Web server



The term web server or webserver can mean one of two things:
 * 1) A computer program that is responsible for accepting HTTP requests from clients (user agents such as web browsers), and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and linked objects (images, etc.).
 * 2) A computer that runs a computer program as described above.

Common features

 * 1) Virtual hosting to serve many web sites using one IP address.
 * 2) Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
 * 3) Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

Origin of returned content
The origin of the content sent by server is known as:
 * static if it comes from an existing file lying on a filesystem;
 * dynamic if it is dynamically generated by some other program or script or application programming interface (API) called by the web server.

Serving static content is usually much faster (from 2 to 100 times) than serving dynamic content, especially if the latter involves data pulled from a database.

Path translation
Web servers are able to map the path component 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 is relative to the Web server's root directory.

Consider the following URL as it would be requested by a client:

http://www.example.com/path/file.html The client's web browser will translate it into a connection to  www.example.com  with the following HTTP 1.1 request: GET /path/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 Unix machines, this is commonly /var/www. The result is the local file system resource:

/var/www/path/file.html

The web server will then read the file, if it exists, and send a response to the client's web browser. The response will describe the content of the file and contain the file itself.

Load limits
A web server (program) has defined load limits, because it can handle only a limited number of concurrent client connections (usually between 2 and 80,000, by default between 500 and 1,000) per IP address (and TCP port) and it can serve only a certain maximum number of requests per second depending on:
 * its own settings;
 * the HTTP request type;
 * content origin (static or dynamic);
 * the fact that the served content is or is not cached;
 * the hardware and software limits of the OS where it is working.

When a web server is near to or over its limits, it becomes overloaded and thus unresponsive.

Kernel-mode and user-mode web servers
A web server can be either implemented into the OS kernel, or in user space (like other regular applications).

An in-kernel web server (like TUX on Linux or Microsoft IIS on Windows) 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 buffers.

Web servers that run in user-mode have to ask the system the permission to use more memory or more CPU resources. Not only 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.

Also applications cannot access the system internal buffers, which is causing useless buffer copies that create another handicap for user-mode web servers.

As a consequence, the only way for a user-mode web server to match kernel-mode performances is to raise the quality of its code to much higher standards than the code used into another web server that runs in the kernel.

This is more difficult under Windows than under Linux where the user-mode overhead is 6 times smaller than under Windows: http://www.acc.umu.se/~bosse/High%20performance%20kernel%20mode%20web%20server%20for%20Windows.pdf

Overload causes
At any time web servers can be overloaded because of:
 * Too much legitimate web traffic (i.e. 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, HW or SW 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: http://images.example.com http://www.example.com
 * 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.:
 * 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;
 * using more efficient computer programs for web servers, etc.;
 * using other workarounds, especially if dynamic content is involved.

Historical notes
In 1989 Tim Berners-Lee proposed to his employer CERN (European Organization for Nuclear Research) a new project, which had the goal of easing the exchange of information between scientists by using a hypertext system. As a result of the implementation of this project, in 1990 Berners-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 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 lots of different social groups of people, first in scientific organizations, then in universities and finally in industry.

In 1994 Tim Berners-Lee decided to constitute the World Wide Web Consortium to regulate the further development of the many technologies involved (HTTP, HTML, etc.) through a standardization process.

Market structure
Given below is a list of top Web server software vendors published in a Netcraft survey in January 2009.



See Category:Web server software for a longer list of HTTP server programs.