Overload causes

At any time web servers can be overloaded because of:

• Too much legitimate web traffic. Thousands or even millions of clients hitting the web site in a short interval of time. (e.g. Slashdot effect);
• DDoS. Distributed Denial of Service attacks;
• 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 and/or web servers;
• Internet web robots. 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 (i.e. DB) failures, etc.; in these cases the remaining web servers get too much traffic and become overloaded.

Overload symptoms

The symptoms of an overloaded web server are:

• requests are served with (possibly long) delays (from 1 second to a few hundred seconds);
• 500, 502, 503, 504 HTTP errors are returned to clients (sometimes also unrelated 404 error or even 408 error may be returned);
• TCP connections are refused or reset (interrupted) before any content is sent to clients;
• in very rare cases, only partial contents are sent (but this behavior may well be considered a bug, even if it usually depends on unavailable system resources).

Anti-overload techniques

To partially overcome above load limits and to prevent overload, most popular web sites use common techniques like:

• managing network traffic, by using:
  • Firewalls to block unwanted traffic coming from bad IP sources or having bad patterns;
  • HTTP traffic managers to drop, redirect or rewrite requests having bad HTTP patterns;
  • Bandwidth management and traffic shaping, in order to smooth down peaks in network usage;
• deploying web cache techniques;
• using different domain names to serve different (static and dynamic) content by separate Web servers