Caching OSM tiles - how to not overload backends?

[tranxene50]

Hello!

Please forgive my bad English, I live in France.

Summary: how to cache - with Varnish - Open Street Map PNG images 
without overloading OSM tiles servers?

The question seems related to Varnish backends and ".max_connections" 
parameter.

A far as I know, if ".max_connections" is reached for a backend, Varnish 
sends 503 http errors.

I understand the logic but would it be possible to queue these incoming 
requests and wait until the selected backend is really available?

backend a_tile                  {
   .host                         = "a.tile.openstreetmap.org";
   .port                         = "80";
   .max_connections              = 2;
}

If Varnish have, let's say 100 incoming requests in 1 second, how can I 
handle this "spike" without overloading the backend?

All my google searches were "dead ends" so I think the question is 
poorly formulated.

Note 1 : using [random|round_robin] directors could be a temporary solution
Note 2 : libvmod-dynamic is great but does not limit backend 
simultaneous connexions

Many thanks for your help!

-- 
tranxene50
tranxene50 at openvz-diff-backups.fr

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