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BaseHTTPServer still writing although client lost network connection

I've implemented a server which accepts requests and after some process the client connects to my server.
The server continuously sends data to client, but if the client lose the network connection (e.g. on my mobile I've disabled the internet access without exiting from the client program), then the server is still writing to the nothing.
I've attached my shortened version of my code logic. Monitoring the input data could be a good idea, but I have some cases when I don't have to wait for any input.
class CustomRequestHandler(BaseHTTPRequestHandler):
def __init__(self, request, client_address, server):
BaseHTTPRequestHandler.__init__(self, request, client_address, server)
def do_GET(self):
try:
readable, writable, exceptional = select.select([self.rfile], [self.wfile], [self.rfile, self.wfile], 0)
for s in readable:
print (s.readline())
for s in writable: #
s.write(b"Data")
except Exception as e:
print(e)
def finish(self, *args, **kw):
print ("Do finish")
class CustomServer(socketserver.ThreadingMixIn, HTTPServer):
pass
def start_server():
httpd = CustomServer((HOST, PORT), CustomRequestHandler)
try:
httpd.allow_reuse_address = True
httpd.serve_forever()
except KeyboardInterrupt:
pass
httpd.server_close()
if __name__ == '__main__':
start_server()
After a while writable became an empty list, but how could I detect if on the client side a network lost occurred? How could I catch the network error?

Your socket is not closed when you cut the network connection. The sender will only get informed when the OS decides that the socket is timed out. This usually takes 30s+.
If on the other hand the receiver program is closed properly, the sender will get notified within milliseconds.
These left open but actually lost connections are a major problem in network programming. There are mitigations but there is no universal ultimate solution to it.

Python: Multithreaded socket server runs endlessly when client stops unexpectedly

I have created a multithreaded socket server to connect many clients to the server using python. If a client stops unexpectedly due to an exception, server runs nonstop. Is there a way to kill that particular thread alone in the server and the rest running
Server:
class ClientThread(Thread):
def __init__(self,ip,port):
Thread.__init__(self)
self.ip = ip
self.port = port
print("New server socket thread started for " + ip + ":" + str(port))
def run(self):
while True :
try:
message = conn.recv(2048)
dataInfo = message.decode('ascii')
print("recv:::::"+str(dataInfo)+"::")
except:
print("Unexpected error:", sys.exc_info()[0])
Thread._stop(self)
tcpServer = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcpServer.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
tcpServer.bind((TCP_IP, 0))
tcpServer.listen(10)
print("Port:"+ str(tcpServer.getsockname()[1]))
threads = []
while True:
print( "Waiting for connections from clients..." )
(conn, (ip,port)) = tcpServer.accept()
newthread = ClientThread(ip,port)
newthread.start()
threads.append(newthread)
for t in threads:
t.join()
Client:
def Main():
s = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.connect((host,int(port)))
while True:
try:
message = input("Enter Command")
s.send(message.encode('ascii'))
except Exception as ex:
logging.exception("Unexpected error:")
break
s.close()

