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Specifying the upstream server by a hostname together with the
"resolve" parameter will make the hostname to be periodically
resolved, and upstream servers added/removed as necessary.
This requires a "resolver" at the "http" configuration block.
The "resolver_timeout" parameter also affects when the failed
DNS requests will be attempted again. Responses with NXDOMAIN
will be attempted again in 10 seconds.
Upstream has a configuration generation number that is incremented each
time servers are added/removed to the primary/backup list. This number
is remembered by the peer.init method, and if peer.get detects a change
in configuration, it returns NGX_BUSY.
Each server has a reference counter. It is incremented by peer.get and
decremented by peer.free. When a server is removed, it is removed from
the list of servers and is marked as "zombie". The memory allocated by
a zombie peer is freed only when its reference count becomes zero.
Co-authored-by: Roman Arutyunyan <arut@nginx.com>
Co-authored-by: Sergey Kandaurov <pluknet@nginx.com>
Co-authored-by: Vladimir Homutov <vl@nginx.com>
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