Important delay with streams (Mozilla EventSource)

[Kernald]

I'm exposing through Traefik an home-assistant instance. It's using Mozilla's EventSource to send events back to the front-end.

If I'm accessing directly the home-assistant instance, events are nearly instant. If I access it through Traefik, they are sent, but with an important delay (between 30 seconds and a minute or two). It looks like a buffer flush occurs every x seconds: if I send a lot of events, they are sent quickly. If I only have one or two, they are queued for a while.

It may be a feature, but not documented anywhere (or at least I couldn't find it). If it is, is there any way to configure the behavior?

[emilevauge]

@Kernald hum, this weird... I would be very interested by reproducing this to investigate.
Could you give me more details ?

[Kernald]

@emilevauge The way I checked this was pretty simple, with curl: my home-assistant instance has been running for long (I was proxying it through nginx until recently). I noticed some delay in the UI, so I subscribed two curl instances to my home-assistant: one through Traefik, the other one directly to the home-assistant webserver (the same port that Traefik backend uses), using this curl arguments exactly: curl -X GET -H "x-ha-access: YOUR_PASSWORD" -H "Content-Type: application/json" http://localhost:8123/api/stream.

I then sent some events from the web UI: the curl instance bound to the home-assistant webserver received them in a second. The other one, bound to Traefik, didn't until a long time (the home-assistant server send some pings every minute or so, some traffic is generated even when idling).

I tried another approach: flood the stream with events. I turned the lights on, off, on… both curl instances got the data really quickly. That's why I'm thinking of a buffer waiting to be full before being flushed.

Note that I didn't found anything relevant on Traefik's debug logs, nor on home-assistant's.

I'm not a web developer at all, so I can't set up a sample web-server right now, but I'll definitely try. You can probably reproduce the issue with a blank home-assistant instance (as it's sending some ping even without any components), which is available on Docker.

Don't hesitate if you need anything more until I can reproduce a minimal example!

[emilevauge]

Thanks @Kernald for your explanations, I will try to reproduce this issue.

[ashmckenzie]

I have a very basic API that uses HTML5 Server Sent Events (using the Content-Type: "text/event-stream" header) and observe the same behaviour. If I hit the application directly I receive an event every 5 secs but through Traefik (1.0), it blocks correctly but instead of receiving events every 5 seconds, I see the backlog of events come through after ~3 mins.

[ashmckenzie]

I created https://github.com/ashmckenzie/traefik_html5sse_example to replicate the issue 😄

[emilevauge]

@ashmckenzie thanks a lot !
Fix is coming.

[bsideup]

Hi @emilevauge,

I use the latest Traefik and it doesn't seem to work.

My server sends transfer-encoding: chunked + Content-Type: text/event-stream, if I access the endpoint, I see the events.
But if I run Traefik it front of it, I don't see any events coming.

Is there any option to disable buffering/caching/whatever completely?
Thanks!

[kelunik]

I have a similar issue, but with a generic content-type, not text/event-stream. An option or header that disables buffering would be really helpful.

[juliens]

@kelunik Can you try with 1.5.0-rc2 ?

[kelunik]

@juliens Sure. Seems all fine there. 👍