Simple TCP proxy

[Morten W. Petersen]

Hi Martin.

I was thinking of doing something with the handle function, but just this
little tweak:

https://github.com/morphex/stp/commit/9910ca8c80e9d150222b680a4967e53f0457b465

made a huge difference in CPU usage.  Hundreds of waiting sockets are now
using 20-30% of CPU instead of 10x that.  So for example making the handle
function exit / stop and wait isn't necessary at this point. It also opens
up the possibility of sending a noop that is appropriate for the given
protocol.

I've not done a lot of thread programming before, but yes, locks can be
used and will be used if necessary. I wasn't sure what data types were
thread safe in Python, and it might be that some variables could be off by
1 or more, if using <= >= checks is an option and that there is no risk of
the variable containing "garbage".

I think with a simple focus, that the project is aimed at one task, will
make it easier to manage even complex matters such as concurrency and
threads.

-Morten

On Wed, Jul 27, 2022 at 11:00 PM Martin Di Paola <martinp.dipaola at gmail.com>
wrote:

>
> On Wed, Jul 27, 2022 at 08:32:31PM +0200, Morten W. Petersen wrote:
> >You're thinking of the backlog argument of listen?
>
>  From my understanding, yes, when you set up the "accepter" socket (the
> one that you use to listen and accept new connections), you can define
> the length of the queue for incoming connections that are not accepted
> yet.
>
> This will be the equivalent of your SimpleQueue which basically puts a
> limits on how many incoming connections are "accepted" to do a real job.
>
> Using skt.listen(N) the incoming connections are put on hold by the OS
> while in your implementation are formally accepted but they are not
> allowed to do any meaningful work: they are put on the SimpleQueue and
> only when they are popped then they will work (send/recv data).
>
> The difference then between the OS and your impl is minimal. The only
> case that I can think is that on the clients' side it may exist a
> timeout for the acceptance of the connection so your proxy server will
> eagerly accept these connections so no timeout is possible(*)
>
> On a side note, you implementation is too thread-naive: it uses plain
> Python lists, integers and boolean variables which are not thread safe.
> It is a matter of time until your server will start behave weird.
>
> One option is that you use thread-safe objects. I'll encourage to read
> about thread-safety in general and then which sync mechanisms Python
> offers.
>
> Another option is to remove the SimpleQueue and the background function
> that allows a connection to be "active".
>
> If you think, the handlers are 99% independent except that you want to
> allow only N of them to progress (stablish and forward the connection)
> and when a handler finishes, another handler "waiting" is activated, "in
> a queue fashion" as you said.
>
> If you allow me to not have a strict queue discipline here, you can achieve
> the same results coordinating the handlers using semaphores. Once again,
> take this email as starting point for your own research.
>
> On a second side note, the use of handlers and threads is inefficient
> because while you have N active handlers sending/receiving data, because
> you are eagerly accepting new connections you will have much more
> handlers created and (if I'm not wrong), each will be a thread.
>
> A more efficient solution could be
>
> 1) accept as many connections as you can, saving the socket (not the
> handler) in the thread-safe queue.
> 2) have N threads in the background popping from the queue a socket and
> then doing the send/recv stuff. When the thread is done, the thread
> closes the socket and pops another from the queue.
>
> So the queue length will be the count of accepted connections but in any
> moment your proxy will not activate (forward) more than N connections.
>
> This idea is thread-safe, simpler, efficient and has the queue
> discipline (I leave aside the usefulness).
>
> I encourage you to take time to read about the different things
> mentioned as concurrency and thread-related stuff is not easy to
> master.
>
> Thanks,
> Martin.
>
> (*) make your proxy server slow enough and yes, you will get timeouts
> anyways.
>
> >
> >Well, STP will accept all connections, but can limit how many of the
> >accepted connections that are active at any given time.
> >
> >So when I bombed it with hundreds of almost simultaneous connections, all
> >of them were accepted, but only 25 were actively sending and receiving
> data
> >at any given time. First come, first served.
> >
> >Regards,
> >
> >Morten
> >
> >On Wed, Jul 27, 2022 at 8:00 PM Chris Angelico <rosuav at gmail.com> wrote:
> >
> >> On Thu, 28 Jul 2022 at 02:15, Morten W. Petersen <morphex at gmail.com>
> >> wrote:
> >> >
> >> > Hi.
> >> >
> >> > I'd like to share with you a recent project, which is a simple TCP
> proxy
> >> > that can stand in front of a TCP server of some sort, queueing
> requests
> >> and
> >> > then allowing n number of connections to pass through at a time:
> >>
> >> How's this different from what the networking subsystem already does?
> >> When you listen, you can set a queue length. Can you elaborate?
> >>
> >> ChrisA
> >> --
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> >>
> >
> >
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-- 
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Twittering at http://twitter.com/blogologue
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