Transports and Protocols

Preface

Transports and Protocols are used by the low-level event loop APIs such as loop.create_connection(). They use callback-based programming style and enable high-performance implementations of network or IPC protocols (e.g. HTTP).

Essentially, transports and protocols should only be used in libraries and frameworks and never in high-level asyncio applications.

This documentation page covers both Transports and Protocols.

はじめに

At the highest level, the transport is concerned with how bytes are transmitted, while the protocol determines which bytes to transmit (and to some extent when).

A different way of saying the same thing: a transport is an abstraction for a socket (or similar I/O endpoint) while a protocol is an abstraction for an application, from the transport's point of view.

Yet another view is the transport and protocol interfaces together define an abstract interface for using network I/O and interprocess I/O.

There is always a 1:1 relationship between transport and protocol objects: the protocol calls transport methods to send data, while the transport calls protocol methods to pass it data that has been received.

Most of connection oriented event loop methods (such as loop.create_connection()) usually accept a protocol_factory argument used to create a Protocol object for an accepted connection, represented by a Transport object. Such methods usually return a tuple of (transport, protocol).

内容

This documentation page contains the following sections:

トランスポート

Transports are classes provided by asyncio in order to abstract various kinds of communication channels.

Transport objects are always instantiated by an ref:asyncio event loop <asyncio-event-loop>.

asyncio implements transports for TCP, UDP, SSL, and subprocess pipes. The methods available on a transport depend on the transport's kind.

トランスポートクラスは スレッド安全ではありません

Transports Hierarchy

class asyncio.BaseTransport

Base class for all transports. Contains methods that all asyncio transports share.

class asyncio.WriteTransport(BaseTransport)

A base transport for write-only connections.

Instances of the WriteTransport class are returned from the loop.connect_write_pipe() event loop method and are also used by subprocess-related methods like loop.subprocess_exec().

class asyncio.ReadTransport(BaseTransport)

A base transport for read-only connections.

Instances of the ReadTransport class are returned from the loop.connect_read_pipe() event loop method and are also used by subprocess-related methods like loop.subprocess_exec().

class asyncio.Transport(WriteTransport, ReadTransport)

Interface representing a bidirectional transport, such as a TCP connection.

The user does not instantiate a transport directly; they call a utility function, passing it a protocol factory and other information necessary to create the transport and protocol.

Instances of the Transport class are returned from or used by event loop methods like loop.create_connection(), loop.create_unix_connection(), loop.create_server(), loop.sendfile(), etc.

class asyncio.DatagramTransport(BaseTransport)

A transport for datagram (UDP) connections.

Instances of the DatagramTransport class are returned from the loop.create_datagram_endpoint() event loop method.

class asyncio.SubprocessTransport(BaseTransport)

An abstraction to represent a connection between a parent and its child OS process.

Instances of the SubprocessTransport class are returned from event loop methods loop.subprocess_shell() and loop.subprocess_exec().

Base Transport

BaseTransport.close()

Close the transport.

If the transport has a buffer for outgoing data, buffered data will be flushed asynchronously. No more data will be received. After all buffered data is flushed, the protocol's protocol.connection_lost() method will be called with None as its argument.

BaseTransport.is_closing()

トランスポートを閉じている最中か閉じていた場合 True を返します。

BaseTransport.get_extra_info(name, default=None)

Return information about the transport or underlying resources it uses.

name is a string representing the piece of transport-specific information to get.

default is the value to return if the information is not available, or if the transport does not support querying it with the given third-party event loop implementation or on the current platform.

For example, the following code attempts to get the underlying socket object of the transport:

sock = transport.get_extra_info('socket')
if sock is not None:
    print(sock.getsockopt(...))

Categories of information that can be queried on some transports:

  • ソケット:
  • SSL ソケット:
    • 'compression': 圧縮アルゴリズムで、ssl.SSLSocket.compression() の結果になります。圧縮されていないときは None になります
    • 'cipher': 3 個の値 (使用されている暗号アルゴリズムの名称、使用が定義されている SSL プロトコルのバージョン、および使用されている秘密鍵のビット数) からなるタプルで、ssl.SSLSocket.cipher() の結果になります
    • 'peercert': ピアの証明書で、ssl.SSLSocket.getpeercert() の結果になります
    • 'sslcontext': ssl.SSLContext のインスタンスになります
    • 'ssl_object': ssl.SSLObject または ssl.SSLSocket インスタンス
  • パイプ:
    • 'pipe': パイプオブジェクトです
  • サブプロセス:
BaseTransport.set_protocol(protocol)

Set a new protocol.

