开发过程中我们经常会遇到nginx 502，503，504错误，这些错误代表什么？什么情况下会出现这些错？下面一一说明（均以nignx+php-fpm举例）：

wikipedia上这么解释：

​​502 Bad Gateway​​

​​The server was acting as a gateway or proxy and received an invalid response from the upstream server.​​

 

​​503 Service Unavailable​​

​​The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.​​

 

​​504 Gateway Time-out​​

​​The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.​​

502 Bad Gateway

fpm进程挂掉或者后端程序过长时间未返回。

编写一个简单的php脚本gateway.php进行测试，内容很简单 <?php sleep(10);echo ‘ok;’?> ，开始下面的测试：

1.启动nginx，不启动fpm，直接 curl http://localhost/gateway.php ，响应502 bad gateway错误且nginx的error log出现错误

​​2017/02/10 19:08:21 [error] 216#216: *84 connect() failed (111: Connection refused) while connecting to upstream, client: 172.17.0.1, server: website80.com, request: "GET /gateway.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "localhost"​​

2.启动fpm，修改php-fpm.conf中request_terminate_timeout的值为5s。继续 curl http://localhost/gateway.php ，响应502 bad gateway。nginx和php-fpm分别报错

​​// nginx error log​​

​​2017/02/10 19:10:57 [error] 246#246: *88 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 172.17.0.1, server: website80.com, request: "GET /gateway.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "localhost"​​

 

​​// php-fpm error log​​

​​[10-Feb-2017 19:10:57] WARNING: [pool www] child 242, script '/home/website/default/gateway.php' (request: "GET /gateway.php") execution timed out (6.205572 sec), terminating​​

​​[10-Feb-2017 19:10:57] WARNING: [pool www] child 242 exited on signal 15 (SIGTERM) after 36.692467 seconds from start​​

503 Service Unavailable

当遇到这个状态码的时候表示服务临时不可用，比如nginx配置了频率限制而client端又超过了配置的限制后就会收到503的响应。

504 Gateway Time-out

nginx的fastcgi模块有一个fastcgi_read_timeout配置，它表示从FastCGI server获取数据的超时时间。如果超过这个配置客户端就是收到504的响应。还以 gateway.php 举例（修改fastcgi_read_timeout的值为5s）：

​​// nginx error log​​

​​2017/02/12 14:57:26 [error] 138#138: *1113 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 172.17.0.1, server: website80.com, request: "GET /gateway.php HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "localhost"​​