Editing Web server (section)

===Causes of overload===
At any time web servers can be overloaded due to one or more of the following causes (e.g.).
* '''Excess legitimate web traffic'''. Thousands or even millions of clients connecting to the website in a short amount of time, 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 worm]]s that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them).
* [[XSS worm]]s can cause high traffic because of millions of infected browsers or web servers.
* [[Internet bot]]s Traffic not filtered/limited on large websites with very few network resources (e.g. [[Bandwidth (computing)|bandwidth]]) and/or hardware resources (CPUs, RAM, disks).
* [[Internet]] (network) slowdowns (e.g. due to packet losses) so that client requests are served more slowly and the number of connections increases so much that server limits are reached.
* Web servers, '''serving dynamic content''', '''waiting for slow responses coming from [[Front and back ends|back-end]] computer(s)''' (e.g. [[database]]s), maybe because of too many queries mixed with too many inserts or updates of DB data; in these cases web servers have to wait for back-end data responses before replying to HTTP clients but during these waits too many new client connections / requests arrive and so they become overloaded.
* Web servers ([[computer]]s) '''partial unavailability'''. This can happen because of required or urgent maintenance or upgrade, hardware or software failures such as [[Front and back ends|back-end]] (e.g. [[database]]) failures; in these cases the remaining web servers may get too much traffic and become overloaded.