Send delay

[SauvageThomas]

Hi !

While working with my ws server I encountered something weird, ServerConnection.send() seems to have a delay under some circonstances. The client and the server are on the same machine.
I'm sorry I don't have code to reproduce the issue, I'm not sure to be able to reproduce it consistently.

Here is the setup

1. The client sends a message
2. The server iterates over some list (here length == 3)
3. The server computes something (can take several seconds)
4. The server sends back a text message for each element
5. The server send a final closing message

The problem, is that the first message is received immediatly by the client, but the last 2 messages are received at the same time (while they were sent at a few seconds interval).
It looks like the packet is waiting for some buffering or it does not have time to execute.

At first, I didn't know if it was due to my client or the server, but I managed to "fix" the issue, so I'm pretty sure that this is due to the server code.

I found two ways to solve it, either :

await ws.send(data)

or waiting for a pong response (with a 0.0 latency) :

pong_waiter = await ws.ping()
latency = await pong_waiter

For sleep, looking at the asyncio doc, it says that might help for long-running tasks, so that might be it

Setting the delay to 0 provides an optimized path to allow other tasks to run. This can be used by long-running functions to avoid blocking the event loop for the full duration of the function call.

Is this a normal behaviour, or am I using the library wrong ?

[aaugustin]

The server computes something (can take several seconds)

It is quite likely that you are running this in the main thread without yielding control. As a consequence, you're blocking the event loop.

Is this a normal behaviour, or am I using the library wrong ?

Possibly you are using asyncio wrong. Does https://docs.python.org/3/library/asyncio-dev.html#running-blocking-code help?

[SauvageThomas]

Wow, thanks. I was using asyncio.create_task, but it seems it was not enough. ThreadPoolExecutor and run_in_executor work fine, thank you.