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


18.5.4.1. Transports
====================

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.

The transport classes are not thread safe.


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()

      Return "True" if the transport is closing or is closed.

      New in version 3.5.1.

   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.

      * socket:

        * "'peername'": the remote address to which the socket is
          connected, result of "socket.socket.getpeername()" ("None"
          on error)

        * "'socket'": "socket.socket" instance

        * "'sockname'": the socket’s own address, result of
          "socket.socket.getsockname()"

      * SSL socket:

        * "'compression'": the compression algorithm being used as a
          string, or "None" if the connection isn’t compressed; result
          of "ssl.SSLSocket.compression()"

        * "'cipher'": a three-value tuple containing the name of the
          cipher being used, the version of the SSL protocol that
          defines its use, and the number of secret bits being used;
          result of "ssl.SSLSocket.cipher()"

        * "'peercert'": peer certificate; result of
          "ssl.SSLSocket.getpeercert()"

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

        * "'ssl_object'": "ssl.SSLObject" or "ssl.SSLSocket"
          instance

      * pipe:

        * "'pipe'": pipe object

      * subprocess:

        * "'subprocess'": "subprocess.Popen" instance

   set_protocol(protocol)

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

      New in version 3.5.3.

   get_protocol()

      Return the current protocol.

      New in version 3.5.3.

   Changed in version 3.5.1: "'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.

   resume_reading()

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


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()

      Return the current size of the output buffer used by the
      transport.

   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.

      Use "set_write_buffer_limits()" to set the limits.

      New in version 3.4.2.

   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)

      Write some *data* bytes to the transport.

      This method does not block; it buffers the data and arranges for
      it to be sent out asynchronously.

   writelines(list_of_data)

      Write a list (or any iterable) of data bytes to the transport.
      This is functionally equivalent to calling "write()" on each
      element yielded by the iterable, but may be implemented more
      efficiently.

   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)

   Send the *data* bytes to the remote peer given by *addr* (a
   transport-dependent target address).  If *addr* is "None", the data
   is sent to the target address given on transport creation.

   This method does not block; it buffers the data and arranges for it
   to be sent out asynchronously.

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()

      Return the subprocess process id as an integer.

   get_pipe_transport(fd)

      Return the transport for the communication pipe corresponding to
      the integer file descriptor *fd*:

      * "0": readable streaming transport of the standard input
        (*stdin*), or "None" if the subprocess was not created with
        "stdin=PIPE"

      * "1": writable streaming transport of the standard output
        (*stdout*), or "None" if the subprocess was not created with
        "stdout=PIPE"

      * "2": writable streaming transport of the standard error
        (*stderr*), or "None" if the subprocess was not created with
        "stderr=PIPE"

      * other *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()".

      On POSIX systems, the function sends SIGKILL to the subprocess.
      On Windows, this method is an alias for "terminate()".

   send_signal(signal)

      Send the *signal* number to the subprocess, as in
      "subprocess.Popen.send_signal()".

   terminate()

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

      On POSIX systems, this method sends SIGTERM to the subprocess.
      On Windows, the Windows API function TerminateProcess() is
      called to stop the subprocess.

   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. Protocols
===================

"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.

Note: 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)

   Called when a connection is made.

   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)

   Called when the connection is lost or closed.

   The argument is either an exception object or "None". The latter
   means a regular EOF is received, or the connection was aborted or
   closed by this side of the connection.

"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()

   Called when the child process has exited.


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

The following callbacks are called on "Protocol" instances:

Protocol.data_received(data)

   Called when some data is received.  *data* is a non-empty bytes
   object containing the incoming data.

   Note: 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.

   Note: 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.

State machine:

   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)

   Called when a datagram is received.  *data* is a bytes object
   containing the incoming data.  *addr* is the address of the peer
   sending the data; the exact format depends on the transport.

DatagramProtocol.error_received(exc)

   Called when a previous send or receive operation raises an
   "OSError".  *exc* is the "OSError" instance.

   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.

Note: 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.

Note: 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.

See also: 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.

See also: 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()

See also: 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.
