from: http://www.wired.com/epicenter/2009/08/twitter-apparently-down/

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Twitter was shut down for hours Thursday morning by what it described as an “ongoing” denial-of-service attack, silencing millions of Tweeters. It was the first major outage the service has suffered in months and possibly the first ever due to sabotage. The outage appeared to begin mid-morning, EST, and affected users around the world. After about three hours, the service was coming back online in fits and starts (updated).

The first official word about the outage came in a terse statement on Twitter’s status blog: “Site is down — We are determining the cause and will provide an update shortly.” That was followed by a more relaxed post on the main Twitter blog by co-founder Biz Stone, which nevertheless gave no indication of how the defense was going — or how long the service might be down.

“On this otherwise happy Thursday morning, Twitter is the target of a denial of service attack,” wrote Stone. “Attacks such as this are malicious efforts orchestrated to DISRUPT and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users. We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate.”

In a denial-of-service attack, a malicious party barrages a server with so many requests that it can’t keep up, or causes it to reset. As a result, legitimate users can only access the server very slowly — or not at all, as appears to be the case here.

Not only was the site down, but client applications that depend on the Twitter API could also not connect to the service, creating a complete Twitter blackout. According to June ComScore numbers Twitter has more than 44 MILLION registered users and its user base has been GROWING rapidly for months as it becomes better known in the mainstream.