Page 296 - DCAP101_BASIC_COMPUTER_SKILLS

Unit 14: Web Server Applications

XSS viruses can cause high traffic because of millions of infected browsers and/or Web Notes
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.

14.1.6 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).
14.1.7 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, i.e.:
http://images.example.com
http://www.example.com
using different domain names and/or computers to separate big files from small and
medium sized files; the idea is to be able to fully cache small and medium sized files
and to efficiently serve big or huge (over 10 - 1000 MB) files by using different settings;
using many Web servers (programs) per computer, each one bound to its own network
card and IP address;
using many Web servers (computers) that are grouped together so that they act or are
seen as one big Web server (see also Load balancer);
adding more hardware resources (i.e. RAM, disks) to each computer;
tuning OS parameters for hardware capabilities and usage;

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