A web server is a computer program that receives HTTP requests from web clients like web browsers. It serves HTTP responses containing web pages and other files. Common features of web servers include supporting HTTP, logging client requests, authentication, handling static and dynamic content, HTTPS support, content compression, and virtual hosting. Web servers have load limits and can become overloaded due to high traffic, DDoS attacks, or other issues. Symptoms of overload include delayed responses and HTTP errors. Techniques to prevent overload include managing network traffic, caching, using multiple servers, and tuning hardware resources.

1. Web Server
A computer program that isresponsible for accepting HTTP requestsfrom web clients, which are known asweb browsers, and serving
them HTTP responsesalong with optional data contents, which usually are web pagessuch as HTML documentsand linked objects
(images, etc.).
Common features
Althoughweb server programsdiffer in detail,they all share some basic commonfeatures.
1. HTTP: every web server program operatesby accepting HTTP requestsfrom the client, and providing an HTTP response to the
client. TheHTTP response usually consistsof an HTML document,but can also be a raw file, an image,or some other type of
document (defined by MIME-types). If some error is found in clientrequest or while tryingto serve it, a web server has to send an error
response which may include some custom HTML or text messagesto better explain the problem to endusers.
2. Logging: usually web servers have also the capability of loggingsome detailedinformation, about client requestsand server
responses, to log files; thisallowsthe webmaster to collect statisticsby running log analyzerson log files.
In practice many web servers implement the following featuresalso:
1. Authentication,optional authorizationrequest (request of user name and password) before allowingaccessto some or all kind of
resources.
2. Handling of static content(filecontent recordedin server'sfilesystem(s)) and dynamic content by supportingone or more related
interfaces(SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP.NET, Server API such as NSAPI, ISAPI, etc.).
3. HTTPS support (by SSL or TLS) to allow secure (encrypted) connectionsto the server on the standard port 443 instead of usual port
80.
4. Content compression (i.e. by gzip encoding) to reduce the size of the responses(to lower bandwidthusage, etc.).
5. Virtual hosting to serve many web sitesusing one IP address.
6. Large file support to be ableto serve fileswhose size is greater than 2 GB on 32 bit OS.
7. Bandwidth throttling to limit thespeed of responsesin order to not saturate the networkand to be able to serve more clients.
Origin of returned content
The originof the content sent by server is called:
• static if it comesfrom an existing filelying on a filesystem;
• dynamic if it isdynamically generated by some other program or script or Application Programming Interfacecalledby the web server.
Serving static content isusually much faster (from 2 to 100 times) than serving dynamic content,especially if the latter in volvesdata
pulled from a database.
Path translation
Web servers are able to map the pathcomponent of a Uniform Resource Locator (URL) into:
• a local file system resource (for static requests);
• an internal or external program name (for dynamic requests).
For a static request the URL path specified by the client isrelative to the Web server'sroot directory.
Consider the followingURL asit would be requested by a client:
http://www.example.com/path/file.html
The client'sweb browser will translate itintoa connection to www.example.com withthe following HTTP 1.1request:
GET /path/file.html HTTP/1.1
Host: www.example.com
The web server on www.example.com will appendthe given pathto the pathof itsroot directory. On Unix machines, thisiscom monly
/var/www/htdocs. The result isthe local file system resource:
/var/www/htdocs/path/file.html
The web server will then read thefile, ifit exists, and send a response to the client'sweb browser. The response will describe the
content of the fileand contain thefile itself.
Load limits
A web server (program) has defined loadlimits, because it can handle only a limited number of concurrent client connections (usually
between 2 and 60,000, by default between 500 and 1,000) per IP address(and IP port) and it can serve only a certain maximum
number of requestsper second depending on:
• its own settings;
• the HTTP request type;
• content origin(static or dynamic);
• the fact that the served content isor is not cached;
• the hardware and software limitsof the OS where it isworking.
When a web server is near to or over its limits, it becomesoverloaded andthusunresponsive.