Switching protocol should only be done when both protocols are documented to support the switch.

BaseTransport.get_protocol()

現在のプロトコルを返します。

Read-only Transports

ReadTransport.is_reading()

Return True if the transport is receiving new data.

バージョン 3.7 で追加.

ReadTransport.pause_reading()

Pause the receiving end of the transport. No data will be passed to the protocol's protocol.data_received() method until resume_reading() is called.

バージョン 3.7 で変更: The method is idempotent, i.e. it can be called when the transport is already paused or closed.

ReadTransport.resume_reading()

Resume the receiving end. The protocol's protocol.data_received() method will be called once again if some data is available for reading.

バージョン 3.7 で変更: The method is idempotent, i.e. it can be called when the transport is already reading.

Write-only Transports

WriteTransport.abort()

Close the transport immediately, without waiting for pending operations to complete. Buffered data will be lost. No more data will be received. The protocol's protocol.connection_lost() method will eventually be called with None as its argument.

WriteTransport.can_write_eof()

Return True if the transport supports write_eof(), False if not.

WriteTransport.get_write_buffer_size()

トランスポートで使用されている出力バッファーの現在のサイズを返します。

WriteTransport.get_write_buffer_limits()

Get the high and low watermarks for write flow control. Return a tuple (low, high) where low and high are positive number of bytes.

水位点の設定は set_write_buffer_limits() で行います。

バージョン 3.4.2 で追加.

WriteTransport.set_write_buffer_limits(high=None, low=None)

Set the high and low watermarks for write flow control.

These two values (measured in number of bytes) control when the protocol's protocol.pause_writing() and protocol.resume_writing() methods are called. If specified, the low watermark must be less than or equal to the high watermark. Neither high nor low can be negative.

pause_writing() is called when the buffer size becomes greater than or equal to the high value. If writing has been paused, resume_writing() is called when the buffer size becomes less than or equal to the low value.

The defaults are implementation-specific. If only the high watermark is given, the low watermark defaults to an implementation-specific value less than or equal to the high watermark. Setting high to zero forces low to zero as well, and causes pause_writing() to be called whenever the buffer becomes non-empty. Setting low to zero causes resume_writing() to be called only once the buffer is empty. Use of zero for either limit is generally sub-optimal as it reduces opportunities for doing I/O and computation concurrently.

Use get_write_buffer_limits() to get the limits.

WriteTransport.write(data)

トランスポートにバイト列 data を書き込みます。

このメソッドはブロックしません; データをバッファーし、非同期に送信する準備を行います。

WriteTransport.writelines(list_of_data)

バイト列のデータのリスト (またはイテラブル) をトランスポートに書き込みます。この振る舞いはイテラブルを yield して各要素で write() を呼び出すことと等価ですが、より効率的な実装となる場合があります。

WriteTransport.write_eof()

Close the write end of the transport after flushing all buffered data. Data may still be received.

This method can raise NotImplementedError if the transport (e.g. SSL) doesn't support half-closed connections.

Datagram Transports

DatagramTransport.sendto(data, addr=None)

リモートピア addr (トランスポート依存の対象アドレス) にバイト列 data を送信します。addrNone の場合、データはトランスポートの作成時に指定された送信先に送られます。

このメソッドはブロックしません; データをバッファーし、非同期に送信する準備を行います。

DatagramTransport.abort()

Close the transport immediately, without waiting for pending operations to complete. Buffered data will be lost. No more data will be received. The protocol's protocol.connection_lost() method will eventually be called with None as its argument.

Subprocess Transports

SubprocessTransport.get_pid()

サブプロセスのプロセス ID (整数) を返します。

SubprocessTransport.get_pipe_transport(fd)

整数のファイル記述子 fd に該当する通信パイプのトランスポートを返します:

  • 0: 標準入力 (stdin) の読み込み可能ストリーミングトランスポート。サブプロセスが stdin=PIPE で作成されていない場合は None
  • 1: 標準出力 (stdout) の書き込み可能ストリーミングトランスポート。サブプロセスが stdout=PIPE で作成されていない場合は None
  • 2: 標準エラー出力 (stderr) の書き込み可能ストリーミングトランスポート。サブプロセスが stderr=PIPE で作成されていない場合は None
  • その他の fd: None
SubprocessTransport.get_returncode()

Return the subprocess return code as an integer or None if it hasn't returned, which is similar to the subprocess.Popen.returncode attribute.

