# `tornado.tcpserver` — Basic `IOStream`-based TCP server

A non-blocking, single-threaded TCP server.

class `tornado.tcpserver.``TCPServer`(ssl_options=None, max_buffer_size=None, read_chunk_size=None)

  A non-blocking, single-threaded TCP server.

  To use `TCPServer`, define a subclass which overrides the `handle_stream` method. For example, a simple echo server could be defined like this:

      from tornado.tcpserver import TCPServer
      from tornado.iostream import StreamClosedError
      from tornado import gen

      class EchoServer(TCPServer):
          async def handle_stream(self, stream, address):
              while True:
                  try:
                      data = await stream.read_until(b"\n")
                      await stream.write(data)
                  except StreamClosedError:
                      break

  To make this server serve SSL traffic, send the `ssl_options` keyword argument with an `ssl.SSLContext` object. For compatibility with older versions of Python `ssl_options` may also be a dictionary of keyword arguments for the `ssl.wrap_socket` method.:

      ssl_ctx = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
      ssl_ctx.load_cert_chain(os.path.join(data_dir, "mydomain.crt"),
                              os.path.join(data_dir, "mydomain.key"))
      TCPServer(ssl_options=ssl_ctx)

  `TCPServer` initialization follows one of three patterns:

  1. `listen`: simple single-process:

         server = TCPServer()
         server.listen(8888)
         IOLoop.current().start()

  2. `bind`/`start`: simple multi-process:

         server = TCPServer()
         server.bind(8888)
         server.start(0)  # Forks multiple sub-processes
         IOLoop.current().start()

     When using this interface, an `IOLoop` must not be passed to the `TCPServer` constructor. `start` will always start the server on the default singleton `IOLoop`.

  3. `add_sockets`: advanced multi-process:

         sockets = bind_sockets(8888)
         tornado.process.fork_processes(0)
         server = TCPServer()
         server.add_sockets(sockets)
         IOLoop.current().start()

     The `add_sockets` interface is more complicated, but it can be used with `tornado.process.fork_processes` to give you more flexibility in when the fork happens. `add_sockets` can also be used in single-process servers if you want to create your listening sockets in some way other than `bind_sockets`.

  New in version 3.1: The `max_buffer_size` argument.

  Changed in version 5.0: The `io_loop` argument has been removed.

  `listen`(port, address='')

    Starts accepting connections on the given port.

    This method may be called more than once to listen on multiple ports. `listen` takes effect immediately; it is not necessary to call `TCPServer.start` afterwards. It is, however, necessary to start the `IOLoop`.

  `add_sockets`(sockets)

    Makes this server start accepting connections on the given sockets.

    The `sockets` parameter is a list of socket objects such as those returned by `bind_sockets`. `add_sockets` is typically used in combination with that method and `tornado.process.fork_processes` to provide greater control over the initialization of a multi-process server.

  `add_socket`(socket)

    Singular version of `add_sockets`. Takes a single socket object.

  `bind`(port, address=None, family=<AddressFamily.AF_UNSPEC: 0>, backlog=128, reuse_port=False)

    Binds this server to the given port on the given address.

    To start the server, call `start`. If you want to run this server in a single process, you can call `listen` as a shortcut to the sequence of `bind` and `start` calls.

    Address may be either an IP address or hostname. If it’s a hostname, the server will listen on all IP addresses associated with the name. Address may be an empty string or None to listen on all available interfaces. Family may be set to either `socket.AF_INET` or `socket.AF_INET6` to restrict to IPv4 or IPv6 addresses, otherwise both will be used if available.

    The `backlog` argument has the same meaning as for `socket.listen`. The `reuse_port` argument has the same meaning as for `bind_sockets`.

    This method may be called multiple times prior to `start` to listen on multiple ports or interfaces.

    Changed in version 4.4: Added the `reuse_port` argument.

  `start`(num_processes=1)

    Starts this server in the `IOLoop`.

    By default, we run the server in this process and do not fork any additional child process.

    If num_processes is `None` or <= 0, we detect the number of cores available on this machine and fork that number of child processes. If num_processes is given and > 1, we fork that specific number of sub-processes.

    Since we use processes and not threads, there is no shared memory between any server code.

    Note that multiple processes are not compatible with the autoreload module (or the `autoreload=True` option to `tornado.web.Application` which defaults to True when `debug=True`). When using multiple processes, no IOLoops can be created or referenced until after the call to `TCPServer.start(n)`.

  `stop`()

    Stops listening for new connections.

    Requests currently in progress may still continue after the server is stopped.

  `handle_stream`(stream, address)

    Override to handle a new `IOStream` from an incoming connection.

    This method may be a coroutine; if so any exceptions it raises asynchronously will be logged. Accepting of incoming connections will not be blocked by this coroutine.

    If this `TCPServer` is configured for SSL, `handle_stream` may be called before the SSL handshake has completed. Use `SSLIOStream.wait_for_handshake` if you need to verify the client’s certificate or use NPN/ALPN.

    Changed in version 4.2: Added the option for this method to be a coroutine.