Occasionally, we will want to forcefully close a transport in a communication channel. For example, even with asynchronous programming and other forms of concurrency, it is possible for your server to be overwhelmed with constant communications from multiple clients. On the other hand, it is undesirable to have the server completely handle some of the sent requests and plainly reject the rest of the requests as soon as the server is at its maximum capacity.

So, instead of keeping the communication open for each and every client connected to the server, we can specify in our protocol that each connection should be closed after a successful communication. We will do this by using the BaseTransport.close() method to forcefully close the calling transport object, which will stop the connection between the server and that specific client. Again, we are modifying the data_received() method of the EchoServerClientProtocol class in Chapter11/example3.py, as follows:

# Chapter11/example3.py

import asyncio

class EchoServerClientProtocol(asyncio.Protocol):
    def connection_made(self, transport):
        peername = transport.get_extra_info('peername')
        print('Connection from {}'.format(peername))
        self.transport = transport

    def data_received(self, data):
        message = data.decode()
        print('Data received: {!r}'.format(message))

        self.transport.write(('Echoed back: {}'.format(message)).encode())

        print('Close the client socket')
        self.transport.close()

loop = asyncio.get_event_loop()
coro = loop.create_server(EchoServerClientProtocol, '127.0.0.1', 8888)
server = loop.run_until_complete(coro)

# Serve requests until Ctrl+C is pressed
print('Serving on {}'.format(server.sockets[0].getsockname()))
try:
    loop.run_forever()
except KeyboardInterrupt:
    pass

# Close the server
server.close()
loop.run_until_complete(server.wait_closed())
loop.close()

Run the script, try to connect to the specified server, and type in some messages, in order to see the changes that we implemented. With our current setup, after a client connects and sends a message to the server, it will receive an echoed message back, and its connection with the server will be closed. The following is the output (again, from the interface of the Telnet program) that I obtained after simulating this process with our current implementation of the protocol:

> telnet 127.0.0.1 8888
Trying 127.0.0.1...
Connected to localhost.
Hello, World!
Echoed back: Hello, World!
Connection closed by foreign host.