SubprocessTransport.kill()

Kill the subprocess.

POSIX システムでは、この関数はサブプロセスに SIGKILL を送信します。Windows では、このメソッドは terminate() の別名です。

See also subprocess.Popen.kill().

SubprocessTransport.send_signal(signal)

サブプロセスにシグナル signal を送信します。subprocess.Popen.send_signal() と同じです。

SubprocessTransport.terminate()

Stop the subprocess.

POSIX システムでは、このメソッドはサブプロセスに SIGTERM を送信します。Windows では、Windows API 関数 TerminateProcess() が呼び出されます。

See also subprocess.Popen.terminate().

SubprocessTransport.close()

Kill the subprocess by calling the kill() method.

If the subprocess hasn't returned yet, and close transports of stdin, stdout, and stderr pipes.

プロトコル

asyncio provides a set of abstract base classes that should be used to implement network protocols. Those classes are meant to be used together with transports.

Subclasses of abstract base protocol classes may implement some or all methods. All these methods are callbacks: they are called by transports on certain events, for example when some data is received. A base protocol method should be called by the corresponding transport.

Base Protocols

class asyncio.BaseProtocol

Base protocol with methods that all protocols share.

class asyncio.Protocol(BaseProtocol)

The base class for implementing streaming protocols (TCP, Unix sockets, etc).

class asyncio.BufferedProtocol(BaseProtocol)

A base class for implementing streaming protocols with manual control of the receive buffer.

class asyncio.DatagramProtocol(BaseProtocol)

The base class for implementing datagram (UDP) protocols.

class asyncio.SubprocessProtocol(BaseProtocol)

The base class for implementing protocols communicating with child processes (unidirectional pipes).

Base Protocol

All asyncio protocols can implement Base Protocol callbacks.

Connection Callbacks

Connection callbacks are called on all protocols, exactly once per a successful connection. All other protocol callbacks can only be called between those two methods.

BaseProtocol.connection_made(transport)

コネクションが作成されたときに呼び出されます。

The transport argument is the transport representing the connection. The protocol is responsible for storing the reference to its transport.

BaseProtocol.connection_lost(exc)

コネクションが失われた、あるいはクローズされたときに呼び出されます。

引数は例外オブジェクトまたは None になります。None のとき、通常の EOF が受信されたか、あるいはコネクションがこちら側から中止またはクローズされたことを意味します。

Flow Control Callbacks

Flow control callbacks can be called by transports to pause or resume writing performed by the protocol.

See the documentation of the set_write_buffer_limits() method for more details.

BaseProtocol.pause_writing()

Called when the transport's buffer goes over the high watermark.

BaseProtocol.resume_writing()

Called when the transport's buffer drains below the low watermark.

If the buffer size equals the high watermark, pause_writing() is not called: the buffer size must go strictly over.

Conversely, resume_writing() is called when the buffer size is equal or lower than the low watermark. These end conditions are important to ensure that things go as expected when either mark is zero.

Streaming Protocols

Event methods, such as loop.create_server(), loop.create_unix_server(), loop.create_connection(), loop.create_unix_connection(), loop.connect_accepted_socket(), loop.connect_read_pipe(), and loop.connect_write_pipe() accept factories that return streaming protocols.

Protocol.data_received(data)

データを受信したときに呼び出されます。data は受信したデータを含む空ではないバイト列オブジェクトになります。

Whether the data is buffered, chunked or reassembled depends on the transport. In general, you shouldn't rely on specific semantics and instead make your parsing generic and flexible. However, data is always received in the correct order.

The method can be called an arbitrary number of times while a connection is open.

However, protocol.eof_received() is called at most once. Once eof_received() is called, data_received() is not called anymore.

Protocol.eof_received()

Called when the other end signals it won't send any more data (for example by calling transport.write_eof(), if the other end also uses asyncio).

This method may return a false value (including None), in which case the transport will close itself. Conversely, if this method returns a true value, the protocol used determines whether to close the transport. Since the default implementation returns None, it implicitly closes the connection.

Some transports, including SSL, don't support half-closed connections, in which case returning true from this method will result in the connection being closed.

ステートマシン:

start -> connection_made
    [-> data_received]*
    [-> eof_received]?
-> connection_lost -> end

Buffered Streaming Protocols

バージョン 3.7 で追加: Important: this has been added to asyncio in Python 3.7 on a provisional basis! This is as an experimental API that might be changed or removed completely in Python 3.8.

Buffered Protocols can be used with any event loop method that supports Streaming Protocols.

