Detect overloaded web, application or database server

If I have a number of web servers, application servers and database servers working at full workloads from time to time, how can I detect the overloaded web, application or database servers?

One way I can image is to monitor the CPU usage of such component or the memory footprint. Is that correct?
Are there any other ways I can detect overloading of the given component?

Jiafan Zhou wrote:
One way I can image is to monitor the CPU usage of such component or the memory footprint. Is that correct?

Yes. We also monitor ping (to make sure the network and hardware are up), HTTP (to make sure the server's responding) and response time (to make sure it's not overwhelmed). We also have some pages that our monitors scan for a particular piece of text that has to be recovered from the database, so we can be sure our application stack is working.
We use a combination of Nagios and ipMonitor (ipMonitor has more tools to monitor Windows machines) which send alerts via email, pager and SMS.

As far as monitoring the CPU usage is concerned, is there any open source project either written in Java or other languages available?

Thanks.

Jiafan Zhou wrote:As far as monitoring the CPU usage is concerned, is there any open source project either written in Java or other languages available?

Thanks.

We use Munin in most of our installations.

http://munin-monitoring.org/