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.

-[ Introduction ]-

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)".

-[ Contents ]-

This documentation page contains the following sections:

* The Transports section documents asyncio "BaseTransport",
  "ReadTransport", "WriteTransport", "Transport", "DatagramTransport",
  and "SubprocessTransport" classes.

* The Protocols section documents asyncio "BaseProtocol", "Protocol",
  "BufferedProtocol", "DatagramProtocol", and "SubprocessProtocol"
  classes.

* The Examples section showcases how to work with transports,
  protocols, and low-level event loop APIs.


Transports
==========

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

Transport objects are always instantiated by an 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.

The transport classes are not thread safe.


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()

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

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:

   * 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

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()

   Return the current protocol.


Read-only Transports
--------------------

ReadTransport.is_reading()

   Return "True" if the transport is receiving new data.

   New in version 3.7.

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.

   Changed in version 3.7: 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.

   Changed in version 3.7: 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()

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

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.

   Use "set_write_buffer_limits()" to set the limits.

   New in version 3.4.2.

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)

   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.

WriteTransport.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.

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)

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


Subprocess Transports
---------------------

SubprocessTransport.get_pid()

   Return the subprocess process id as an integer.

SubprocessTransport.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"

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.

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

   See also "subprocess.Popen.kill()".

SubprocessTransport.send_signal(signal)

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

SubprocessTransport.terminate()

   Stop the subprocess.

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

   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.


Protocols
=========

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)

   Called when a connection is made.

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

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.

-[ 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)

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

   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.

State machine:

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


Buffered Streaming Protocols
----------------------------

New in version 3.7: **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.

State machine:

   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)

   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 could not be delivered to its
   recipient. In many conditions though, undeliverable datagrams will
   be silently dropped.

Note:

  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()

   Called when the child process has exited.


Examples
========


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 EchoServerProtocol(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: EchoServerProtocol(),
           '127.0.0.1', 8888)

       async with server:
           await server.serve_forever()


   asyncio.run(main())

See also:

  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):
           self.message = message
           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),
           '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())

See also:

  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, on_con_lost):
           self.message = message
           self.on_con_lost = on_con_lost
           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("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()

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

       transport, protocol = await loop.create_datagram_endpoint(
           lambda: EchoClientProtocol(message, on_con_lost),
           remote_addr=('127.0.0.1', 9999))

       try:
           await 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, on_con_lost):
           self.transport = None
           self.on_con_lost = on_con_lost

       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()
       on_con_lost = loop.create_future()

       # 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(on_con_lost), 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())

See also:

  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.
