Web server

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The term Web server can mean one of two things:

  1. A computer that is responsible for accepting HTTP requests from clients, which are known 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 program that provides the functionality described in the first sense of the term.


[edit] Common features

Although Web server programs differ in detail, they all share some basic common features.

  1. HTTP responses to HTTP requests: every Web server program operates by accepting HTTP requests from the network, and providing an HTTP response to the requester. The HTTP response typically consists of an HTML document, but can also be a raw text file, an image, or some other type of document; if something bad is found in client request or while trying to serve the request, a Web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.
  2. Logging: usually Web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the Webmaster to collect statistics by running log analyzers on log files.

In practice many Web servers implement the following features too.

  1. Configurability of available features by configuration files or even by an external user interface.
  2. Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.
  3. Handling of not only static content (file content recorded in server's filesystem(s)) but of dynamic content too by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).
  4. Module support, in order to allow the extension of server capabilities by adding or modifying software modules which are linked to the server software or that are dynamically loaded (on demand) by the core server.
  5. HTTPS support (by SSL or TLS) in order to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.
  6. Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).
  7. Virtual Host to serve many web sites using one IP address.
  8. Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
  9. Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

[edit] Origin of returned content

The origin of the content sent by server is called:

  • 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 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.

[edit] Path translation

Web servers usually translate the path component of a Uniform Resource Locator (URL) into a local file system resource. 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:


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/htdocs. The result is the local file system resource:


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.

[edit] Performances

Web servers (programs) are supposed to serve requests quickly from more than one TCP/IP connection at a time.

Main key performance parameters (measured under a varying load of clients and requests per client), are:

  • number of requests per second (depending on the type of request, etc.);
  • latency time in milliseconds for each new connection or request;
  • throughput in bytes per second (depending on file size, cached or non cached content, available network bandwidth, etc.).

Above three parameters vary noticeably depending on the number of active connections, so a fourth parameter is the concurrency level supported by a Web server under a specific configuration.

Last but not least, the specific server model used to implement a Web server program can bias the performance and scalability level that can be reached.

[edit] 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 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 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.

[edit] 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);
  • DDoS (Distributed Denial of Service) attacks;
  • Computer worms that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them);
  • 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 / 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 of course they become overloaded.

[edit] Overload Symptoms

The symptoms of an overloaded Web server are:

  • requests are served with noticeably (long) delays (from 1 second to a few hundreds of 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 before any content is sent to clients.

[edit] Anti Overload Techniques

To partially overcome above load limits and to prevent the overload scenario, 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 HW resources (i.e. RAM, disks) to each computer;
  • tuning OS parameters for HW capabilities and usage;
  • using more efficient computer programs for Web servers, etc.;
  • using other workarounds, specially if dynamic content is involved.

[edit] Historical note

Image:First Web Server.jpg
This NeXTcube used by Berners-Lee at CERN became the first Web server.

In 1989 Tim Berners-Lee proposed to his employer CERN (European center 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, Berners-Lee wrote two programs: a browser called WorldWideWeb and the world's first Web server, which ran on NeXTSTEP. Today, this machine is on exhibition at CERN's public museum, Microcosm.

First web server in U.S. had been installed on December 12, 1991 at SLAC [1]

[edit] Software

The four top most common Web or HTTP server programs are:

There are thousands of different Web server programs available; many of them are specialized for some uses and can be tailored to satisfy specific needs.

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

[edit] Statistics

The most popular Web servers, used for public Web sites, are tracked by Netcraft Web Server Survey, with details given by Netcraft Web Server Reports.

The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX, Windows NT and Apple Mac. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.

Apache has been the most popular Web server on the Internet since April of 1996. The November 2005 Netcraft Web Server Survey found that more than 70% of the Web sites on the Internet were using Apache, thus making it more widely used than all other Web servers combined. The Apache HTTP Server is a project of the Apache Software Foundation

Another site providing statistics is SecuritySpace ([2]), which also provides a detailed breakdown for each version of Web server: [3]

[edit] See also

[edit] External links

bs:Web server ca:Servidor web cs:Webový server da:Webserver de:Webserver es:Servidor web fr:Serveur HTTP it:Web server he:שרת דפי אינטרנט lv:Tīmekļa serveris hu:Webszerver nl:Webserver ja:Webサーバ pl:Serwer WWW ru:Веб-сервер simple:Web server fi:WWW-palvelin th:เว็บเซิร์ฟเวอร์ tr:Web sunucusu zh:網頁伺服器

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