Web server
A web server is server software, or hardware dedicated to running this software, that can satisfy client requests on the World Wide Web. A web server can, in general, contain one or more websites. A web server processes incoming network requests over HTTP and several other related protocols.
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 the text content.
servers are installed together being used for the Wikimedia Foundation.
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 major 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 generate HTML documents dynamically as opposed to returning static documents. The former is primarily used for retrieving or modifying information from databases. The latter is typically much faster and more easily cached but cannot deliver dynamic content.
Web servers can frequently 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 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 March 1989 Sir 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 Web browser called WorldWideWeb
- The world's first web server, later known as CERN httpd, which ran on NeXTSTEP
In 1994 Berners-Lee decided to constitute the World Wide Web Consortium to regulate the further development of the many technologies involved through a standardization process.
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
Consider the following URL as it would be requested by a client over HTTP:
The client's user agent will translate it into a connection to with the following HTTP/2 request:
GET /path/file.html HTTP/2
The web server on will append the given path to the path of its root directory. On an Apache server, this is commonly . The result is the local file system resource:
/home/www/path/file.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 incorporated into the OS kernel, or in user space.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 limitation 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 depending on:- its own settings,
- the HTTP request type,
- whether the content is static or dynamic,
- whether the content is cached, or compressed, and
- the hardware and software limitations of the OS of the computer on which the web server runs.
Causes of overload
At any time web servers can be overloaded due to:- Excess 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 worms can cause high traffic because of millions of infected browsers 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
- 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
- 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 or IP addresses to serve different content by separate web servers, e.g.:
- *
http://images.example.com - *
http://example.com - Using different domain names 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 internet servers per computer, each one bound to its own network card and IP address
- Using many internet 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
February 2019
Below are the latest statistics of the market share of all sites of the top web servers on the Internet by W3Techs.
| Product | Vendor | Percent |
| Apache | Apache | 44.3% |
| nginx | NGINX, Inc. | 41.0% |
| IIS | Microsoft | 8.9% |
| LiteSpeed Web Server | LiteSpeed Technologies | 3.9% |
| GWS | 0.9% |
All other web servers are used by less than 1% of the websites.
July 2018
Below are the latest statistics of the market share of all sites of the top web servers on the Internet by W3Techs.
| Product | Vendor | Percent |
| Apache | Apache | 45.9% |
| nginx | NGINX, Inc. | 39.0% |
| IIS | Microsoft | 9.5% |
| LiteSpeed Web Server | LiteSpeed Technologies | 3.4% |
| GWS | 1.0% |
All other web servers are used by less than 1% of the websites.
February 2017
Below are the latest statistics of the market share of all sites of the top web servers on the Internet by Netcraft.
| Product | Vendor | January 2017 | Percent | February 2017 | Percent | Change | Chart color |
| IIS | Microsoft | 821,905,283 | 45.66% | 773,552,454 | 43.16% | −2.50 | |
| Apache | Apache | 387,211,503 | 21.51% | 374,297,080 | 20.89% | −0.63 | |
| nginx | NGINX, Inc. | 317,398,317 | 17.63% | 348,025,788 | 19.42% | 1.79 | |
| GWS | 17,933,762 | 1.00% | 18,438,702 | 1.03% | 0.03 |
February 2016
Below are the latest statistics of the market share of all sites of the top web servers on the Internet by Netcraft.
| Product | Vendor | January 2016 | Percent | February 2016 | Percent | Change | Chart color |
| Apache | Apache | 304,271,061 | 33.56% | 306,292,557 | 32.80% | 0.76 | |
| IIS | Microsoft | 262,471,886 | 28.95% | 278,593,041 | 29.83% | 0.88 | |
| nginx | NGINX, Inc. | 141,443,630 | 15.60% | 137,459,391 | 16.61% | −0.88 | |
| GWS | 20,799,087 | 2.29% | 20,640,058 | 2.21% | −0.08 |
Apache, IIS and Nginx are the most used web servers on the World Wide Web.