Twisted Socket Send Message Immediately

Question

I am making an iPhone application that communicates to a Twisted socket and it works great when I have one message to send. However, my issue is I need to send many different bits of information to the app. Here is my code.

            if numrows == 1:
                #Did login
                msg = "%s: Login Credentials Success" % _UDID
                print msg
                for c in self.factory.clients:
                    c.message(msg)
                    time.sleep(0.5)

                for result in results:
                    for i in range(1, 6):
                        msg = "%s:L%d;%s" % (_UDID, i, result[i])
                        print msg
                        for c in self.factory.clients:
                            c.message(msg)
                            time.sleep(0.5)
            else:
                msg = "%s: Login Credentials Failed" % _UDID
                print msg
                for c in self.factory.clients:
                    c.message(msg)
                    time.sleep(0.5)

            cursor.close()
            database.close()

        #print msg

        #for c in self.factory.clients:
            #c.message(msg)

def message(self, message):
    self.transport.write(message)

Say I were to send just the first msg, and every other msg didn't exist along with the print and for methods below each msg, the message Login Credentials Success would be sent to the app. But if put in the rest like how you are seeing it, nothing goes though because it sends it all at once, even with putting a time.sleep in the code.

The app checks the response every .05 seconds or less. Even though that the login credentials is on the top, it doesn't go through because there is more info that is being sent afterwards, but without all the info after the credentials message, it would go through.

I am desperate in finding the answer to this. I've tried everything I can think of. The app is not the issue, it's the Python.

Thanks.

Answer 1

At a risk of offending by contradicting, you may want to reexamine the claim that your app is not the problem. It sounds like you are expecting to have complete control over the content of each outgoing TCP packet, and that your app is depending on packet boundaries to determine the message boundaries. This isn't a very good approach to networking in general; some network intermediaries may split up (fragment) or even combine packets along the way, which would confuse your app. Remember that TCP is not a packet protocol: it is a stream protocol, so you're only really guaranteed that the octets you sent will be received in order, if at all.

A common approach to dealing with messages of varying size is to prefix each message with an X-bit big-endian size field stating how large the following message is. The receiving end of the communication reads X bits first, then reads 'size' octets after that to get the full message (blocking until that point, and leaving any additional information in the buffer for the next message handler to get).

I wouldn't mind helping more with your Twisted code, but it may very well be that it's already written properly. At least, I recommend not depending on trying to make Twisted flush network write buffers immediately. While it may help make your code appear to work right, right now, it will really just be hiding problems that will surface later.

Answer 2

Your issue appears to be that Twisted buffers the data you write to it.

Apparently, there is no easy way to force the data to be sent on its own without refactoring a great deal of your code. See Twisted transport.write and Using Twisted's twisted.web classes, how do I flush my outgoing buffers?.

Without knowing what your code looks like before the snipped you pasted and, according to the accepted answer of the last link:

• define the wait function just as it is in the accepted answer
• make sure that your class inherits from http.Request
• decorate your method message with defer.inlineCallbacks
• add yield wait(N) (for a value of N that you have to test and determine) after calls to write on the message method

I don't have enough experience with Twisted to know which of those steps are needed and which are just curft from the code of the original answer that doesn't apply to your code.

It may be possible (and easier) though, to re-write the iPhone application to accept the kind of messages that get sent when they include multiple writes in a single message.