When will primary server come back in http upstream module?

[linbo liao]

Hi,

Refer to http://nginx.org/en/docs/http/ngx_http_upstream_module.html#server ,
if all primary server are unavailable, backup server will handle request.

I have two question?
1.  What's the meaning of unavailable?
2.  When will primary server come back, after fail_timeout?

Thanks,
Linbo
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