varnish (plus) http/2 and websockets, server sent events

[Guillaume Quintard]

Hi Tom,

I have a hard time believing this doesn't work with pass, but let's check.

Would you be able to run `varnishlig -g raw` as the transaction is going on
? (You don't want extra traffic otherwise things will be hard to follow)


-- 
Guillaume Quintard

On Mon, Jun 23, 2025, 08:31 Tom Anheyer | BerlinOnline <
Tom.Anheyer at berlinonline.de> wrote:

> Good morning,
>
> OK, one more. Server-Sent-Events are streaming GET requests. I've tried
>
> sub vcl_recv {
>
> 	…
>
> 	return(pass);
>
> }
>
>
> sub vcl_backend_fetch {
>
> 	set bereq.first_byte_timeout = 30s;
>
> 	set bereq.between_bytes_timeout = 30s;
>
> }
>
>
> sub vcl_backend_response {
>
> 	set beresp.do_esi = false;
>
> 	set beresp.do_stream = true;
>
> }
>
>
> without success. The request is forwarded to the SSE server but comes
> never back and does not show up in varnishlog. Is there any chance to get
> this work without `pipe`?
>
> thanks in advance
> tom
>
> Am Freitag, dem 20.06.2025 um 15:05 +0200 schrieb Guillaume Quintard:
>
> Always a pleasure! Let us know if you have any further questions!
>
> --
> Guillaume Quintard
>
> On Fri, Jun 20, 2025, 13:30 Tom Anheyer | BerlinOnline <
> Tom.Anheyer at berlinonline.de> wrote:
>
>
> Hi Guillaume,
>
> thank you for your very quick (as always) and clear answer.
>
> tom
>
> Am Freitag, dem 20.06.2025 um 11:37 +0200 schrieb Guillaume Quintard:
> > Hi Tom,
> > This is a base behavior, so Varnish Enterprise inherits it, but it's
> present
> > in Varnish Cache too.
> > Piping involves foregoing HTTP parsing and just passing all the
> connection
> > bytes back and forth. It's possible on HTTP 1.x since there's only at
> most one
> > request per connection, but this breaks with HTTP2 multiplexing, so
> Varnish
> > does the best thing it can and uses a pass instead.
> > Interestingly, H/2 didn't plan an upgrade path to websockets (or to
> anything
> > else, really) for a relatively sane reason: H/2 requires TLS and TLS has
> ALPN
> > that allows you to go to websockets directly, without the upgrade hop.
> > So, to my (maybe outdated) knowledge of H/2, upgrading to websockets
> isn't
> > possible.
> > Please let me know if I'm not being clear.
> > --
> > Guillaume Quintard
> >
> > On Fri, Jun 20, 2025, 11:15 Tom Anheyer | BerlinOnline
> > <Tom.Anheyer at berlinonline.de> wrote:
> > > Hello,
> > >
> > > I don't know if this is specific to varnish plus:
> > >
> > >
> > > If I enable pipe-mode for server sent events in vcl_recv this way:
> > >
> > > if (…) {
> > >     set req.http.connection = "close";
> > >     return(pipe);
> > > }
> > >
> > > the log shows:
> > >
> > > -   VCL_return     pipe
> > > -   VCL_call       HASH
> > > -   VCL_return     lookup
> > > -   VCL_Error      vcl_recv{} returns pipe/connect for HTTP/2 request.
> > > Doing pass.
> > > -   VCL_call       PASS
> > >
> > >
> > > Is it possible to use server sent events, websockets with http/2
> requests?
> > >
> > >
> > > I've opened a varnish software ticket but I thin
> <https://www.google.com/maps/search/pened+a+varnish+software+ticket+but+I+thin?entry=gmail&source=g>k
> that's public interest.
> > >
> > > tom
> > >
>
>
> --
>
> Tom Anheyer
> Senior Developer
>
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