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When I was running my 11.500 TGPs network I had a single server hosted on different datacenter only for checking if some sites were down.
It's relatively easy to make such a system... but of course you need to know how. Putting all the sites into a big array/database and check them in bulk every 5 minutes is not a good solution and you are very lucky if you get only 30% false positives that way.
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