tornado.tcpserver — Basic IOStream-based TCP server

A non-blocking, single-threaded TCP server.

class tornado.tcpserver.TCPServer(ssl_options: Dict[str, Any] | SSLContext | None = None, max_buffer_size: int | None = None, read_chunk_size: int | None = None)[source]

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

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

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.SSLContext.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 initialization follows one of three patterns:

  1. listen: single-process:

    async def main():
        server = TCPServer()
        await asyncio.Event.wait()

    While this example does not create multiple processes on its own, when the reuse_port=True argument is passed to listen() you can run the program multiple times to create a multi-process service.

  2. add_sockets: multi-process:

    sockets = bind_sockets(8888)
    async def post_fork_main():
        server = TCPServer()
        await asyncio.Event().wait()

    The add_sockets interface is more complicated, but it can be used with tornado.process.fork_processes to run a multi-process service with all worker processes forked from a single parent. 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.

    Note that when using this pattern, nothing that touches the event loop can be run before fork_processes.

  3. bind/start: simple deprecated multi-process:

    server = TCPServer()
    server.start(0)  # Forks multiple sub-processes

    This pattern is deprecated because it requires interfaces in the asyncio module that have been deprecated since Python 3.10. Support for creating multiple processes in the start method will be removed in a future version of Tornado.

Added in version 3.1: The max_buffer_size argument.

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

listen(port: int, address: str | None = None, family: AddressFamily = AddressFamily.AF_UNSPEC, backlog: int = 128, flags: int | None = None, reuse_port: bool = False) None[source]

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 event loop if it is not already running.

All arguments have the same meaning as in tornado.netutil.bind_sockets.

Changed in version 6.2: Added family, backlog, flags, and reuse_port arguments to match tornado.netutil.bind_sockets.

add_sockets(sockets: Iterable[socket]) None[source]

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: socket) None[source]

Singular version of add_sockets. Takes a single socket object.

bind(port: int, address: str | None = None, family: AddressFamily = AddressFamily.AF_UNSPEC, backlog: int = 128, flags: int | None = None, reuse_port: bool = False) None[source]

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.

Changed in version 6.2: Added the flags argument to match bind_sockets.

Deprecated since version 6.2: Use either listen() or add_sockets() instead of bind() and start().

start(num_processes: int | None = 1, max_restarts: int | None = None) None[source]

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).

Values of num_processes other than 1 are not supported on Windows.

The max_restarts argument is passed to fork_processes.

Changed in version 6.0: Added max_restarts argument.

Deprecated since version 6.2: Use either listen() or add_sockets() instead of bind() and start().

stop() None[source]

Stops listening for new connections.

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

handle_stream(stream: IOStream, address: tuple) Awaitable[None] | None[source]

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.