Python newbie looking into communications...

[Sam Marrocco]

Peter Hansen wrote:

> 
> What headaches do you feel are involved in "maintaining" TCP connections?
> 
> So far as I know, there is *nothing* that must be done by application-
> level code to "maintain" an open connection, except perhaps send and
> receive data using it...
> 


Perhaps "maintaining" was the wrong term to use....

Since I've been barraged with "don't use UDP" replies, here's a little 
background on what the app does. Be gentle--this has been my first 
experience with machine-machine communications.

-The Controller app runs on Windows, and is written (mainly) in vb.net.
-The Node app is a small program that must be portable to windows, 
linux, and irix. I chose python for a bunch of reasons, the main one 
being portability. The node app may run on dozens (or more) machines at 
once.

I started designing the system with TCP in mind--the controller 
launches, establishes a connection with each node, and communications 
can commence. But soon I realized that:

If the Controller was a TCP Server and the nodes were TCP Clients, the 
Controller/server had to be running first, then the Nodes/Clients. If 
the Controller/Server was restarted (which happens often)then things 
went awry. Also, the Nodes/clients do not necessarily know *who* the 
Controller/Server is, so they receive that information within their data 
from the Controller/Server--then send info back to that machine. Nodes 
and Controller can be rebooted at any time, and the apps restarted.

When I started playing with UDP, I realized that, yes, the "lack of 
guarantee" that the data would get there was an issue--which I could 
correct for by including acknowledgments and a little handshaking 
between machines. But it seemed well worth it for the ability to have a 
"server-less" communications arrangement that didn't gag when the server 
machine disappeared. UDP peer-peer comm seemed a worthy compromise. I'm 
not dealing with large amounts of data--usually small strings, and they 
aren't pushed around the network very fast....usually a dialog might be 
something like:

Controller: MachineA, run this calculation.
MachineA: Okay, controller, I'm doing that calculation.
Controller: Displays information on what MachineA is doing.
MachineA: Okay, controller, I'm done. Here's the result.
Controller: Okay MachineA. Displays info.
process repeats.....

In my perfect world, all I really wanted was a way to say
SendToMachine(machinename, data)
and an event to catch data
DataReceived(MachineName, data)

But the most important thing was that any machine on the network could 
be restarted at any time, *without* having to have a server app always 
running.

-- 
==================================================================
Sam J. Marrocco
Sr. Visual Effects Artist/R&D
Travelling Pictures/GTN
Inferno, Flame, Houdini, Maya, All that cool stuff!
"The fact that no one understands you doesn't make you an artist."
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