We Keep Coding

Threading in Python 3: Spawning a new thread for a specific process

So I've used a bit of threading in python 2.6, but I'm confused how to do it in python 3, and it's been a while. Basically, I have a function that I pass 2 variables. I want to thread that function so that every time it's called, it creates a new thread instance, and when it finishes, the thread closes. It shouldn't be that difficult, but for some reason, after reading a ton of documentation, I can't seem to figure it out. Here's an example.
```
def accept_connection(clientsocket, addr):
print("Got a connection from %s" % str(addr)) #some serverside debugging output
msg = clientsocket.recv(1024) #recieve a message from the client
ClientAppend = str(addr) + str(msg, 'ascii')
Clients.append(ClientAppend)
print(Clients)
accept_connection(clientsocket, addr)
In case it's unclear, I want the accept_connection() function to be threaded, so that I can accept multiple connections at once and they aren't denied because one is already being processed. I'm not really sure how to do this, and I appreciate any and all help I can get.

So what you're looking for has already been nicely described in this here. You want to look into multithread tcp file transfer on localhost.
They have a nice tutorial from scratch which you can use here.
They have a five part tutorial with really simple examples which are well explained. I feel the tutorial would explain much better than I could.

Python parallel processing from client-server application

I have a web application (django) that stores in mysql database PID numbers of processes from remote Linux machine. I designed a simple server-client application that talking to remote server and getting me some data about given PID number (cpu%, mem%) ... this data is from 5s interval.
But there is a performance problem .... I have 200 pids to check and every of them takes ~5 sec and they are processing in the for loop. So I have situation where I`m waiting 200*5 sec minimum
Can somebody advise me how to make it parallel processing? So my application will be able to fetch for example 50 pids at one time ... I believe python client - server library can handle multiple requests coming to the server.
I want to archive something like:
for pid in my_200_pid_list:
// Some parallel magic to not wait and pass another 49...
result[pid] = askforprocess(pid)
My client code:
def askforprocess(processpid):
#Create TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Connect on host and port provided in command line arguments
server_address = ('172.16.1.105', int('5055'))
sock.connect(server_address)
# Send the data
try:
message = processpid
sock.sendall(message)
data = sock.recv(2048)
finally:
sock.close()
return data

In general, it's best to do stuff like this using a single thread when possible. You just have to make sure your functions don't block other functions. The builtin lib that comes to mind is select. Unfortunately, it's a bit difficult to explain and I haven't used it in quite some time. Hopefully this link will help you understand it http://pymotw.com/2/select/.
You can also use the multiprocessing lib and poll each pid in a separate thread. This can be very difficult to manage if you plan to scale out further! Use threads only as a last resort (this is my usual rule of thumb when it comes to threads). https://docs.python.org/2/library/multiprocessing.html#module-multiprocessing
from multiprocessing import Process
def askforprocess(processpid):
#Create TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Connect on host and port provided in command line arguments
server_address = ('172.16.1.105', int('5055'))
sock.connect(server_address)
# Send the data
try:
message = processpid
sock.sendall(message)
data = sock.recv(2048)
finally:
sock.close()
return data
if __name__ == '__main__':
info('main line')
p = Process(target=askforprocess, args=(processpid,))
p.start()
Lastly, there's Twisted library which is probably the most difficult to understand, but defiantly makes concurrent (not necessarily parallel) functions easy to write. Only bad thing is you'd probably have to rewrite your entire app in order to use Twisted. Don't be put off by this fact, try to use it if you can.
Hope that helps.

Use threads to process your requests in parallel: https://docs.python.org/2/library/threading.html

In this Python 3 client-server example, client can't send more than one message

This is a simple client-server example where the server returns whatever the client sends, but reversed.
Server:
import socketserver
class MyTCPHandler(socketserver.BaseRequestHandler):
def handle(self):
self.data = self.request.recv(1024)
print('RECEIVED: ' + str(self.data))
self.request.sendall(str(self.data)[::-1].encode('utf-8'))
server = socketserver.TCPServer(('localhost', 9999), MyTCPHandler)
server.serve_forever()
Client:
import socket
import threading
s = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.connect(('localhost',9999))
def readData():
while True:
data = s.recv(1024)
if data:
print('Received: ' + data.decode('utf-8'))
t1 = threading.Thread(target=readData)
t1.start()
def sendData():
while True:
intxt = input()
s.send(intxt.encode('utf-8'))
t2 = threading.Thread(target=sendData)
t2.start()
I took the server from an example I found on Google, but the client was written from scratch. The idea was having a client that can keep sending and receiving data from the server indefinitely.
