Immediate ServerSentEvent response with blocking generator

[lilac stump]

Hi,
I’m implementing an endpoint to open a notification stream using ServerSentEvent replies. I use an asynchronous generator that is waiting a trigger (in my case a publish on a channel).
I m wondering if I could be able to send an immediate HTTP 200 OK reply even if I’m waiting ?

async def notification_publisher(self, channels: ChannelsPlugin) -> AsyncGenerator[SSEData, None]:
        subscriber = await channels.subscribe(["general"])
        async for message in subscriber.iter_events():
            yield message

@get("/{dynamic_id:uuid}/events", status_code=HTTP_200_OK)
    async def subscribe_to_notifications(self, dynamic_id: UUID, channels: ChannelsPlugin) -> ServerSentEvent:
        return ServerSentEvent(self.notification_publisher(channels))

In other words, is it possible to generate a response before the first yield is reached ?
Possible to decorrelate the reply to the GET request and the notifications sending ?

Thank you
Florian

[lofty fog]

Notes for Immediate ServerSentEvent response with blocking generator

At your assistance

@lilac stump

No Response?

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Closing

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MCVE

Please include an MCVE so that we can reproduce your issue locally.

[reef ocean]

No, it's not

You can either send a regular response or stream

You can't do both

(not a litestar thing, that's an HTTP thing 🙂)

Can you explain what you're trying to achieve?

I'm sure there's some way to handle this

[lilac stump]

What I explain below comes from a “system specification” that is not written by REST API nor HTTP specialist.

I have a client that send a GET request to a specific endpoint to open a notification stream in order to “allow” the server to push notifications.
Specs requires that the servers replies with an HTTP 200 with no content.
Then, on specific event, server will push notifications on the stream.

It seems that the standard authorizes it

[neat blaze]

I m wondering if I could be able to send an immediate HTTP 200 OK reply even if I’m waiting ?
isnt this what happens by default?

[lilac stump]

Seems the case in your example, not what I get when I test on my side.

Could you test with something close to my code ?

[lilac stump]

@neat blaze could you share the code of your endpoint

[reef ocean]

Good catch! I was assuming @lilac stump was asking to send an additional response on top of that

[reef ocean]

Have you verified this isn't your client doing something wrong?

If it's receiving the event stream, it should also have received the initial response

[lilac stump]

I tried again with postman and I don’t get any response until I publish a message on the “general” channel.
I was not able to reproduce @neat blaze example. Get an exception on the line: await sleep()

[hushed lava]

post a mvce so we can talk

[neat blaze]

i didnt use the channels thing, i wonder if that could be somehow the issue?

[lilac stump]

Even with a simple generator like yours, I don’t get an initial response (before the asyncio.sleep)

@hushed lava

import json
from collections.abc import AsyncGenerator
from litestar import Litestar, Controller, get, post
from litestar.channels import ChannelsPlugin
from litestar.channels.backends.memory import MemoryChannelsBackend
from litestar.response import ServerSentEvent
from litestar.status_codes import HTTP_200_OK, HTTP_204_NO_CONTENT
from litestar.types import SSEData

class NotificationController(Controller):
    path = "/notifications"

    @post("/{dynamic_id:int}", status_code=HTTP_204_NO_CONTENT)
    async def publish_notif(self, dynamic_id: int, channels: ChannelsPlugin) -> None:
 
        channels.publish(json.dumps({"test_key": "value"}), "general")

    async def notification_publisher(self, channels: ChannelsPlugin) -> AsyncGenerator[SSEData, None]:
        subscriber = await channels.subscribe(["general"])
        async for message in subscriber.iter_events():
            yield message

    @get("/{dynamic_id:int}/events", status_code=HTTP_200_OK)
    async def subscribe_to_notifications(self, dynamic_id: int, channels: ChannelsPlugin) -> ServerSentEvent:
        """
        """
        return ServerSentEvent(self.notification_publisher(channels))


channels_plugin = ChannelsPlugin(
    backend=MemoryChannelsBackend(),
    channels=["general"],
    arbitrary_channels_allowed=True
)

app = Litestar(
    path="/api",
    route_handlers=[NotificationController],
    plugins=[channels_plugin]
)

[hushed lava]

i changed nothing in your code, seems to work

[lilac stump]

Yeah it works. The issue is that to get a HTTP 200 Ok response, I need to wait for a publish, that’s not what I want. I want to get a response immediately when the SSE generator is started.

[hushed lava]

i can have a 200 without the need to publish, its working fine

what version of litestar you use

what trace you got

logs

[lilac stump]

Litestar 2.14

Serving by granian

It seems to work indeed with a browser but don’t get the same output with postman. Could you try ?

[hushed lava]

Ok so it seems like a postman issue

I don't use that