apache - Varnish triggering 503 without timeout -


disclaimer: in moment migrating servers windows linux, see unusual architecture here.

i have following communication between our backend servers:

client -> apache httpd (decrypts ssl - running windows server) -> varnish cache (forward proxy , http cache - running ubuntu 16) -> apache tomcat (application server - running windows server).

we have 200 users active @ peak time , ajax call verify user connection every second.

from time time, our clients receives 503 (backend fetch failed - guru meditation) ajax request. first thought kind of application server slowdown increase timeouts follows:

backend xxx {     .host = "<%= $host_api %>";     .port = "<%= $host_port %>";     .first_byte_timeout = 1200s;     .between_bytes_timeout = 1200s;     .connect_timeout  = 10s; }

but problem continued happen. investigated modifying apache access log log time request spent on backend:

the configuration was:

logformat "%h %l %u %t \"%r\" %>s %b **%t/%d**" commomwithtime

so time shown *time_in_seconds/time_in_miliseconds*

when 503 error, receive following line: 

200.201.175.53 - - [11/sep/2017:17:16:06 -0300] "get /sistema/verificardesconexao?_=1505154046861 http/1.1" 503 286 **0/0**

so apache telling me varnish returned 503 error instantaneously! in case, impossible timeout error, right?

here 1 varnishlog message 503 error (i had modify sensitive parts):

*   << bereq    >> 179699819 -   begin          bereq 179699818 pass -   timestamp      start: 1505161516.061772 0.000000 0.000000 -   bereqmethod    -   berequrl       /sistema/verificardesconexao?_=1505153165280 -   bereqprotocol  http/1.1 -   bereqheader    host: 10.1.1.135 -   bereqheader    accept: application/json, text/javascript, */*; q=0.01 -   bereqheader    x-requested-with: xmlhttprequest -   bereqheader    user-agent: mozilla/5.0 (windows nt 6.1; win64; x64) applewebkit/537.36 (khtml, gecko) chrome/59.0.3071.115 safari/537.36 -   bereqheader    content-type: application/json;charset=iso-8859-1 -   bereqheader    referer: https://www.sistema.com.br/sistema/ -   bereqheader    accept-encoding: gzip, deflate, br -   bereqheader    accept-language: pt-br,pt;q=0.8,en-us;q=0.6,en;q=0.4 -   bereqheader    cookie: jsessionid=0334d2460bd8e8ae9299e546aeb2838f; logo=axis; idiomaselecionado=pt_br -   bereqheader    x-forwarded-host: www.sistema.com.br -   bereqheader    x-forwarded-server: www.sistema.com.br -   bereqheader    x-forwarded-for: [ip], 10.1.1.177 -   bereqheader    x-varnish: 179699819 -   vcl_call       backend_fetch -   vcl_return     fetch -   fetcherror     no backend connection -   timestamp      beresp: 1505161516.062073 0.000301 0.000301 -   timestamp      error: 1505161516.062079 0.000307 0.000006 -   berespprotocol http/1.1 -   berespstatus   503 -   berespreason   service unavailable -   berespreason   backend fetch failed -   berespheader   date: mon, 11 sep 2017 20:25:16 gmt -   berespheader   server: varnish -   vcl_call       backend_error -   berespheader   content-type: text/html; charset=utf-8 -   berespheader   retry-after: 5 -   vcl_return     deliver -   storage        malloc transient -   objprotocol    http/1.1 -   objstatus      503 -   objreason      backend fetch failed -   objheader      date: mon, 11 sep 2017 20:25:16 gmt -   objheader      server: varnish -   objheader      content-type: text/html; charset=utf-8 -   objheader      retry-after: 5 -   length         286 -   bereqacct      0 0 0 0 0 0 -   end

which other cases varnish 503 error happen , how should identify , solve it?

it seems varnish cannot reach or connect backend server log says

fetcherror no backend connection

can check can ping backend varnish , inititate http connection ?


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