Multiple iPhone APN messages, single connection

Question

I have a strange issue. I'm trying to get Apple Push Notifications working with Python. I can connect and send individual messages without a problem. The issues pop up when I start sending more than one message, but it's more bizarre than even that.

I'm testing with multiple devices... some iPhones and some iPod Touches. I can send multiple messages to the iPhones without a hitch, but if I have an iPod Touch device id in the list, any message that goes after will fail.

So if I send 4 messages in sequence like this:

1 - iPhone
2 - iPhone
3 - ipod Touch
4 - iPhone

1 and 2 will be delivered, 3 and 4 fail.

Using the same device ID's, if I move any of the iPod Touch device ID's to be the first message, all messages will fail. Likewise, if I only send to iPhones, all messages will succeed.

Here's the code I'm testing with, in it's current state, I would only get the first two messages, the last two will fail every time.

import struct, ssl, json, sys, time, socket, binascii
from optparse import OptionParser

class PushSender(object):

    def __init__(self, host, cert, key):
        self.apnhost = (host, 2195)
        self.sock = ssl.wrap_socket(socket.socket(socket.AF_INET, socket.SOCK_STREAM),
                                    keyfile = key,
                                    certfile = cert,
                                    do_handshake_on_connect=False)
        self.sock.connect(self.apnhost)
        while True:
            try:
                self.sock.do_handshake()
                break
            except ssl.SSLError, err:
                if err.args[0] == ssl.SSL_ERROR_WANT_READ:
                    select.select([self.sock], [], [])
                elif err.args[0] == ssl.SSL_ERROR_WANT_WRITE:
                    select.select([], [self.sock], [])
                else:
                    raise

    def send_message(self, token, message):
        payload = {'aps':{'alert':message}}
        token = binascii.unhexlify(token)
        payloadstr = json.dumps(payload, separators=(',',':'))
        payloadLen = len(payloadstr)
        fmt = "!BH32sH%ds" % payloadLen
        notification = struct.pack(fmt, 0, 32, token, payloadLen, payloadstr)
        self.sock.write(notification)
        self.sock.


    def close(self):
        self.sock.close()

def main():
    parser = OptionParser()
    parser.add_option("-c", "--certificate", dest="cert",
                      metavar="FILE",
                      help="Certificate file", )

    parser.add_option("-p", "--privatekey", dest="key",
                      metavar="FILE",
                      help="Key file", )
    parser.add_option("--host", help="apn host", dest='host')
    (options, args) = parser.parse_args()

    sender = PushSender(options.host, options.cert, options.key)

    iphone1 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    print 'Sending iPhone #1 a message.'
    print sender.send_message(iphone1,'Hey iPhone #1.')

    iphone2 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    print 'Sending iPhone #2 a message.'
    print sender.send_message(iphone2,'Hey iPhone #2.')

    ipod1 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    print 'Sending iPod #1 a message.'
    print sender.send_message(ipod1,'Hey iPod #1.')

    iphone3 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    print 'Sending iPhone #3 a message.'
    print sender.send_message(iphone3,'Hey iPhone #3.')

    sender.close()

if __name__=="__main__":
    main()

Any help would be appreciated...

Answer 1

Apple will silently drop your connection if it receives an invalid device token or a message that's too long. The next couple of messages after that will fail because they're just sent into the ether, essentially - the connection is closed, but the TCP window isn't exhausted.

At Urban Airship, where I work, we have a debug mode to use when people test their applications with push on our service. This will pause for a little bit after sending a message to ensure that this wasn't the issue - if the connection drops, we know it's an issue with the device token and report an error as such. A similar method might be a good way for you to check that this is, or is not, what's going on. Obviously this kills throughput and so we don't recommend it for a production environment.