What do you use to monitor your web application?
chuhnk: 1. I dont actively use Apache extended status or req/sec monitoring because it slows down serving requests. Instead I look in the logs every minute and feed the count into ganglia.2. Again apache logs into ganglia. New Relic for rails. Firebug and yslow when testing.3. Exception notifier in rails,  ErrorDocument in apache that triggers a php script which notifies by email. Log4j in java.4. Redis, OpenMQ for queueing, Resque for background jobs5. MySQL only because at the time postgres did not support replication.6. AlertSite7. Monit8. MonitI am always on the look out for better ways to monitor the servers at work. I never really liked nagios. Monit is extremely easy to use and is very good at what it does. MMonit is a management console that can centralize the monitoring of all servers. Ganglia is great for metrics, flickr can vouch for that one.