How could you detect programmatically when your server is over capacity in ASP.NET?

Question

One of the interesting things about Twitter is the over capacity, fail whale. My question is, programmatically, how can they detect when their system is over capacity? Is there a special type of exception that gets thrown in this case?

Answer 1

I believe its the routers/load balancer's detect this for twitter. If a machine or group of machines have large number of exceptions or return some HTTP 5xx errors, then the load balancers fail over to a "fail whale" server.

Answer 2

There are any number of things that could be used to determine this - it'll depend on the system and what metrics the devs decide to use. A few examples:

• vBulletin, a PHP-based forum system can shut itself down if the Unix load average hits a certain (admin-selected) value
• Some systems that involve queuing (as Twitter does) can monitor the size of the queue and shut out users if the queue grows too large
• Some systems have the servers doing the actual processing behind a proxy or load balancer. If they go offline, the proxy or load balancer can redirect traffic to an error page like the failwhale.

Answer 3

Presumably, they've done basic load-testing so they have a solid idea of how much they can process before slowing down unacceptably or even crashing.

Answer 4

Diskeeper tells you that your system is stressed if you've used over a certain portion of your virtual memory, which I thought was interesting at the time.