一、proxy转发模块的超时设置
proxy_connect_timeout
Defines a timeout for establishing a connection with a proxied server.?
语法 proxy_connect_timeout time?
默认值 60s
上下文 http server location
说明 该指令设置与upstream server的连接超时时间，有必要记住，这个超时不能超过75秒。
这个不是等待后端返回页面的时间，那是由proxy_read_timeout声明的。如果你的upstream服务器起来了，但是hanging住了(例如，没有足够的线程处理请求，所以把你的请求放到请求池里稍后处理)，那么这个声明是没有用的，因为与upstream服务器的连接已经建立了。
This directive assigns a timeout for the connection to the proxyserver. This is not the time until the server returns the pages, this is the proxy_read_timeout statement. If your proxyserver is up, but hanging (e.g. it does not have enough threads to process your request so it puts you in the pool of connections to deal with later), then this statement will not help as the connection to the server has been made. It is necessary to keep in mind that this time out cannot be more than 75 seconds.
proxy_read_timeout
语法 proxy_read_timeout time?
默认值 60s
上下文 http server location
说明 该指令设置与代理服务器的读超时时间。它决定了nginx会等待多长时间来获得请求的响应。这个时间不是获得整个response的时间，而是两次reading操作的时间。(我的理解它指的是同一响应下的两个资源之间的时间，例如客户端请求了server下的a资源和b资源，这个两次reading操作的时间就是指请求得到a之后与得到b之前的时间)
This directive sets the read timeout for the response of the proxied server. It determines how long NGINX will wait to get the response to a request. The timeout is established not for entire response, but only between two operations of reading.
In contrast to proxy_connect_timeout, this timeout will catch a server that puts you in it's connection pool but does not respond to you with anything beyond that. Be careful though not to set this too low, as your proxy server might take a longer time to respond to requests on purpose (e.g. when serving you a report page that takes some time to compute). You are able though to have a different setting per location, which enables you to have a higher proxy_read_timeout for the report page's location.
If the proxied server nothing will communicate after this time, then nginx is shut connection.
proxy_send_timeout
语法 proxy_send_timeout time?
默认值 60s
上下文 http server location
说明 这个指定设置了发送请求给upstream服务器的超时时间。超时设置不是为了整个发送期间，而是在两次write操作期间。如果超时后，upstream没有收到新的数据，nginx会关闭连接
send_timeout
默认：send_timeout 60;
send_timeout?指定客户端的响应超时时间。这个设置不会用于整个转发器，而是在两次客户端读取操作之间。如果在这段时间内，客户端没有读取任何数据，nginx就会关闭连接。
Directive assigns response timeout to client. Timeout is established not on entire transfer of answer, but only between two operations of reading, if after this time client will take nothing, then nginx is shutting down the connection.
二、负载均衡配置时的2个参数
max_fails=number
sets the number of unsuccessful attempts to communicate with the server that should happen in the duration set by the fail_timeout parameter to consider the server unavailable for a duration also set by the fail_timeout parameter. By default, the number of unsuccessful attempts is set to 1. The zero value disables the accounting of attempts. What is considered an unsuccessful attempt is defined by the proxy_next_upstream, fastcgi_next_upstream, uwsgi_next_upstream, scgi_next_upstream, memcached_next_upstream, and grpc_next_upstream directives.
fail_timeout=time
sets?the time during which the specified number of unsuccessful attempts to communicate with the server should happen to consider the server unavailable;
and the period of time the server will be considered unavailable.
By default, the parameter is set to 10 seconds.
???这2个参数一起配合，来控制nginx怎样认为upstream中的某个server是失效的当在fail_timeout的时间内，某个server连接失败了max_fails次，则nginx会认为该server不工作了。同时，在接下来的 fail_timeout时间内，nginx不再将请求分发给失效的server。
参考：
https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_connect_timeout
https://blog.csdn.net/weixin_37839711/article/details/81902978