Too much web traffic can dramatically slow down or even prevent all access to a web site. This is caused by more file requests going to the server than it can handle and may be an intentional attack on the site or simply caused by over-popularity. Large scale web sites with numerous servers can often cope with the traffic required and it is more likely that smaller services are affected by traffic overload.
Denial of service attacks
Denial-of-service attacks (DoS attacks) have forced web sites to close after a malicious attack, flooding the site with more requests than it could cope with. Viruses have also been used to co-ordinate large scale distributed denial-of-service attacks.
Sudden popularity
A sudden burst of publicity may accidentally cause a web traffic overload. A news item in the media, a quickly propagating email, or a link from a popular site may cause such a boost in visitors (sometimes called a flash crowd) that overwhelms the site.
Web sites have been forced to close after an unexpected mass increase of traffic, particularly those run by an individual leasing the bandwidth from an ISP or hosting site. Some sites backed by large companies running their own servers have also been caught out by the problems of overpopularity. When first announced, the Vision of Britain Through Time site, containing information taken from the 1901 UK census, was advertised on numerous television programmes and causing such interest that the site had to be taken offline until different arrangements were made to cope with the traffic. The site was hosted by a project at the University of Edinburgh and they had not foreseen the amount of bandwidth and the server load that would be required. Ironically, by the time the site was able to cope with the traffic both the interest and the free advertisements of the site had greatly slowed, giving them excess capacity.
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