BufferedProtocol implementations allow explicit manual allocation and control of the receive buffer. Event loops can then use the buffer provided by the protocol to avoid unnecessary data copies. This can result in noticeable performance improvement for protocols that receive big amounts of data. Sophisticated protocol implementations can significantly reduce the number of buffer allocations.

The following callbacks are called on BufferedProtocol instances:

BufferedProtocol.get_buffer(sizehint)

Called to allocate a new receive buffer.

sizehint is the recommended minimum size for the returned buffer. It is acceptable to return smaller or larger buffers than what sizehint suggests. When set to -1, the buffer size can be arbitrary. It is an error to return a buffer with a zero size.

get_buffer() must return an object implementing the buffer protocol.

BufferedProtocol.buffer_updated(nbytes)

Called when the buffer was updated with the received data.

nbytes is the total number of bytes that were written to the buffer.

BufferedProtocol.eof_received()

See the documentation of the protocol.eof_received() method.

get_buffer() can be called an arbitrary number of times during a connection. However, protocol.eof_received() is called at most once and, if called, get_buffer() and buffer_updated() won't be called after it.

ステートマシン:

start -> connection_made
    [-> get_buffer
        [-> buffer_updated]?
    ]*
    [-> eof_received]?
-> connection_lost -> end

Datagram Protocols

Datagram Protocol instances should be constructed by protocol factories passed to the loop.create_datagram_endpoint() method.

DatagramProtocol.datagram_received(data, addr)

データグラムを受信したときに呼び出されます。data は受信データを含むバイトオブジェクトです。addr はデータを送信するピアのアドレスです; 正確な形式はトランスポートに依存します。

DatagramProtocol.error_received(exc)

直前の送信あるいは受信が OSError を送出したときに呼び出されます。excOSError のインスタンスになります。

This method is called in rare conditions, when the transport (e.g. UDP) detects that a datagram could not be delivered to its recipient. In many conditions though, undeliverable datagrams will be silently dropped.

注釈

On BSD systems (macOS, FreeBSD, etc.) flow control is not supported for datagram protocols, because there is no reliable way to detect send failures caused by writing too many packets.

The socket always appears 'ready' and excess packets are dropped. An OSError with errno set to errno.ENOBUFS may or may not be raised; if it is raised, it will be reported to DatagramProtocol.error_received() but otherwise ignored.

Subprocess Protocols

Datagram Protocol instances should be constructed by protocol factories passed to the loop.subprocess_exec() and loop.subprocess_shell() methods.

SubprocessProtocol.pipe_data_received(fd, data)

Called when the child process writes data into its stdout or stderr pipe.

fd is the integer file descriptor of the pipe.

data is a non-empty bytes object containing the received data.

SubprocessProtocol.pipe_connection_lost(fd, exc)

Called when one of the pipes communicating with the child process is closed.

fd is the integer file descriptor that was closed.

SubprocessProtocol.process_exited()

子プロセスが終了したときに呼び出されます。

使用例

TCP Echo Server

Create a TCP echo server using the loop.create_server() method, send back received data, and close the connection:

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))

        print('Send: {!r}'.format(message))
        self.transport.write(data)

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


async def main():
    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    server = await loop.create_server(
        lambda: EchoServerClientProtocol(),
        '127.0.0.1', 8888)

    async with server:
        await server.serve_forever()


asyncio.run(main())

参考

The TCP echo server using streams example uses the high-level asyncio.start_server() function.

TCP Echo Client

A TCP echo client using the loop.create_connection() method, sends data, and waits until the connection is closed:

import asyncio


class EchoClientProtocol(asyncio.Protocol):
    def __init__(self, message, on_con_lost, loop):
        self.message = message
        self.loop = loop
        self.on_con_lost = on_con_lost

    def connection_made(self, transport):
        transport.write(self.message.encode())
        print('Data sent: {!r}'.format(self.message))

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

    def connection_lost(self, exc):
        print('The server closed the connection')
        self.on_con_lost.set_result(True)


async def main():
    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    on_con_lost = loop.create_future()
    message = 'Hello World!'

    transport, protocol = await loop.create_connection(
        lambda: EchoClientProtocol(message, on_con_lost, loop),
        '127.0.0.1', 8888)

    # Wait until the protocol signals that the connection
    # is lost and close the transport.
    try:
        await on_con_lost
    finally:
        transport.close()


asyncio.run(main())

参考

The TCP echo client using streams example uses the high-level asyncio.open_connection() function.