Sorry about a very, very long answer but here goes.
There are quite a many issues with your code. First of all, your client does not actually close the socket, as s.close() will never get executed. Your loop is interrupted at break and anything that follows it will be ignored. So change the order of these statements for the sake of good programming but it has nothing to do with your problem.
Your server code is wrong in quite a many ways. As it is currently written, it never exits. Your threads also do not work right. I have fixed your code so that it is a working, multithreaded server, but it still does not exit as I have no idea what would be the trigger to make it exit. But let us start from the main loop:
while True:
print( "Waiting for connections from clients..." )
(conn, (ip,port)) = tcpServer.accept()
newthread = ClientThread(conn, ip,port)
newthread.daemon = True
newthread.start()
threads.append(newthread) # Do we need this?
for t in threads:
t.join()
I have added passing of conn to your client thread, the reason of which becomes apparent in a moment. However, your while True loop never breaks, so you will never enter the for loop where you join your threads. If your server is meant to be run indefinitely, this is not a problem at all. Just remove the for loop and this part is fine. You do not need to join threads just for the sake of joining them. Joining threads only allows your program to block until a thread has finished executing.
Another addition is newthread.daemon = True. This sets your threads to daemonic, which means they will exit as soon as your main thread exits. Now your server responds to control + c even when there are active connections.
If your server is meant to be never ending, there is also no need to store threads in your main loop to threads list. This list just keeps growing as a new entry will be added every time a client connects and disconnects, and this leaks memory as you are not using the threads list for anything. I have kept it as it was there, but there still is no mechanism to exit the infinite loop.
Then let us move on to your thread. If you want to simplify the code, you can replace the run part with a function. There is no need to subclass Thread in this case, but this works so I have kept your structure:
class ClientThread(Thread):
def __init__(self,conn, ip,port):
Thread.__init__(self)
self.ip = ip
self.port = port
self.conn = conn
print("New server socket thread started for " + ip + ":" + str(port))
def run(self):
while True :
try:
message = self.conn.recv(2048)
if not message:
print("closed")
try:
self.conn.close()
except:
pass
return
try:
dataInfo = message.decode('ascii')
print("recv:::::"+str(dataInfo)+"::")
except UnicodeDecodeError:
print("non-ascii data")
continue
except socket.error:
print("Unexpected error:", sys.exc_info()[0])
try:
self.conn.close()
except:
pass
return
First of all, we store conn to self.conn. Your version used a global version of conn variable. This caused unexpected results when you had more than one connection to the server. conn is actually a new socket created for the client connection at accept, and this is unique to each thread. This is how servers differentiate between client connections. They listen to a known port, but when the server accepts the connection, accept creates another port for that particular connection and returns it. This is why we need to pass this to the thread and then read from self.conn instead of global conn.
Your server "hung" upon client connetion errors as there was no mechanism to detect this in your loop. If the client closes connection, socket.recv() does not raise an exception but returns nothing. This is the condition you need to detect. I am fairly sure you do not even need try/except here but it does not hurt - but you need to add the exception you are expecting here. In this case catching everything with undeclared except is just wrong. You have also another statement there potentially raising exceptions. If your client sends something that cannot be decoded with ascii codec, you would get UnicodeDecodeError (try this without error handling here, telnet to your server port and copypaste some Hebrew or Japanese into the connection and see what happens). If you just caught everything and treated as socket errors, you would now enter the thread ending part of the code just because you could not parse a message. Typically we just ignore "illegal" messages and carry on. I have added this. If you want to shut down the connection upon receiving a "bad" message, just add self.conn.close() and return to this exception handler as well.
Then when you really are encountering a socket error - or the client has closed the connection, you will need to close the socket and exit the thread. You will call close() on the socket - encapsulating it in try/except as you do not really care if it fails for not being there anymore.
And when you want to exit your thread, you just return from your run() loop. When you do this, your thread exits orderly. As simple as that.
Then there is yet another potential problem, if you are not only printing the messages but are parsing them and doing something with the data you receive. This I do not fix but leave this to you.
TCP sockets transmit data, not messages. When you build a communication protocol, you must not assume that when your recv returns, it will return a single message. When your recv() returns something, it can mean one of five things:
The client has closed the connection and nothing is returned
There is exactly one full message and you receive that
There is only a partial message. Either because you read the socket before the client had transmitted all data, or because the client sent more than 2048 bytes (even if your client never sends over 2048 bytes, a malicious client would definitely try this)
There are more than one messages waiting and you received them all
As 4, but the last message is partial.
Most socket programming mistakes are related to this. The programmer expects 2 to happen (as you do now) but they do not cater for 3-5. You should instead analyse what was received and act accordingly. If there seems to be less data than a full message, store it somewhere and wait for more data to appear. When more data appears, concatenate these and see if you now have a full message. And when you have parsed a full message from this buffer, inspect the buffer to see if there is more data there - the first part of the next message or even more full messages if your client is fast and server is slow. If you process a message and then wipe the buffer, you might have wiped also bytes from your next message.

How to reconnect to RabbitMQ?

My python script constantly has to send messages to RabbitMQ once it receives one from another data source. The frequency in which the python script sends them can vary, say, 1 minute - 30 minutes.
Here's how I establish a connection to RabbitMQ:
rabt_conn = pika.BlockingConnection(pika.ConnectionParameters("some_host"))
channel = rbt_conn.channel()
I just got an exception
pika.exceptions.ConnectionClosed
How can I reconnect to it? What's the best way? Is there any "strategy"? Is there an ability to send pings to keep a connection alive or set timeout?
Any pointers will be appreciated.

