Deliver on HIT, otherwise redirect using "503; Location: ..."

[Mark Staudinger]

Hi Anton,

Have you looked into the "do_stream" feature of Varnish?  This will begin  
serving the content to the visitor without waiting for the entire object  
to be downloaded and stored in cache.  Set in vcl_backend_response.

https://github.com/mattiasgeniar/varnish-4.0-configuration-templates/blob/master/default.vcl

Cheers,
Mark

On Sat, 17 Dec 2016 19:05:48 -0500, Anton Berezhkov  
<bubonic.pestilence at gmail.com> wrote:

> Hello.
>
> Switched to Varnish from Nginx for additional functionality and better  
> control of handling requests.
> But still can't implement what i want. And I want simple behaviour  
> "Redirect on MISS/PASS".
> I want to use VC for deploying quick "cdn" servers for our  
> mp4-video-servers (used for HTML5 players), without need to store all  
> files on this quick (ssd, upto 2x480GB space, full database about 6TB).
>
> Currently we have 6 servers with SATA HDDs and hitting iowait like a  
> trucks :)
>
> Examples:
> - Request -> Varnish -> HIT: serve it using Varnish.
> - Request -> Varnish -> MISS: start caching data from backend, and  
> instantly reply to client: `Location: http://backend/$req.url"
> - Request -> Varnish -> UPDATE: see `-> MISS` behaviour.
>
> From my perspective, i should do this "detach & reply redirect"  
> somewhere in `vcl_miss` OR `vcl_backend_fetch`, because if i understood  
> correctly https://www.varnish-cache.org/docs/4.1/reference/states.html,  
> i need vcl_backend_response to keep run in background (as additional  
> thread) while doing return(synth(...)) to redirect user.
>
> Similiar thing is "hitting stale content while object is updating".
> But in my case "replying redirect while object is updating".
>
> Also, i pray to implement this without writing additional scripts, why?  
> I could do external php/ruby checker/cache-pusher with nginx & etc. But  
> scared by performance downgrade :(
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