At any time web servers can be overloaded due to:
- Excess legitimate web traffic. Thousands or even millions of clients connecting to the web site in a short interval, e.g., Slashdot effect;
- Distributed Denial of Service attacks. A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer or network resource unavailable to its intended users;
- Computer worms that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them)
- XSS viruses can cause high traffic because of millions of infected browsers or web servers;
- Internet bots Traffic not filtered/limited on large web sites with very few resources (bandwidth, etc.);
- Internet (network) slowdowns, so that client requests are served more slowly and the number of connections increases so much that server limits are reached;
- Web servers (computers) partial unavailability. This can happen because of required or urgent maintenance or upgrade, hardware or software failures, back-end (e.g., database) failures, etc.; in these cases the remaining web servers get too much traffic and become overloaded.
