proxy_next_upstream表示对于后端服务器，出现问题后将会转移到另外一台服务器
proxy_next_upstream
语法: proxy_next_upstream [error|timeout|invalid_header|http_500|http_503|http_404|off]
默认值: proxy_next_upstream error timeout
上下文: http, server, location
Directive determines, in what cases the request will be transmitted to the next server:
- error ― an error has occurred while connecting to the server, sending a request to it, or reading its response;
- timeout ― occurred timeout during the connection with the server, transfer the requst or while reading response from the server;
- invalid_header ― server returned a empty or incorrect answer;
- http_500 ― server returned answer with code 500
- http_503 ― server returned answer with code 503
- http_404 ― server returned answer with code 404
- off ― it forbids the request transfer to the next server
Transferring the request to the next server is only possible when nothing has been transferred to the client -- that is, if an error or timeout arises in the middle of the transfer of the request, then it is not possible to retry the current request on a different server.