how to handle tcp client socket auto reconnect in python asyncio?

Question

I am using python asyncio streams to connect to several socket servers, but when the server is down, my code can't auto reconnect.

What I need is that, when the server is down, my script will try to reconnect every 5 seconds, until connected and start to parse the data again.

import asyncio

server1 = {'host': '192.168.1.51', 'port': 11110}
server2 = {'host': '192.168.1.52', 'port': 11110}


async def tcp_client(host, port, loop):
    print('connect to server {} {}'.format(host, str(port)))
    reader, writer = await asyncio.open_connection(host, port, loop=loop)

    while True:
        data = await reader.read(100)
        print('raw data received: {}'.format(data))

        await asyncio.sleep(0.1)


loop = asyncio.get_event_loop()
try:
    for server in [server1, server2]:
        loop.run_until_complete(tcp_client(server['host'], server['port'], loop))
        print('task added: connect to server {} {}'.format(server['host'], server['port']))
finally:
    loop.close()
    print('loop closed')

First of all your code will not run: you can't access items in a dictionary using dot notation - use `server['host']` or `server.get('host')`. — mhawke, May 26 '17 at 08:20
Another problem is that `loop.run_until_complete()` runs until it, well, completes, i.e. it blocks. Thus only one server will be connected at a time. — mhawke, May 26 '17 at 08:23
And another problem is that `tcp_client()` needs to check if the connection is closed. You can tell that when `data` is the empty string - then return from the function. — mhawke, May 26 '17 at 08:24
1. I have corrected the codes. 2. So I need to use loop.call_soon() instead of loop.run_until_complete() to run the task? 3. I can use len(data) == 0 to check if the connection is closed? — Desmond Chen, May 26 '17 at 08:31
2. Consider using [asyncio.gather](https://docs.python.org/3/library/asyncio-task.html#asyncio.gather) or [asyncio.wait](https://docs.python.org/3/library/asyncio-task.html#asyncio.wait) 3. See [StreamReader.at_eof](https://docs.python.org/3/library/asyncio-stream.html?highlight=stream%20api#asyncio.StreamReader.at_eof) — Vincent, May 26 '17 at 10:49

Vincent · Accepted Answer · 2017-05-26T17:39:19.810

5

You can handle reconnection by simply looping over a try/except statement.

Additionally, asyncio.wait_for can be used to set a timeout on the read operation.

Consider this working example:

import asyncio

async def tcp_client(host, port):
    reader, writer = await asyncio.open_connection(host, port)
    try:
        while not reader.at_eof():
            data = await asyncio.wait_for(reader.read(100), 3.0)
            print('raw data received: {}'.format(data))
    finally:
        writer.close()

async def tcp_reconnect(host, port):
    server = '{} {}'.format(host, port)
    while True:
        print('Connecting to server {} ...'.format(server))
        try:
            await tcp_client(host, port)
        except ConnectionRefusedError:
            print('Connection to server {} failed!'.format(server))
        except asyncio.TimeoutError:
            print('Connection to server {} timed out!'.format(server))
        else:
            print('Connection to server {} is closed.'.format(server))
        await asyncio.sleep(2.0)

async def main():
    servers = [('localhost', 8888), ('localhost', 9999)]
    coros = [tcp_reconnect(host, port) for host, port in servers]
    await asyncio.gather(*coros)

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())
    loop.close()

edited May 26 '17 at 17:39

answered May 26 '17 at 11:13

Vincent

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I used your code but failed. When I unplug my servers and plugin them again, connection didn't restarted again. below is the log: 2017-05-26 21:39:54,239 - Connecting to server 10.0.1.68 1100 ... 2017-05-26 21:39:54,240 - Connecting to server 10.0.1.69 1100 ... 2017-05-26 21:39:54,977 - raw data received: b'030 1495834968 00.000 00.004 00.010 00.007 00.004 00.000 00.000 00.000\n' – Desmond Chen May 26 '17 at 13:48
@DesmondChen Maybe another exception is raised when you unplug your servers, you can try to replace `ConnectionRefusedError` with `Exception`. Also, using `await asyncio.gather(*coros)` will cause your program to stop at the first unexpected exception. – Vincent May 26 '17 at 14:13
I changed "ConnectionRefusedError to Exception" and "await asyncio.wait(coros)" to "await asyncio.gather(*coros)". But it still doesn't work. I think that it is stuck at async def tcp_client(host, port) and no exception raised. – Desmond Chen May 26 '17 at 14:50
@DesmondChen You might want to run your tests with netcat servers (e.g. run `nc -l 8888`, write a message to stdin, press enter and interrupt with CTRL+C) to see if the problem comes from your servers. – Vincent May 26 '17 at 14:56
It seems that when the buffer is empty, read.at_eof() is still False. – Desmond Chen May 26 '17 at 15:31
@DesmondChen If the connection has been closed, `reader.at_eof()` should be True. Do you expect the server or the client to close the connection? – Vincent May 26 '17 at 15:41
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/145222/discussion-between-desmond-chen-and-vincent). – Desmond Chen May 26 '17 at 16:07

how to handle tcp client socket auto reconnect in python asyncio?

1 Answers1