Topic guides¶
Auto ping¶
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’ 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
...
Auto pong¶
Available since 1.6
By default picows always replies to incoming PING messages with PONG.
This is controlled by enable_auto_pong argument to ws_connect
and ws_create_server. If disabled, PING messages must be handled
manually from on_ws_frame.
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())
...
Measuring/checking round-trip time¶
Available since 1.5
picows allows to conveniently measure round-trip time to a remote peer using
measure_roundtrip_time. This is done by sending PING request multiple
times and measuring response delay.
Checkout an okx_roundtrip_time.py example of how to measure RTT to a popular OKX crypto-currency exchange and initiate reconnect if it doesn’t satisfy a predefined threshold.
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, transport: picows.WSTransport, msg_type: picows.WSMsgType, payload: bytearray):
# Called after concatenating a message from multiple frames
pass
Before using this code snippet, consider verifying what the remote peer is sending. It’s quite common for clients and servers to never fragment their messages. Additionally, control messages like PING, PONG, and 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 echo_client_cython.pyx of a simple echo client that is written in Cython.
Enable debug logs¶
picows logs using Python’s standard logging module under picows.* logger. You may use any available way to set log level to PICOWS_DEBUG_LL (=9) to enable debug logging.
# Either set global log level
logging.basicConfig(level=picows.PICOWS_DEBUG_LL)
# Or set picows logger log level only
logging.basicConfig(level=logging.INFO)
logging.getLogger("picows").setLevel(picows.PICOWS_DEBUG_LL)
Exceptions handling¶
When talking about how library deals with exceptions, there are 2 questions that must be addressed:
What kinds of exceptions can the library functions throws?
picows may raise any exception that the underlying system calls may raise.
For example, ConnectionResetError from ws_connect or BrokenPipeError
from WSTransport.send.
picows does not wrap these exceptions in its own special exception type.
Additionally, ws_connect may raise WSError in cases of websocket
negotiation errors.
In general, WSError is reserved for errors specific to websockets only.
There is also a special exception, asyncio.CancelledError, which any coroutine can raise when it is externally cancelled. Sometimes you need to handle this exception manually. For example, in a reconnection loop where you want to reconnect on any error, the loop should break on asyncio.CancelledError.
What happens if a user callback raises an exception, and how does the library handle it?
This is described in the documentation of each particular method.
In most cases, picows will send a CLOSE frame with an INTERNAL_ERROR close code and disconnect.
However, for on_ws_frame, it is possible to override it by setting disconnect_on_error=False
in ws_connect/ws_create_server.