18.5.4. Transports and protocols (callback based API)
*****************************************************

**Source code:** Lib/asyncio/transports.py

**Source code:** Lib/asyncio/protocols.py


18.5.4.1. 传输
==============

Transports are classes provided by "asyncio" in order to abstract
various kinds of communication channels.  You generally won't
instantiate a transport yourself; instead, you will call an
"AbstractEventLoop" method which will create the transport and try to
initiate the underlying communication channel, calling you back when
it succeeds.

Once the communication channel is established, a transport is always
paired with a protocol instance.  The protocol can then call the
transport's methods for various purposes.

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

传输类属于 线程不安全 。

在 3.6 版更改: The socket option "TCP_NODELAY" is now set by default.


18.5.4.1.1. BaseTransport
-------------------------

class asyncio.BaseTransport

   Base class for transports.

   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 "connection_lost()" method will be called with "None"
      as its argument.

   is_closing()

      返回 "True" ，如果传输正在关闭或已经关闭。

      3.5.1 新版功能.

   get_extra_info(name, default=None)

      Return optional transport information.  *name* is a string
      representing the piece of transport-specific information to get,
      *default* is the value to return if the information doesn't
      exist.

      This method allows transport implementations to easily expose
      channel-specific information.

      * 套接字:

        * "'peername'": 套接字链接时的远端地址，
          "socket.socket.getpeername()" 方法的结果 (出错时为 "None" )

        * "'socket'": "socket.socket" 实例

        * "'sockname'": 套接字本地地址， "socket.socket.getsockname()"
          方法的结果

      * SSL套接字

        * "'compression'": 用字符串指定压缩算法，或者链接没有压缩时为
          "None"  ；"ssl.SSLSocket.compression()" 的结果。

        * "'cipher'": 一个三值的元组，包含使用密码的名称，定义使用的
          SSL协议的版本，使用的加密位数。  "ssl.SSLSocket.cipher()" 的
          结果。

        * "'peercert'": 远端凭证；  "ssl.SSLSocket.getpeercert()" 结果
          。

        * "'sslcontext'": "ssl.SSLContext" 实例

        * "'ssl_object'": "ssl.SSLObject" 或 "ssl.SSLSocket" 实例

      * 管道：

        * "'pipe'": 管道对象

      * 子进程：

        * "'subprocess'": "subprocess.Popen" 实例

   set_protocol(protocol)

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

      3.5.3 新版功能.

   get_protocol()

      返回当前协议。

      3.5.3 新版功能.

   在 3.5.1 版更改: "'ssl_object'" info was added to SSL sockets.


18.5.4.1.2. ReadTransport
-------------------------

class asyncio.ReadTransport

   Interface for read-only transports.

   pause_reading()

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

      在 3.6.7 版更改: 这个方法幂等的， 它可以在传输已经暂停或关闭时调
      用。

   resume_reading()

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

      在 3.6.7 版更改: 这个方法幂等的， 它可以在传输已经准备好读取数据
      时调用。


18.5.4.1.3. WriteTransport
--------------------------

class asyncio.WriteTransport

   Interface for write-only transports.

   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 "connection_lost()" method
      will eventually be called with "None" as its argument.

   can_write_eof()

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

   get_write_buffer_size()

      返回传输使用输出缓冲区的当前大小。

   get_write_buffer_limits()

      Get the *high*- and *low*-water limits for write flow control.
      Return a tuple "(low, high)" where *low* and *high* are positive
      number of bytes.

      使用 "set_write_buffer_limits()" 设置限制。

      3.4.2 新版功能.

   set_write_buffer_limits(high=None, low=None)

      Set the *high*- and *low*-water limits for write flow control.

      These two values (measured in number of bytes) control when the
      protocol's "pause_writing()" and "resume_writing()" methods are
      called. If specified, the low-water limit must be less than or
      equal to the high-water limit.  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-
      water limit is given, the low-water limit defaults to an
      implementation-specific value less than or equal to the high-
      water limit.  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.

   write(data)

      将一些 *data* 字节串写入传输。

      此方法不会阻塞；它会缓冲数据并安排其被异步地发出。

   writelines(list_of_data)

      将数据字节串的列表（或任意可迭代对象）写入传输。 这在功能上等价
      于在可迭代对象产生的每个元素上调用 "write()"，但其实现可能更为高
      效。

   write_eof()

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

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


18.5.4.1.4. DatagramTransport
-----------------------------

DatagramTransport.sendto(data, addr=None)

   将 *data* 字节串发送到 *addr* (基于传输的目标地址) 所给定的远端对等
   方。 如果 *addr* 为 "None"，则将数据发送到传输创建时给定的目标地址
   。

   此方法不会阻塞；它会缓冲数据并安排其被异步地发出。

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 "connection_lost()" method will
   eventually be called with "None" as its argument.


