upstream resin {
 #ip_hash;  server 10.10.69.206:8088;
 server 10.10.69.206:8089 backup;
}
upstream edtold {
 #ip_hash;

server 10.10.69.206:8089;
}

location ~ ^/pic/(.*)$ {    #proxy_set_header Host bjctc.azure.sohu.com;

   #proxy_set_header Authorization IfI9RK0wiqEAsRKYBYLz/Q==:oRq3wTvlScelecBPOTYUnA==;    #proxy_pass http://10.10.89.33/peakviewcnc/$1;    more_set_headers "Content-type:image/jpeg";    proxy_next_upstream http_500 http_502 http_503 http_504 error timeout invalid_header;    proxy_pass http://resin;
    proxy_intercept_errors on;
    #recursive_error_pages on;     expires 7d; } error_page? 400 = @backup;
error_page? 405 = @backup; location  @backup{
    #access_log   off;
 if ($request_method = POST) {
    return 404;
 }  proxy_pass  http://edtold;
    #rewrite?? "^/pic/(.*)$" /pic/$1 break;     expires 7d; } 利用了upstream和? proxy_next_upstream 。当主的服务无相应或者当机就启用backup服务，当主机链接进入后出现错误如：http_500 http_502 http_503 http_504 error timeout invalid_header也用backup的， 当出现400和405错误也启用backup。