Topic guides

Making data interface async

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.

If you just want to turn non-async callbacks into async, the most efficient approach is to use eager tasks available since python 3.13. Eager tasks do not wait for the next event loop cycle and get executed immediately. See echo_client_async_callbacks.py illustrating this approach.

If you need an async receive_message(), similar to what aiohttp and websockets offer, then you would have to use asyncio.Queue. The latency penalty will become bigger, since awaiting coroutine can only be woken up on the next event loop cycle and message payload will always have to be copied. See echo_client_async_iteration.py illustrating this approach.

picows lets you choose the best possible approach for your project. Very often turning async is not really necessary on the data path. With picows you can delay this and do it only when necessary, for example, only when you actually have to start some async operation.

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.

Important

Consider verifying what the remote peer is sending. It’s very common for clients and servers to never fragment their messages. In such case Frame == Message. Additionally, control messages like PING, PONG, and CLOSE are never fragmented.

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)

echo_client_fragmented_msg.py demonstrates how to correctly split messages and how to assemble them back.

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 the library deals with exceptions, there are two questions that must be addressed:

What kinds of exceptions can the library functions throw?

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?

In most cases, picows initiates websocket shutdown:

On the client side, the first exception raised by a user handler is stored internally, transferred to WSTransport.wait_disconnected, and re-raised when await transport.wait_disconnected() completes. This makes handler failures observable from your top-level coroutine.

On the server side, callback exceptions are logged and not re-raised via wait_disconnected (there is no per-client await path on server internals).

For on_ws_frame, this behavior is configurable via disconnect_on_exception in ws_connect/ws_create_server:

  • disconnect_on_exception=True (default): exception triggers disconnect, and on client side it is re-raised by wait_disconnected.

  • disconnect_on_exception=False: exception is only logged, connection stays open.

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 behavior is controlled by three 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 message 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())

        ...

Graceful websocket shutdown

According to RFC 6455, graceful websocket shutdown is a CLOSE handshake: one side sends a CLOSE frame, the peer replies with CLOSE, and then both sides close the underlying TCP connection.

picows does not perform full websocket CLOSE handshake automatically:

If you want graceful websocket shutdown, handle CLOSE explicitly in your listener:

class Listener(picows.WSListener):

    def initiate_close(self, transport: picows.WSTransport):
        transport.send_close(picows.WSCloseCode.OK, b"done")
        transport.disconnect()

    def on_ws_frame(self, transport: picows.WSTransport, frame: picows.WSFrame):
        if frame.msg_type == picows.WSMsgType.CLOSE:
            # If peer initiates close, echo CLOSE and then disconnect.
            # If CLOSE is a reply to our CLOSE, it safe to call send_close and disconnect again.
            # They will be ignored.
            transport.send_close(frame.get_close_code(), frame.get_close_message())
            transport.disconnect()
            return

        ...

graceful_shutdown.py contains a complete runnable example.

You do not need extra guards around those calls:

Disconnect behavior and asyncio transport semantics

WSTransport.send, WSTransport.send_ping, WSTransport.send_pong, and WSTransport.send_close eventually rely on asyncio transport write(), which may buffer data.

By default, WSTransport.disconnect calls asyncio transport close(). This attempts to flush data that has already been enqueued by previous send calls before the socket is closed (subject to OS/kernel behavior and network conditions).

For immediate teardown, call WSTransport.disconnect with graceful=False. This maps to asyncio transport abort() and closes the connection without waiting for buffered outgoing data.

Measuring/checking round-trip time

Available since 1.5

picows allows you to conveniently measure round-trip time to a remote peer using measure_roundtrip_time. This is done by sending PING requests multiple times and measuring response delay.

Check out okx_roundtrip_time.py example of how to measure RTT to a popular OKX crypto-currency exchange and initiate reconnect if it does not satisfy a predefined threshold.

Dealing with slow clients

When a server pushes messages faster than a client can consume them, the write side of the connection eventually hits transport high watermark limits. On the server side, per-client listeners can react to this by overriding WSListener.pause_writing and WSListener.resume_writing.

This allows implementing backpressure-aware producers: pause message generation while writing is paused and resume only when the transport drains.

slow_client_backpressure.py demonstrates how pause_writing/resume_writing are triggered and how to stop the producer while the client is slow.

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.

Using proxies

Available since 1.13

ws_connect supports HTTP, SOCKS4 and SOCKS5 proxies via python-socks. Use the proxy argument with a proxy URL. HTTPS proxy URLs (https://...) are not currently supported:

transport, listener = await ws_connect(
    ClientListener,
    "ws://127.0.0.1:9000/",
    proxy="socks5://user:password@127.0.0.1:1080",
)

When connecting to wss:// URLs through a proxy, picows establishes a tunnel through the proxy and then performs the TLS handshake with the websocket server.

Hostname resolution generally happens at the proxy, unless it is SOCK4. SOCK4 is an old protocol, where CONNECT request doesn’t support host names, only IP addresses. SOCK4 hostname resolution is performed at the client.

Basic auth is supported. Login and password can be specified in the proxy URL.

Currently, picows does not attempt to use system proxy settings. If you want to use system-wide proxy settings, get them using getproxies and pass one as the proxy argument.

Setting socket options

If you need custom TCP socket tuning, use on_ws_connected callback and adjust the raw socket there.

import socket
from picows import WSListener, WSTransport

class Listener(WSListener):
    ...
    def on_ws_connected(transport: WSTransport):
        sock: socket.socket = transport.underlying_transport.get_extra_info("socket")
        # Example: enlarge kernel socket buffers
        sock.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, 1 * 1024 * 1024)
        sock.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, 1 * 1024 * 1024)

Note

picows already enables TCP_NODELAY and, when available on the platform, TCP_QUICKACK to reduce latency by default.