18.5.4.1.5. BaseSubprocessTransport
-----------------------------------

class asyncio.BaseSubprocessTransport

   get_pid()

      将子进程的进程 ID 以整数形式返回。

   get_pipe_transport(fd)

      返回对应于整数文件描述符 *fd* 的通信管道的传输:

      * "0": 标准输入 (*stdin*) 的可读流式传输，如果子进程创建时未设置
        "stdin=PIPE" 则为 "None"。

      * "1": 标准输出writable streaming transport of the standard
        output (*stdout*) 的可写流式传输，如果子进程创建时未设置
        "stdout=PIPE" 则为 "None"。

      * "2": 标准错误 (*stderr*) 的可写流式传输，如果子进程创建时未设
        置 "stderr=PIPE" 则为 "None"

      * 其他 *fd*: "None"

   get_returncode()

      Return the subprocess returncode as an integer or "None" if it
      hasn't returned, similarly to the "subprocess.Popen.returncode"
      attribute.

   kill()

      Kill the subprocess, as in "subprocess.Popen.kill()".

      在 POSIX 系统中，函数会发送 SIGKILL 到子进程。 在 Windows 中，此
      方法是 "terminate()" 的别名。

   send_signal(signal)

      发送 *signal* 编号到子进程，与 "subprocess.Popen.send_signal()"
      一样。

   terminate()

      Ask the subprocess to stop, as in
      "subprocess.Popen.terminate()". This method is an alias for the
      "close()" method.

      在 POSIX 系统中，此方法会发送 SIGTERM 到子进程。 在 Windows 中，
      则会调用 Windows API 函数 TerminateProcess() 来停止子进程。

   close()

      Ask the subprocess to stop by calling the "terminate()" method
      if the subprocess hasn't returned yet, and close transports of
      all pipes (*stdin*, *stdout* and *stderr*).


18.5.4.2. 协议
==============

"asyncio" provides base classes that you can subclass to implement
your network protocols.  Those classes are used in conjunction with
transports (see below): the protocol parses incoming data and asks for
the writing of outgoing data, while the transport is responsible for
the actual I/O and buffering.

When subclassing a protocol class, it is recommended you override
certain methods.  Those methods are callbacks: they will be called by
the transport on certain events (for example when some data is
received); you shouldn't call them yourself, unless you are
implementing a transport.

注解:

  All callbacks have default implementations, which are empty.
  Therefore, you only need to implement the callbacks for the events
  in which you are interested.


18.5.4.2.1. Protocol classes
----------------------------

class asyncio.Protocol

   The base class for implementing streaming protocols (for use with
   e.g. TCP and SSL transports).

class asyncio.DatagramProtocol

   The base class for implementing datagram protocols (for use with
   e.g. UDP transports).

class asyncio.SubprocessProtocol

   The base class for implementing protocols communicating with child
   processes (through a set of unidirectional pipes).


18.5.4.2.2. Connection callbacks
--------------------------------

These callbacks may be called on "Protocol", "DatagramProtocol" and
"SubprocessProtocol" instances:

BaseProtocol.connection_made(transport)

   链接建立时被调用。

   The *transport* argument is the transport representing the
   connection.  You are responsible for storing it somewhere (e.g. as
   an attribute) if you need to.

BaseProtocol.connection_lost(exc)

   链接丢失或关闭时被调用。

   方法的参数是一个异常对象或为 "None"。 后者意味着收到了常规的 EOF，
   或者连接被连接的一端取消或关闭。

"connection_made()" and "connection_lost()" are called exactly once
per successful connection.  All other callbacks will be called between
those two methods, which allows for easier resource management in your
protocol implementation.

The following callbacks may be called only on "SubprocessProtocol"
instances:

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 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()

   子进程退出时被调用。


18.5.4.2.3. Streaming protocols
-------------------------------

The following callbacks are called on "Protocol" instances:

Protocol.data_received(data)

   当收到数据时被调用。 *data* 为包含入站数据的非空字节串对象。

   注解:

     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
     enough.  However, data is always received in the correct order.

Protocol.eof_received()

   Called when the other end signals it won't send any more data (for
   example by calling "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, closing the transport is up to the protocol.
   Since the default implementation returns "None", it implicitly
   closes the connection.

   注解:

     Some transports such as SSL don't support half-closed
     connections, in which case returning true from this method will
     not prevent closing the connection.

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

状态机：

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


18.5.4.2.4. Datagram protocols
------------------------------

The following callbacks are called on "DatagramProtocol" instances.

DatagramProtocol.datagram_received(data, addr)

   当接收到数据报时被调用。 *data* 是包含传入数据的字节串对象。 *addr*
   是发送数据的对等端地址；实际的格式取决于具体传输。

DatagramProtocol.error_received(exc)

   当前一个发送或接收操作引发 "OSError" 时被调用。  *exc* 是 "OSError"
   的实例。

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


18.5.4.2.5. Flow control callbacks
----------------------------------

These callbacks may be called on "Protocol", "DatagramProtocol" and
"SubprocessProtocol" instances:

BaseProtocol.pause_writing()

   Called when the transport's buffer goes over the high-water mark.