RabbitMQ uses heartbeats to detect and close "dead" connections and to prevent network devices (firewalls etc.) from terminating "idle" connections. From version 3.5.5 on, the default timeout is set to 60 seconds (previously it was ~10 minutes). From the docs:
Heartbeat frames are sent about every timeout / 2 seconds. After two missed heartbeats, the peer is considered to be unreachable.
The problem with Pika's BlockingConnection is that it is unable to respond to heartbeats until some API call is made (for example, channel.basic_publish(), connection.sleep(), etc).
The approaches I found so far:
Increase or deactivate the timeout
RabbitMQ negotiates the timeout with the client when establishing the connection. In theory, it should be possible to override the server default value with a bigger one using the heartbeat_interval argument, but the current Pika version (0.10.0) uses the min value between those offered by the server and the client. This issue is fixed on current master.
On the other hand, is possible to deactivate the heartbeat functionality completely by setting the heartbeat_interval argument to 0, which may well drive you into new issues (firewalls dropping connections, etc)
Reconnecting
Expanding on #itsafire's answer, you can write your own publisher class, letting you reconnect when required. An example naive implementation:
import logging
import json
import pika
class Publisher:
EXCHANGE='my_exchange'
TYPE='topic'
ROUTING_KEY = 'some_routing_key'
def __init__(self, host, virtual_host, username, password):
self._params = pika.connection.ConnectionParameters(
host=host,
virtual_host=virtual_host,
credentials=pika.credentials.PlainCredentials(username, password))
self._conn = None
self._channel = None
def connect(self):
if not self._conn or self._conn.is_closed:
self._conn = pika.BlockingConnection(self._params)
self._channel = self._conn.channel()
self._channel.exchange_declare(exchange=self.EXCHANGE,
type=self.TYPE)
def _publish(self, msg):
self._channel.basic_publish(exchange=self.EXCHANGE,
routing_key=self.ROUTING_KEY,
body=json.dumps(msg).encode())
logging.debug('message sent: %s', msg)
def publish(self, msg):
"""Publish msg, reconnecting if necessary."""
try:
self._publish(msg)
except pika.exceptions.ConnectionClosed:
logging.debug('reconnecting to queue')
self.connect()
self._publish(msg)
def close(self):
if self._conn and self._conn.is_open:
logging.debug('closing queue connection')
self._conn.close()
Other possibilities
Other possibilities which I yet didn't explore:
Using an asynchronous adapter for publishing
Keeping your RabbitMQ connection and your "publish" code on a background thread, which calls periodically connection.sleep() to responde to server heartbeats.

Dead simple: some pattern like this.
import time
while True:
try:
communication_handles = connect_pika()
do_your_stuff(communication_handles)
except pika.exceptions.ConnectionClosed:
print 'oops. lost connection. trying to reconnect.'
# avoid rapid reconnection on longer RMQ server outage
time.sleep(0.5)
You will probably have to re-factor your code, but basically it is about catching the exception, mitigate the problem and continue doing your stuff.
The communication_handles contain all the pika elements like channels, queues and whatever that your stuff needs to communicate with RabbitMQ via pika.

How can I write a socket server in a different thread from my main program (using gevent)?