Overload causes
At any time web servers can be overloaded because of:
• Too much legitimateweb traffic (i.e. thousandsor even millionsof clientshittingthe web site in a short interval of time. e.g.Slashdot
effect);
• DDoS (Distributed Denial of Service) attacks;
• Computer wormsthat sometimescause abnormal traffic because of millionsof infectedcomputers(not coordinated among them);
• XSS viruses can cause high traffic because of millionsof infected browsersand/or web servers;
• Internet web robotstraffic not filtered/limitedon largeweb siteswith very few resources (bandwidth, etc.);
• Internet (network) slowdowns, so that client requestsare served more slowly and the number of connectionsincreasesso much that
server limitsare reached;
• Web servers (computers) partial unavailability,thiscan happen because of requiredor urgent maintenance or upgrade, HW or SW
failures, back-end (i.e. DB) failures, etc.; in these casesthe remainingweb servers get too much traffic andbecomeoverloaded.

2. Overload symptoms
The symptomsof 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 errorsare returned to clients(sometimesalso unrelated404 error or even 408 error may be returned);
• TCP connectionsare refused or reset (interrupted) before any contentissent to clients;
• in very rare cases, only partial contentsare sent (but thisbehavior may well be considered a bug, even ifit usually dependson
unavailable system resources).
Anti-overload techniques
To partially overcome above loadlimitsand to prevent overload, most popular web sitesuse common techn iqueslike:
• managingnetworktraffic, by using:
o Firewallsto blockunwanted traffic coming from bad IP sourcesor having bad patterns;
o HTTP traffic managersto drop, redirect or rewrite requestshaving badHTTP patterns;
o Bandwidth management and traffic shaping, inorder to smooth down peaksin networkusage;
• deployingweb cache techniques;
• using different domain namesto serve different (static and dynamic) content by separate Web servers, i.e.:
o http://images.example.com
o
o http://www.example.com
o
• using different domain namesand/or computersto separate big filesfrom small andmedium sized files; the ideaisto be ab le to fully
cache small and medium sized filesand to efficiently serve big or huge (over 10 - 1000 MB) filesby using different settings;
• using many Web servers (programs) per computer, each one boundto itsown networkcard 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 parametersfor hardware capabilitiesand usage;
• using more efficient computer programsfor web servers, etc.;
• using other workarounds, especially if dynamic content isinvolved.
Historical notes
In 1989 Tim Berners-Lee proposed to hisemployer CERN (European Organization for Nuclear Research) a new project, which hadthe
goal of easing the exchangeof information betweenscientistsby using a hypertext system. As a result of the implementationof this
project, in 1990Berners-Lee wrote two programs:
• a browser called WorldWideWeb;
• the world's first web server, later known as CERN HTTPd, which ran on NeXTSTEP.
Between 1991 and1994the simplicity and effectiveness of early technologiesused to surf and exchangedata throughthe World Wide
Web helped to port them to many different operatingsystems and spread their use among lotsof different social groupsof peo ple, first
in scientific organizations, then in universitiesand finally in industry.
In 1994 Tim Berners-Lee decidedto constitutethe World WideWeb Consortium to regulate thefurther development of the many
technologiesinvolved (HTTP, HTML,etc.) througha standardizationprocess
A Web server is a programthat uses HTTP (Hypertext Transfer Protocol) to serve the files that form Web pages to users, in
response to their requests, w hich are forwarded by their computers' HTTP clients. Dedicated computers and appliances may be
referred to as Web servers as well.
What software is on a web server?
Category:Web server software. FromWikipedia, the free encyclopedia. A web server (sometimes called an HTTP server or
application server) is a programthat serves content using the HTTP protocol. This content is frequently in the formof HTML
documents, images, and other webresources, but can include any type of file.
What are the different types of Web servers?
Major ones are Netscape's iPlanet, Bea's Web Logic and IBM's WebSphere.
 Apache HTTP Server.
 Internet Information Services.
 lighttpd.
 Sun Java System Web Server.
 Jigsaw Server.