今天早上apache无法访问，查看日志发现
[Thu Feb 21 09:47:48 2008] [error] (111)Connection refused: proxy: HTTP: attempt to connect to 127.0.0.1:3000 (localhost) failed
[Thu Feb 21 09:47:48 2008] [error] ap_proxy_connect_backend disabling worker for (localhost)
[Thu Feb 21 09:47:48 2008] [error] proxy: HTTP: disabled connection for (localhost)
3000端口运行的是ruby的radiant cms系统，所以便怀疑radiant启动失败，便手工启动该服务
[root@localhost ~]# service mongrel_cluster start
Starting all mongrel_clusters…
** !!! PID file tmp/pids/mongrel.3000.pid already exists. Mongrel could be running already. Check your log/mongrel.3000.log for errors.
** !!! Exiting with error. You must stop mongrel and clear the .pid before I’ll attempt a start.
** !!! PID file tmp/pids/mongrel.4000.pid already exists. Mongrel could be running already. Check your log/mongrel.4000.log for errors.
** !!! Exiting with error. You must stop mongrel and clear the .pid before I’ll attempt a start.
[root@localhost ~]# service mongrel_cluster start
Starting all mongrel_clusters…
** !!! PID file tmp/pids/mongrel.4000.pid already exists. Mongrel could be running already. Check your log/mongrel.4000.log for errors.
** !!! Exiting with error. You must stop mongrel and clear the .pid before I’ll attempt a start.
看意思应该是一些临时文件存在，所以无法启动，到该目录下删除，重新启动，果然可以。
发现apache如果在httpd中转向到其他web server，如果那个web server启动失败，则同时会导致apache无法启动，
所以如果在linux下使用service自动启动各种服务，需要注意服务的启动顺序。