I'm developing a Flask/gevent WSGIserver webserver that needs to communicate (in the background) with a hardware device over two sockets using XML.
One socket is initiated by the client (my application) and I can send XML commands to the device. The device answers on a different port and sends back information that my application has to confirm. So my application has to listen to this second port.
Up until now I have issued a command, opened the second port as a server, waited for a response from the device and closed the second port.
The problem is that it's possible that the device sends multiple responses that I have to confirm. So my solution was to keep the port open and keep responding to incoming requests. However, in the end the device is done sending requests, and my application is still listening (I don't know when the device is done), thereby blocking everything else.
This seemed like a perfect use case for a thread, so that my application launches a listening server in a separate thread. Because I'm already using gevent as a WSGI server for Flask, I can use the greenlets.
The problem is, I have looked for a good example of such a thing, but all I can find is examples of multi-threading handlers for a single socket server. I don't need to handle a lot of connections on the socket server, but I need it launched in a separate thread so it can listen for and handle incoming messages while my main program can keep sending messages.
The second problem I'm running into is that in the server, I need to use some methods from my "main" class. Being relatively new to Python I'm unsure how to structure it in a way to make that possible.
class Device(object):
def __init__(self, ...):
self.clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
def _connect_to_device(self):
print "OPEN CONNECTION TO DEVICE"
try:
self.clientsocket.connect((self.ip, 5100))
except socket.error as e:
pass
def _disconnect_from_device(self):
print "CLOSE CONNECTION TO DEVICE"
self.clientsocket.close()
def deviceaction1(self, ...):
# the data that is sent is an XML document that depends on the parameters of this method.
self._connect_to_device()
self._send_data(XMLdoc)
self._wait_for_response()
return True
def _send_data(self, data):
print "SEND:"
print(data)
self.clientsocket.send(data)
def _wait_for_response(self):
print "WAITING FOR REQUESTS FROM DEVICE (CHANNEL 1)"
self.serversocket.bind(('10.0.0.16', 5102))
self.serversocket.listen(5) # listen for answer, maximum 5 connections
connection, address = self.serversocket.accept()
# the data is of a specific length I can calculate
if len(data) > 0:
self._process_response(data)
self.serversocket.close()
def _process_response(self, data):
print "RECEIVED:"
print(data)
# here is some code that processes the incoming data and
# responds to the device
# this may or may not result in more incoming data
if __name__ == '__main__':
machine = Device(ip="10.0.0.240")
Device.deviceaction1(...)
This is (globally, I left out sensitive information) what I'm doing now. As you can see everything is sequential.
If anyone can provide an example of a listening server in a separate thread (preferably using greenlets) and a way to communicate from the listening server back to the spawning thread, it would be of great help.
Thanks.
EDIT:
After trying several methods, I decided to use Pythons default select() method to solve this problem. This worked, so my question regarding the use of threads is no longer relevant. Thanks for the people who provided input for your time and effort.

Hope it can provide some help, In example class if we will call tenMessageSender function then it will fire up an async thread without blocking main loop and then _zmqBasedListener will start listening on separate port untill that thread is alive. and whatever message our tenMessageSender function will send, those will be received by client and respond back to zmqBasedListener.
Server Side
import threading
import zmq
import sys
class Example:
def __init__(self):
self.context = zmq.Context()
self.publisher = self.context.socket(zmq.PUB)
self.publisher.bind('tcp://127.0.0.1:9997')
self.subscriber = self.context.socket(zmq.SUB)
self.thread = threading.Thread(target=self._zmqBasedListener)
def _zmqBasedListener(self):
self.subscriber.connect('tcp://127.0.0.1:9998')
self.subscriber.setsockopt(zmq.SUBSCRIBE, "some_key")
while True:
message = self.subscriber.recv()
print message
sys.exit()
def tenMessageSender(self):
self._decideListener()
for message in range(10):
self.publisher.send("testid : %d: I am a task" %message)
def _decideListener(self):
if not self.thread.is_alive():
print "STARTING THREAD"
self.thread.start()
Client
import zmq
context = zmq.Context()
subscriber = context.socket(zmq.SUB)
subscriber.connect('tcp://127.0.0.1:9997')
publisher = context.socket(zmq.PUB)
publisher.bind('tcp://127.0.0.1:9998')
subscriber.setsockopt(zmq.SUBSCRIBE, "testid")
count = 0
print "Listener"
while True:
message = subscriber.recv()
print message
publisher.send('some_key : Message received %d' %count)
count+=1
Instead of thread you can use greenlet etc.

How to know using BaseHTTPRequestHandler that client closed connection

I am writing http server that can serve big files to client.
While writing to wfile stream it is possible that client closes connection and my server gets socket error (Errno 10053).
Is it possible to stop writing when client closes connection?

You can add these methods to your BaseHTTPRequestHandler class so that you can know if the client closed the connection:
def handle(self):
"""Handles a request ignoring dropped connections."""
try:
return BaseHTTPRequestHandler.handle(self)
except (socket.error, socket.timeout) as e:
self.connection_dropped(e)
def connection_dropped(self, error, environ=None):
"""Called if the connection was closed by the client. By default
nothing happens.
"""
# add here the code you want to be executed if a connection
# was closed by the client
In the second method: connection_dropped, you can add some code that you want to be executed each time a socket error (e.g. client closed the connection) occures.