Long-lived connections#

Since the WebSocket protocol is intended for real-time communications over long-lived connections, it is desirable to ensure that connections don’t break, and if they do, to report the problem quickly.

Connections can drop as a consequence of temporary network connectivity issues, which are very common, even within datacenters.

Furthermore, WebSocket builds on top of HTTP/1.1 where connections are short-lived, even with Connection: keep-alive. Typically, HTTP/1.1 infrastructure closes idle connections after 30 to 120 seconds.

As a consequence, proxies may terminate WebSocket connections prematurely when no message was exchanged in 30 seconds.

Keepalive in websockets#

To avoid these problems, websockets runs a keepalive and heartbeat mechanism based on WebSocket Ping and Pong frames, which are designed for this purpose.

It loops through these steps:

  1. Wait 20 seconds.

  2. Send a Ping frame.

  3. Receive a corresponding Pong frame within 20 seconds.

If the Pong frame isn’t received, websockets considers the connection broken and closes it.

This mechanism serves two purposes:

  1. It creates a trickle of traffic so that the TCP connection isn’t idle and network infrastructure along the path keeps it open (“keepalive”).

  2. It detects if the connection drops or becomes so slow that it’s unusable in practice (“heartbeat”). In that case, it terminates the connection and your application gets a ConnectionClosed exception.

Timings are configurable with the ping_interval and ping_timeout arguments of connect() and serve(). Shorter values will detect connection drops faster but they will increase network traffic and they will be more sensitive to latency.

Setting ping_interval to None disables the whole keepalive and heartbeat mechanism.

Setting ping_timeout to None disables only timeouts. This enables keepalive, to keep idle connections open, and disables heartbeat, to support large latency spikes.

Why doesn’t websockets rely on TCP keepalive?

TCP keepalive is disabled by default on most operating systems. When enabled, the default interval is two hours or more, which is far too much.

Keepalive in browsers#

Browsers don’t enable a keepalive mechanism like websockets by default. As a consequence, they can fail to notice that a WebSocket connection is broken for an extended period of time, until the TCP connection times out.

In this scenario, the WebSocket object in the browser doesn’t fire a close event. If you have a reconnection mechanism, it doesn’t kick in because it believes that the connection is still working.

If your browser-based app mysteriously and randomly fails to receive events, this is a likely cause. You need a keepalive mechanism in the browser to avoid this scenario.

Unfortunately, the WebSocket API in browsers doesn’t expose the native Ping and Pong functionality in the WebSocket protocol. You have to roll your own in the application layer.

Latency issues#

Latency between a client and a server may increase for two reasons:

  • Network connectivity is poor. When network packets are lost, TCP attempts to retransmit them, which manifests as latency. Excessive packet loss makes the connection unusable in practice. At some point, timing out is a reasonable choice.

  • Traffic is high. For example, if a client sends messages on the connection faster than a server can process them, this manifests as latency as well, because data is waiting in flight, mostly in OS buffers.

    If the server is more than 20 seconds behind, it doesn’t see the Pong before the default timeout elapses. As a consequence, it closes the connection. This is a reasonable choice to prevent overload.

    If traffic spikes cause unwanted timeouts and you’re confident that the server will catch up eventually, you can increase ping_timeout or you can disable keepalive entirely with ping_interval=None.

    The same reasoning applies to situations where the server sends more traffic than the client can accept.

You can monitor latency as follows:

import asyncio
import logging
import time

async def log_latency(websocket, logger):
    t0 = time.perf_counter()
    pong_waiter = await websocket.ping()
    await pong_waiter
    t1 = time.perf_counter()
    logger.info("Connection latency: %.3f seconds", t1 - t0)

asyncio.create_task(log_latency(websocket, logging.getLogger()))