nginx 502

[Answer]

log_format access  '$remote_addr - $remote_user [$time_local] "$request" $http_host $status $body_bytes_sent "$http_referer" "$http_x_forwarded_for" "$upstream_addr" "$upstream_status" $upstream_cache_status "$upstream_http_content_type" "$upstream_response_time" > $request_time "$http_user_agent" ';

180.106.101.115 - - [25/Oct/2016:22:53:52 +0800] "GET /Code?callback=a&userName=aa05ee5b9fdb HTTP/1.1" ab.com 502 67 "https://ab.com/login/ulogin.php?callback=loginCallback" "-" "192.168.0.116:443" "200" - "text/json; charset=UTF-8" "0.044" > 0.044 "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.80 Safari/537.36 Core/1.47.1322.400 QQBrowser/9.4.9211.400"


Why is upstream_status returned to the 200 state, while status returns is 502?

Does it mean that the backend server is returned to normal, and nginx returns to the client is 502 bad gateway? Which aspect should proceed to look into the problem?
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