Sending the first message with the client works. But when I try to send a second message, I get this error:
ConnectionAbortedError: [WinError 10053] An established connection was
aborted by the software in your host machine
What am I doing wrong?

For TCPServer, the handle method of the handler gets called once to handle the entire session. This may not be entirely clear from the documentation, but socketserver is, like many libraries in the stdlib, meant to serve as clear sample code as well as to be used directly, which is why the docs link to the source, where you can clearly see that it's only going to call handle once per connection (TCPServer.get_request is defined as just calling accept on the socket).
So, your server receives one buffer, sends back a response, and then quits, closing the connection.
To fix this, you need to use a loop:
def handle(self):
while True:
self.data = self.request.recv(1024)
if not self.data:
print('DISCONNECTED')
break
print('RECEIVED: ' + str(self.data))
self.request.sendall(str(self.data)[::-1].encode('utf-8'))
A few side notes:
First, using BaseRequestHandler on its own only allows you to handle one client connection at a time. As the introduction in the docs says:
These four classes process requests synchronously; each request must be completed before the next request can be started. This isn’t suitable if each request takes a long time to complete, because it requires a lot of computation, or because it returns a lot of data which the client is slow to process. The solution is to create a separate process or thread to handle each request; the ForkingMixIn and ThreadingMixIn mix-in classes can be used to support asynchronous behaviour.
Those mixin classes are described further in the rest of the introduction, and farther down the page, and at the bottom, with a nice example at the end. The docs don't make it clear, but if you need to do any CPU-intensive work in your handler, you want ForkingMixIn; if you need to share data between handlers, you want ThreadingMixIn; otherwise it doesn't matter much which you choose.
Note that if you're trying to handle a large number of simultaneous clients (more than a couple dozen), neither forking nor threading is really appropriate—which means TCPServer isn't really appropriate. For that case, you probably want asyncio, or a third-party library (Twisted, gevent, etc.).
Calling str(self.data) is a bad idea. You're just going to get the source-code-compatible representation of the byte string, like b'spam\n'. What you want is to decode the byte string into the equivalent Unicode string: self.data.decode('utf8').
There's no guarantee that each sendall on one side will match up with a single recv on the other side. TCP is a stream of bytes, not a stream of messages; it's perfectly possible to get half a message in one recv, and two and a half messages in the next one. When testing with a single connection on localhost with the system under light load, it will probably appear to "work", but as soon as you try to deploy any code that assumes that each recv gets exactly one message, your code will break. See Sockets are byte streams, not message streams for more details. Note that if your messages are just lines of text (as they are in your example), using StreamRequestHandler and its rfile attribute, instead of BaseRequestHandler and its request attribute, solves this problem trivially.
You probably want to set server.allow_reuse_address = True. Otherwise, if you quit the server and re-launch it again too quickly, it'll fail with an error like OSError: [Errno 48] Address already in use.

Kill a tcp connection in a separate thread in Python

So here's the problem, I have a small server script in Python that is supposed to accept multiple clients and based on the message they are sending, receiving a certain command back to them. It's a simple concept and it's working like I want to, with one really big problem: I put each connection on hold and in separate thread, and I want when a certain connected users puts EXIT to close the connection...Which works, with one really big problem - the thread is kept alive and there is no way to kill it and that really bothers me.
sock = socket()
sock.bind((host,port))
sock.listen(50)
def clientthread(conn):
while True:
data = conn.recv(1024).strip()
if(data == "HELO"):
conn.send("HELO")
elif(data == "EXIT"):
conn.close()
break
return
while True:
conn,addr = sock.accept()
start_new_thread(clientthread, (conn,))
conn.close()
sock.close()
I searched of a way to terminate a thread but just couldn't find it, .join() is not working here since it detects the thread as "dummy", it does not recognize the __stop() and since a couple of searches on google for this topic I'm really out of options. Any idea? I'll be really grateful, thanks.

