How to make "too much load" messages

I see these kinds of messages on twitter and other places.... ho开发者_StackOverflow社区w do they work? Is it MySQL connections?

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You can measure response time of each request (or a given fraction of the requests). If the response time goes up above a threshold you start rejecting requests.

If you measure SQL connection etc you need to know where your bottle necks are at a given moment. If your application is limited by bandwidth between your servers or something else counting SQL connections will not help you.

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It might be shown if the database refused a connection or if the system load (read from /proc/loadavg) is too high. It could also be shown if too many users are logged in - but that's only likely for web applications where logged in users are expensive.. certain games for example where lots of state information must be kept while someone is logged in.