The term web server, also written as Web server, can refer to either the hardware or the software
that helps to deliver web content that can be accessed through the Internet.
The most common use of web servers is to host websites, but there are other uses such as
gaming, data storage, running enterprise applications, handling email, FTP, or other web uses.
Overview The primary function of a web server is to
store, process and deliver web pages to clients. The communication between client and server
takes place using the Hypertext Transfer Protocol. Pages delivered are most frequently HTML documents,
which may include images, style sheets and scripts in addition to text content.
A user agent, commonly a web browser or web crawler, initiates communication by making
a request for a specific resource using HTTP and the server responds with the content of
that resource or an error message if unable to do so. The resource is typically a real
file on the server's secondary storage, but this is not necessarily the case and depends
on how the web server is implemented. While the primary function is to serve content,
a full implementation of HTTP also includes ways of receiving content from clients. This
feature is used for submitting web forms, including uploading of files.
Many generic web servers also support server-side scripting using Active Server Pages, PHP,
or other scripting languages. This means that the behaviour of the web server can be scripted
in separate files, while the actual server software remains unchanged. Usually, this
function is used to create HTML documents dynamically as opposed to returning static
documents. The former is primarily used for retrieving and/or modifying information from
databases. The latter is typically much faster and more easily cached but cannot deliver
dynamic content. Web servers are not always used for serving
the World Wide Web. They can also be found embedded in devices such as printers, routers,
webcams and serving only a local network. The web server may then be used as a part
of a system for monitoring and/or administering the device in question. This usually means
that no additional software has to be installed on the client computer, since only a web browser
is required. History
In 1989 Tim Berners-Lee proposed a new project to his employer CERN, with the goal of easing
the exchange of information between scientists by using a hypertext system. The project resulted
in Berners-Lee writing two programs in 1990: A browser called WorldWideWeb.
The world's first web server, later known as CERN httpd, which ran on NeXTSTEP
Between 1991 and 1994, the simplicity and effectiveness of early technologies used to
surf and exchange data through the World Wide Web helped to port them to many different
operating systems and spread their use among scientific organizations and universities,
and then to industry. In 1994 Tim Berners-Lee decided to constitute
the World Wide Web Consortium to regulate the further development of the many technologies
involved through a standardization process. Common features
Virtual hosting to serve many web sites using one IP address
Large file support to be able to serve files whose size is greater than 2 GB on 32 bit
OS Bandwidth throttling to limit the speed of
responses in order to not saturate the network and to be able to serve more clients
Server-side scripting to generate dynamic web pages, still keeping web server and website
implementations separate from each other Path translation
Web servers are able to map the path component of a Uniform Resource Locator into:
A local file system resource An internal or external program name
For a static request the URL path specified by the client is relative to the web server's
root directory. Consider the following URL as it would be
requested by a client:
http:www.example.comfile.html
The client's user agent will translate it into a connection to www.example.com with
the following HTTP 1.1 request:
GET file.html HTTP/1.1 Host: www.example.com
The web server on www.example.com will append the given path to the path of its root directory.
On an Apache server, this is commonly www. The result is the local file system resource:
wwwfile.html
The web server then reads the file, if it exists and sends a response to the client's
web browser. The response will describe the content of the file and contain the file itself
or an error message will return saying that the file does not exist or is unavailable.
Kernel-mode and user-mode web servers A web server can be either implemented into
the OS kernel, or in user space. An in-kernel web server will usually work
faster, because, as part of the system, it can directly use all the hardware resources
it needs, such as non-paged memory, CPU time-slices, network adapters, or buffers.
Web servers that run in user-mode have to ask the system for permission to use more
memory or more CPU resources. Not only do these requests to the kernel take time, but
they are not always satisfied because the system reserves resources for its own usage
and has the responsibility to share hardware resources with all the other running applications.
Executing in user mode can also mean useless buffer copies which are another handicap for
user-mode web servers. Load limits
A web server has defined load limits, because it can handle only a limited number of concurrent
client connections per IP address and it can serve only a certain maximum number of requests
per second depending on: its own settings,
the HTTP request type, whether the content is static or dynamic,
whether the content is cached, and the hardware and software limitations of the
OS of the computer on which the web server runs.
When a web server is near to or over its limit, it becomes unresponsive.
Causes of overload At any time web servers can be overloaded
because of: Too much 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 or distributed denial-of-service 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; XSS viruses can cause high traffic because
of millions of infected browsers and/or web servers;
Internet bots Traffic not filtered/limited on large web sites with very few resources;
Internet slowdowns, so that client requests are served more slowly and the number of connections
increases so much that server limits are reached; Web servers partial unavailability. This can
happen because of required or urgent maintenance or upgrade, hardware or software failures,
back-end failures, etc.; in these cases the remaining web servers get too much traffic
and become overloaded. Symptoms of overload
The symptoms of an overloaded web server are: Requests are served with delays.
The web server returns an HTTP error code, such as 500, 502, 503, 504, 408, or even 404,
which is inappropriate for an overload condition. The web server refuses or resets TCP connections
before it returns any content. In very rare cases, the web server returns
only a part of the requested content. This behavior can be considered a bug, even if
it usually arises as a symptom of overload. Anti-overload techniques
To partially overcome above average 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
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 files by using different settings
Using many web servers per computer, each one bound to its own network card and IP address
Using many web servers that are grouped together behind a load balancer so that they act or
are seen as one big web server Adding more hardware resources to each computer
Tuning OS parameters for hardware capabilities and usage
Using more efficient computer programs for web servers, etc.
Using other workarounds, especially if dynamic content is involved
Market share
Below are the latest statistics of the market share of the top web servers on the Internet
by Netcraft Survey April, May 2014. Apache, IIS and Nginx are the most used web
servers on the Internet. See also
Application server Comparison of web server software
HTTP compression MaidSafe
Open source web application SSI, CGI, SCGI, FastCGI, PHP, Java Servlet,
JavaServer Pages, ASP, ASP.NET, SAPI Variant object
Virtual hosting Web hosting service
Web service References
External links RFC 2616, the Request for Comments document
that defines the HTTP 1.1 protocol. C64WEB.COM — Commodore 64 running as a
web server using Contiki How to set up Web Server on Windows, Linux,
and Mac Using Apache