Topic guides¶
Auto ping-pong¶
Available since 1.4
The WebSocket protocol includes special frame types, WSMsgType.PING and WSMsgType.PONG, which are useful for detecting stale connections.
From the user’s perspective, these frames function like regular frames and may contain payload data. When one side receives a PING frame, it must respond with a PONG frame that includes the same payload as the PING.
picows offers an efficient ‘auto ping-pong’ mechanism to automatically send a PING to the remote peer after a specified period of inactivity and to handle and verify PONG responses. If no PONG is received, the WebSocket will be disconnected.
This behaviour is controlled by the 3 parameters passed to ws_connect or ws_create_server:
await ws_connect(...,
enable_auto_ping=True, # disabled by default
auto_ping_idle_timeout=2, # send ping after 2 seconds of inactivity
auto_ping_reply_timeout=1 # expect pong reply within 1 second
)
Furthermore, it is possible to customize what will be ping and pong frames. Apart from PING/PONG msg types other common options are:
TEXT frames with ‘ping’ and ‘pong’ payload.
TEXT frames with full json payload like {“op”: “ping”} and {“op”: “pong”}
Customization is done by overloading WSListener send_user_specific_ping and is_user_specific_pong methods.
class ClientListener(picows.WSListener):
...
def send_user_specific_ping(transport: picows.WSTransport):
transport.send(picows.WSMsgType.TEXT, b"ping")
# default implementation does:
# transport.send_ping()
def is_user_specific_pong(frame: picows.WSFrame):
return frame.msg_type == picows.WSMsgType.TEXT and frame.get_payload_as_memoryview() == b"pong"
# default implementation does:
# return frame.msg_type == picows.WSMsgType.PONG
Please note that is_user_specific_pong is designed to be fast, as it is called for every incoming message before the on_ws_frame invocation.
A common pitfall is parsing the payload with a JSON parser twice.
If this applies to your use case, it’s better to delay the determination of a pong until after the payload has been parsed in on_ws_frame.
class ClientListener(picows.WSListener):
...
def send_user_specific_ping(transport: picows.WSTransport):
transport.send(picows.WSMsgType.TEXT, b'{"op":"ping"}')
def is_user_specific_pong(frame: picows.WSFrame):
# It is inefficient to do json.loads(frame.get_payload_as_utf8_text()) here.
# Because we would have to do it again in on_ws_frame
return False
def on_ws_frame(transport: picows.WSTransport, frame: picows.WSFrame):
if frame.msg_type == picows.WSMsgType.TEXT:
obj = json.loads(frame.get_payload_as_utf8())
if obj["op"] == "pong":
# Notify transport that pong reply has been received
transport.notify_user_specific_pong_received()
return
# Process other operations
...
Additionally, you must manually respond to incoming PING frames.
The auto-ping mechanism only handles sending PING frames to the remote peer and processing PONG replies;
it does not handle replying to incoming PING frames.
class ClientListener(picows.WSListener):
...
def on_ws_frame(transport: picows.WSTransport, frame: picows.WSFrame):
if frame.msg_type == picows.WSMsgType.PING:
transport.send_pong(frame.get_payload_as_bytes())
...
Message fragmentation¶
In the WebSocket protocol, there is a distinction between messages and frames. A message can be split across multiple frames, and reassembling them is done by concatenating the frame payloads.
picows does not attempt to concatenate frames automatically, as the most efficient way to handle this may vary depending on the specific use case.
Message fragmentation works as follows:
Unfragmented message:
WSFrame(msg_type=WSMsgType.<actual message type>, fin=True)
Fragmented message:
WSFrame(msg_type=WSMsgType.<actual message type>, fin=False)
WSFrame(msg_type=WSMsgType.CONTINUATION, fin=False)
...
# the last frame of the message
WSFrame(msg_type=WSMsgType.CONTINUATION, fin=True)
Here is the naive way to implement concatenation:
class ClientListener(picows.WSListener):
def __init__(self):
self._full_msg == bytearray()
self._full_msg_type = picows.WSMsgType.TEXT
def on_ws_frame(transport: picows.WSTransport, frame: picows.WSFrame):
... # Handle PING/PONG/CLOSE control frames first
if frame.fin:
if self._full_msg:
# This is the last fragment of the message because fin is set
# and there were previous fragments
assert frame.msg_type == picows.WSMsgType.CONTINUATION
self._full_msg += frame.get_payload_as_memoryview()
self.on_concatenated_message(transport, self._full_msg_type, self._full_msg)
self._full_msg.clear()
else:
# This is the only fragment of the message because fin is set
# and there was not previous fragments
self.on_unfragmented_message(transport, frame)
else:
if not self._full_msg:
# First fragment determine the whole message type
self._full_msg_type == frame.msg_type
# Accumulate payload from multiple fragments
self._full_msg += frame.get_payload_as_memoryview()
return
def on_unfragmented_message(self, transport: picows.WSTransport, frame: picows.WSFrame):
# Called for the simple case when a frame is a whole message
pass
def on_concatenated_message(self, msg_type: picows.WSMsgType, payload: bytearray):
# Called after concatenating a message from multiple frames
pass
Before blindly coping this code, consider checking what remote peer is sending. It is very common that clients and servers never fragment their messages. Also control messages PING/PONG/CLOSE are never fragmented.
Async iteration¶
The on_ws_* methods in WSListener are non-async for performance reasons. There are several factors that make a non-async interface significantly faster than an async one:
Implementing an async interface requires queuing data for later processing by a coroutine, which then needs to be woken up by the event loop. This introduces a substantial delay in processing and adds extra overhead for the event loop.
Since data cannot be processed immediately from the read buffer, it would need to be copied, which eliminates the advantage of zero-copy.
Regular Cython class methods can be overloaded very efficiently (equivalent to a C function call via a vtable), which is not possible for async class methods.
In summary, you can build an async interface on top of a non-async one and accept the performance trade-off when needed. However, if the interface is async-only, you cannot avoid this performance penalty.
Here is a one way to implement async iteration using asyncio.Queue:
class ClientListener(picows.WSListener):
def __init__(self):
self.msg_queue = asyncio.Queue()
...
def on_ws_frame(transport: picows.WSTransport, frame: picows.WSFrame):
if frame.msg_type == picows.WSMsgType.TEXT:
obj = json.loads(frame.get_payload_as_utf8_text())
self.msg_queue.put_nowait(obj)
def on_ws_disconnected(transport: picows.WSTransport):
# Push None to indicate the end of the stream
self.msg_queue.put_nowait(None)
async def some_async_function():
transport, listener = await ws_connect(ClientListener, ...)
while True:
msg = await listener.msg_queue.get()
listener.msg_queue.task_done()
if msg is None:
# client disconnected
:else
# Otherwise process message in async context
Another approach would be to just use asyncio.Loop.create_task:
async def process_message(msg):
...
class ClientListener(picows.WSListener):
def on_ws_frame(transport: picows.WSTransport, frame: picows.WSFrame):
if frame.msg_type == picows.WSMsgType.TEXT:
msg = json.loads(frame.get_payload_as_utf8_text())
asyncio.get_running_loop().create_task(process_message(msg))
Consider using it together with eager task factory.
Using Cython interface¶
picows classes and enums are Cython extension types. If you are using Cython in your project, you can access picows type definitions and some extra functionality by importing picows.pxd that is installed with the library.
Check out an example of a simple echo client that is written in Cython.