[Varnish] #518: Default backend health right after launch

[Varnish]

#518: Default backend health right after launch
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 Reporter:  rts     |        Owner:       
     Type:  defect  |       Status:  new  
 Priority:  normal  |    Milestone:       
Component:  build   |      Version:  trunk
 Severity:  normal  |   Resolution:       
 Keywords:          |  
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Comment (by kb):

 I'll take single-malt scotch instead of cash? :-)

 The attached patch creates a backend flag to change the initial health of
 backends upon varnishd startup:
 {{{
 backend foo {
         .initial_health = 1;
 }
 }}}
 The backend healthy flag internally is an unsigned int, so I kept the same
 type.  A value > 0 here will cause this backend to default to healthy.
 Also, the initial "Probe" output at startup will note "initially healthy"
 if this flag is set.

 The backend will immediately go "sick" if a single health check fails*,
 until the window is flushed.  This seems like the safest way to implement
 this option, since a bad host will only receive hits until the first probe
 can execute against it, which in my testing was nearly immediate.  Healthy
 hosts will continue to pass and stay healthy.

 I'm not using probed backends in production right now, but I'm running one
 instance of the patched version at ~1,000 requests per second (at 8% of a
 single 2.5G Xeon) and it's stable.  YMMV, protect yourself, etc.

 I thought about having this flag apply to the director, and backends would
 inherit this central flag...  but while setting this for every backend in
 the config is a little verbose, it just seems cleaner, conceptually.  And
 a per-backend setting seems cleaner and more flexible than a command-line
 flag.

 TODO: I should probably add "-p initial_health=1", as it fits the defaults
 like between_bytes_timeout.

 Comments?  Thoughts?
 --
 Ken.

 *(I'm marking the "oldest" .threshold probes in the .window as "pass" and
 the others "fail", so a single probe fail will cause sickness until the
 window has flushed. The BITMAP()/vt->happy stuff in
 bin/varnishd/cache_backend_poll.[ch] made my face bleed, but look there
 for specifics of the implementation.)

-- 
Ticket URL: <http://varnish.projects.linpro.no/ticket/518#comment:4>
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