.end_headers()
        s.wfile.write(body)

    def do_POST(s):
        """Respond to a POST request."""
        global instances
        length = int(s.headers.getheader('content-length'))
        field_data = s.rfile.read(length)
        fields = urlparse.parse_qs(field_data)
        print "Instance %s just registered\n" % fields['hostname'][0]
        instances.append(fields['hostname'][0])
        print "Total number of instances registered: %d" % len(set(instances))
        # Send back a HTTP 200 so the instance will stop trying to register
        s.send_response(200)

instances = []
PORT = 8000
Handler = MyHandler

httpd = SocketServer.TCPServer(("", PORT), Handler)
print "serving at port", PORT
httpd.serve_forever()