[Varnish] #553: Check backends with 'OPTIONS *' request

[Varnish]

#553: Check backends with 'OPTIONS *' request
----------------------+-----------------------------------------------------
 Reporter:  ask       |       Owner:  phk          
     Type:  defect    |      Status:  new          
 Priority:  normal    |   Milestone:               
Component:  varnishd  |     Version:  trunk        
 Severity:  normal    |    Keywords:  loadbalancing
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 Perlbal has a great feature where it checks the backend with an 'OPTIONS
 *' request at the start of each backend keep-alive session.  It'd be nice
 to have this in Varnish, too.

 To make this reasonable Perlbal also keeps reusing this same backend
 connection until disconnected, until a configurable number of requests
 have been done or until there are more than N idle backend connections
 (and it'll disconnect some.

 It makes the "bad backend server" detection almost perfect and it makes
 "perfect load balancing" trivial -- just only allow CPUs * 1.5
 simultaneous connections (or whatever) on each backend server and let
 Varnish try opening as many connections as it'd like.

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Ticket URL: <http://varnish.projects.linpro.no/ticket/553>
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