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From Emanuele Rocca <...@wikimedia.org>
Subject total_server_connections incremented by plugins
Date Wed, 20 Mar 2019 16:45:28 GMT
I'm a bit confused by the total_client_connections and
total_server_connections metrics.

My understanding is that total_client_connections refers to inbound connections
made to ATS, while total_server_connections is about outbound connections made
by ATS to origin servers.

I do however see proxy.process.http.total_server_connections increasing
each time there's an inbound connection handled by plugins such as
stats_over_http and healthchecks.

The metric is incremented by HttpServerSession::new_connection, and I see the
following backtrace for *actual* new origin server connections:

 0x506120 : _ZN17HttpServerSession14new_connectionEP14NetVConnection+0x0/0x400 [/usr/bin/traffic_server]
 0x4f0fa7 : _ZN6HttpSM22state_http_server_openEiPv+0x6a7/0x890 [/usr/bin/traffic_server]
 0x4e2c02 : _ZN6HttpSM12main_handlerEiPv+0x1b2/0x260 [/usr/bin/traffic_server]
 0x688ac0 : _ZN18UnixNetVConnection9connectUpEP7EThreadi+0x300/0x4a0 [/usr/bin/traffic_server]
 0x681260 : _ZN16UnixNetProcessor19connect_re_internalEP12ContinuationPK8sockaddrP12NetVCOptions+0x5b0/0x730
[/usr/bin/traffic_server]
 0x4ea3b5 : _ZN6HttpSM19do_http_server_openEb+0x1945/0x1ec0 [/usr/bin/traffic_server]

Connections that end up incrementing the metric due to plugins have a different backtrace:

 0x506120 : _ZN17HttpServerSession14new_connectionEP14NetVConnection+0x0/0x400 [/usr/bin/traffic_server]
 0x4f0fa7 : _ZN6HttpSM22state_http_server_openEiPv+0x6a7/0x890 [/usr/bin/traffic_server]
 0x4e2c02 : _ZN6HttpSM12main_handlerEiPv+0x1b2/0x260 [/usr/bin/traffic_server]
 0x64d119 : _ZN12PluginVCCore10connect_reEP12Continuation+0x89/0x120 [/usr/bin/traffic_server]
 0x4e8d1a : _ZN6HttpSM19do_http_server_openEb+0x2aa/0x1ec0 [/usr/bin/traffic_server]

This seems a bug to me (I'd expect total_server_connections not to go up when
the connection is handled by stats_over_http.so / healthchecks.so), but please
do let me know if I am missing something.

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