AFAIK, you can't kill a thread from another - you have to arrange for the thread-to-be-killed to notice some flag has changed, and terminate itself.
BTW, your socket code looks a little off - you need a loop around your send's and recv's unless you use something like twisted or bufsock. IMO, bufsock is much easier and less error prone than twisted, but I may be biased because I wrote bufsock. http://stromberg.dnsalias.org/~strombrg/bufsock.html
The problem with what I'm seeing is that TCP reserves the right to split or aggregate transmission units. Usually it won't, but under high load, or with a changing Path MTU, or even just Nagle, it probably will.

Assuming you're using Python v2.4+, you should be using the newer Threading module. Check out a tutorial on it here - It explains the use of the threading module you're using now and how and why you should use the newer Threading module.

python socket server/client protocol with unstable client connection

I have a threaded python socket server that opens a new thread for each connection.
The thread is a very simple communication based on question and answer.
Basically client sends initial data transmission, server takes it run an external app that does stuff to the transmission and returns a reply that the server will send back and the loop will begin again until client disconnects.
Now because the client will be on a mobile phone thus an unstable connection I get left with open threads no longer connected and because the loop starts with recv it is rather difficult to break on lost connectivity this way.
I was thinking on adding a send before the recv to test if connection is still alive but this might not help at all if the client disconnects after my failsafe send as the client sends a data stream every 5 seconds only.
I noticed the recv will break sometimes but not always and in those cases I am left with zombie threads using resources.
Also this could be a solid vulnerability for my system to be DOSed.
I have looked through the python manual and Googled since thursday trying to find something for this but most things I find are related to client and non blocking mode.
Can anyone point me in the right direction towards a good way on fixing this issue?
Code samples:
Listener:
serversocket = socket(AF_INET, SOCK_STREAM)
serversocket.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
serversocket.bind(addr)
serversocket.listen(2)
logg("Binded to port: " + str(port))
# Listening Loop
while 1:
clientsocket, clientaddr = serversocket.accept()
threading.Thread(target=handler, args=(clientsocket, clientaddr,port,)).start()
# This is useless as it will never get here
serversocket.close()
Handler:
# Socket connection handler (Threaded)
def handler(clientsocket, clientaddr, port):
clientsocket.settimeout(15)
# Loop till client closes connection or connection drops
while 1:
stream = ''
while 1:
ending = stream[-6:] # get stream ending
if ending == '.$$$$.':
break
try:
data = clientsocket.recv(1)
except:
sys.exit()
if not data:
sys.exit()
# this is the usual point where thread is closed when a client closes connection normally
stream += data
# Clear the line ending
stream = base64.b64encode(stream[:-6])
# Send data to be processed
re = getreply(stream)
# Send response to client
try:
clientsocket.send(re + str('.$$$$.'))
except:
sys.exit()
As you can see there are three conditions that at least one should trigger exit if connection fails but sometimes they do not.

Sorry, but I think that threaded idea in this case is not good. As you do not need to process/do a lot of stuff in these threads (workers?) and most of the time these threads are waiting for socket (is the blocking operation, isn't it?) I would advice to read about event-driven programming. According to sockets this pattern is extremly useful, becouse you can do all stuff in one thread. You are communicate with one socket at a time, but the rest of connections are just waiting to data so there is almost no loss. When you send several bytes you just check that maybe another connection requires carrying. You can read about select
and epoll.
In python there is several libraries to play with this nicly:
libev (c library wrapper) - pyev
tornado
twisted
I used tornado in some projects and it is done this task very good. Libev is nice also, but is a c-wrapper so it is a little bit low-level (but very nice for some tasks).

So you should use socket.settimeout(float) with the clientsocket like one of the comments suggested.
The reason you don't see any difference is, when you call socket.recv(bufsize[, flags]) and the timeout runs out an socket.timeout exception is thrown and you catch that exception and exit.
try:
data = clientsocket.recv(1)
except:
sys.exit()
should be somthing like:
try:
data = clientsocket.recv(1)
except timeout:
#timeout occurred
#handle it
clientsocket.close()
sys.exit()