UDP Echo Server

A UDP echo server, using the loop.create_datagram_endpoint() method, sends back received data:

import asyncio


class EchoServerProtocol:
    def connection_made(self, transport):
        self.transport = transport

    def datagram_received(self, data, addr):
        message = data.decode()
        print('Received %r from %s' % (message, addr))
        print('Send %r to %s' % (message, addr))
        self.transport.sendto(data, addr)


async def main():
    print("Starting UDP server")

    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    # One protocol instance will be created to serve all
    # client requests.
    transport, protocol = await loop.create_datagram_endpoint(
        lambda: EchoServerProtocol(),
        local_addr=('127.0.0.1', 9999))

    try:
        await asyncio.sleep(3600)  # Serve for 1 hour.
    finally:
        transport.close()


asyncio.run(main())

UDP Echo Client

A UDP echo client, using the loop.create_datagram_endpoint() method, sends data and closes the transport when it receives the answer:

import asyncio


class EchoClientProtocol:
    def __init__(self, message, loop):
        self.message = message
        self.loop = loop
        self.transport = None
        self.on_con_lost = loop.create_future()

    def connection_made(self, transport):
        self.transport = transport
        print('Send:', self.message)
        self.transport.sendto(self.message.encode())

    def datagram_received(self, data, addr):
        print("Received:", data.decode())

        print("Close the socket")
        self.transport.close()

    def error_received(self, exc):
        print('Error received:', exc)

    def connection_lost(self, exc):
        print("Connection closed")
        self.on_con_lost.set_result(True)


async def main():
    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    message = "Hello World!"
    transport, protocol = await loop.create_datagram_endpoint(
        lambda: EchoClientProtocol(message, loop),
        remote_addr=('127.0.0.1', 9999))

    try:
        await protocol.on_con_lost
    finally:
        transport.close()


asyncio.run(main())

Connecting Existing Sockets

Wait until a socket receives data using the loop.create_connection() method with a protocol:

import asyncio
import socket


class MyProtocol(asyncio.Protocol):

    def __init__(self, loop):
        self.transport = None
        self.on_con_lost = loop.create_future()

    def connection_made(self, transport):
        self.transport = transport

    def data_received(self, data):
        print("Received:", data.decode())

        # We are done: close the transport;
        # connection_lost() will be called automatically.
        self.transport.close()

    def connection_lost(self, exc):
        # The socket has been closed
        self.on_con_lost.set_result(True)


async def main():
    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    # Create a pair of connected sockets
    rsock, wsock = socket.socketpair()

    # Register the socket to wait for data.
    transport, protocol = await loop.create_connection(
        lambda: MyProtocol(loop), sock=rsock)

    # Simulate the reception of data from the network.
    loop.call_soon(wsock.send, 'abc'.encode())

    try:
        await protocol.on_con_lost
    finally:
        transport.close()
        wsock.close()

asyncio.run(main())

参考

The watch a file descriptor for read events example uses the low-level loop.add_reader() method to register an FD.

The register an open socket to wait for data using streams example uses high-level streams created by the open_connection() function in a coroutine.

loop.subprocess_exec() and SubprocessProtocol

An example of a subprocess protocol used to get the output of a subprocess and to wait for the subprocess exit.

The subprocess is created by th loop.subprocess_exec() method:

import asyncio
import sys

class DateProtocol(asyncio.SubprocessProtocol):
    def __init__(self, exit_future):
        self.exit_future = exit_future
        self.output = bytearray()

    def pipe_data_received(self, fd, data):
        self.output.extend(data)

    def process_exited(self):
        self.exit_future.set_result(True)

async def get_date():
    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    code = 'import datetime; print(datetime.datetime.now())'
    exit_future = asyncio.Future(loop=loop)

    # Create the subprocess controlled by DateProtocol;
    # redirect the standard output into a pipe.
    transport, protocol = await loop.subprocess_exec(
        lambda: DateProtocol(exit_future),
        sys.executable, '-c', code,
        stdin=None, stderr=None)

    # Wait for the subprocess exit using the process_exited()
    # method of the protocol.
    await exit_future

    # Close the stdout pipe.
    transport.close()

    # Read the output which was collected by the
    # pipe_data_received() method of the protocol.
    data = bytes(protocol.output)
    return data.decode('ascii').rstrip()

if sys.platform == "win32":
    asyncio.set_event_loop_policy(
        asyncio.WindowsProactorEventLoopPolicy())

date = asyncio.run(get_date())
print(f"Current date: {date}")

See also the same example written using high-level APIs.