BaseProtocol.resume_writing()

   Called when the transport's buffer drains below the low-water mark.

"pause_writing()" and "resume_writing()" calls are paired --
"pause_writing()" is called once when the buffer goes strictly over
the high-water mark (even if subsequent writes increases the buffer
size even more), and eventually "resume_writing()" is called once when
the buffer size reaches the low-water mark.

注解:

  If the buffer size equals the high-water mark, "pause_writing()" is
  not called -- it must go strictly over. Conversely,
  "resume_writing()" is called when the buffer size is equal or lower
  than the low-water mark.  These end conditions are important to
  ensure that things go as expected when either mark is zero.

注解:

  On BSD systems (OS X, FreeBSD, etc.) flow control is not supported
  for "DatagramProtocol", because send failures caused by writing too
  many packets cannot be detected easily.  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.


18.5.4.2.6. Coroutines and protocols
------------------------------------

Coroutines can be scheduled in a protocol method using
"ensure_future()", but there is no guarantee made about the execution
order.  Protocols are not aware of coroutines created in protocol
methods and so will not wait for them.

To have a reliable execution order, use stream objects in a coroutine
with "yield from". For example, the "StreamWriter.drain()" coroutine
can be used to wait until the write buffer is flushed.


18.5.4.3. Protocol examples
===========================


18.5.4.3.1. TCP echo client protocol
------------------------------------

TCP echo client  using the "AbstractEventLoop.create_connection()"
method, send data and wait until the connection is closed:

   import asyncio

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

       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')
           print('Stop the event loop')
           self.loop.stop()

   loop = asyncio.get_event_loop()
   message = 'Hello World!'
   coro = loop.create_connection(lambda: EchoClientProtocol(message, loop),
                                 '127.0.0.1', 8888)
   loop.run_until_complete(coro)
   loop.run_forever()
   loop.close()

The event loop is running twice. The "run_until_complete()" method is
preferred in this short example to raise an exception if the server is
not listening, instead of having to write a short coroutine to handle
the exception and stop the running loop. At "run_until_complete()"
exit, the loop is no longer running, so there is no need to stop the
loop in case of an error.

参见:

  The TCP echo client using streams example uses the
  "asyncio.open_connection()" function.


18.5.4.3.2. TCP echo server protocol
------------------------------------

TCP echo server using the "AbstractEventLoop.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()

   loop = asyncio.get_event_loop()
   # Each client connection will create a new protocol instance
   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()

"Transport.close()" can be called immediately after
"WriteTransport.write()" even if data are not sent yet on the socket:
both methods are asynchronous. "yield from" is not needed because
these transport methods are not coroutines.

参见:

  The TCP echo server using streams example uses the
  "asyncio.start_server()" function.


18.5.4.3.3. UDP echo client protocol
------------------------------------

UDP echo client using the
"AbstractEventLoop.create_datagram_endpoint()" method, send data and
close the transport when we received the answer:

   import asyncio

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

       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("Socket closed, stop the event loop")
           loop = asyncio.get_event_loop()
           loop.stop()

   loop = asyncio.get_event_loop()
   message = "Hello World!"
   connect = loop.create_datagram_endpoint(
       lambda: EchoClientProtocol(message, loop),
       remote_addr=('127.0.0.1', 9999))
   transport, protocol = loop.run_until_complete(connect)
   loop.run_forever()
   transport.close()
   loop.close()


18.5.4.3.4. UDP echo server protocol
------------------------------------

UDP echo server using the
"AbstractEventLoop.create_datagram_endpoint()" method, send 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)

   loop = asyncio.get_event_loop()
   print("Starting UDP server")
   # One protocol instance will be created to serve all client requests
   listen = loop.create_datagram_endpoint(
       EchoServerProtocol, local_addr=('127.0.0.1', 9999))
   transport, protocol = loop.run_until_complete(listen)

   try:
       loop.run_forever()
   except KeyboardInterrupt:
       pass

   transport.close()
   loop.close()


18.5.4.3.5. Register an open socket to wait for data using a protocol
---------------------------------------------------------------------

Wait until a socket receives data using the
"AbstractEventLoop.create_connection()" method with a protocol, and
then close the event loop

   import asyncio
   try:
       from socket import socketpair
   except ImportError:
       from asyncio.windows_utils import socketpair

   # Create a pair of connected sockets
   rsock, wsock = socketpair()
   loop = asyncio.get_event_loop()

   class MyProtocol(asyncio.Protocol):
       transport = None

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

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

           # We are done: close the transport (it will call connection_lost())
           self.transport.close()

       def connection_lost(self, exc):
           # The socket has been closed, stop the event loop
           loop.stop()

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

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

   # Run the event loop
   loop.run_forever()

   # We are done, close sockets and the event loop
   rsock.close()
   wsock.close()
   loop.close()

参见:

  The watch a file descriptor for read events example uses the low-
  level "AbstractEventLoop.add_reader()" method to register the file
  descriptor of a socket.

  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.
