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 Web pages, which are usually HTML documents and linked objects (images, etc.).
  2. A computer program that provides the functionality described in the first sense of the term.


Contents

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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 human people (people trying to fetch a web page, etc.).
  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.
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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, specially if the latter involves data pulled from a database.

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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: 

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

/var/www/htdocs/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.

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Concurrency

Web servers (programs) often use concurrent programming techniques, sometime even in combination with finite state machines and non-blocking I/O, to increase the responsiveness and the scalability of the server when providing pages to multiple clients simultaneously.

These techniques are useful not only to reduce response latencies in the common cases (i.e. disk I/O waits, non cached accesses, etc.) but also to fully deploy some hardware capabilities, such as:

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Server Models

A webserver program, as any other server, can be implemented by using one of these server models:

  • single process, finite state machine and non blocking or even asynchronous I/O;
  • multi process, finite state machine and no blocking or even asynchronous I/O;
  • single process, forking a new process for each request;
  • multi process, with adaptive pre-forking of processes;
  • single process, multithreaded;
  • multi process, multithreaded.

The following sub-chapters summarize pros and contras of the main various models.

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Finite state machine servers

To minimize the context switches and to maximize the scalability, many small webservers are implemented as a single process (or at most as a process per CPU) and a finite state machine. Every task is split into two or more small steps that are executed as needed (typically on demand); by keeping the internal state of each connection and by using non-blocking I/O or asynchronous I/O, it is possible to implement ultra fast web servers, at least for serving static content.

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Threaded-based servers

Many web servers are multithreaded. This means that inside each server's process, there are two or more threads, each one able to execute its own task independently from the others.

When a user visits a web site, a web server will use a thread to serve the page to that user. If another user visits the site while the previous user is still being served, the web server can serve the second visitor by using a different thread. Thus, the second user does not have to wait for the first visitor to be served. This is very important because not all users have the same speed Internet connection. A slow user should not delay all other visitors from downloading a web page.

Threads are often used to serve dynamic content.

For better performance, threads used by web servers and other Internet services are typically pooled and reused to eliminate even the small overhead associated with creating a thread.

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Process-based servers

For reliability and security reasons, some web servers using multiple processes (rather than multiple threads within a single process) still remain in production use, such as Apache 1.3.

A pool of processes are used, and reused, until a certain threshold of requests have been served by a process before it is replaced by a new one.

Because threads share a main process context, a crashing thread may more easily crash the whole application, and a buffer overflow can have more disastrous consequences.

Moreover, a memory leak in system libraries which are out of the control of the application programmer cannot be dealt with using threads, but are appropriately dealt with using a pool of processes with a limited life time (because OS automatically frees all the allocated memory, requested by a process, when the process dies).

Another problem relates to the wide variety of third party libraries which might be used by an application (a PHP extension library for instance) which might not be thread safe.

Using multiple processes also allows to deal with situations which can benefit from privilege separation techniques to achieve better security and to work around some OS limits which very often are per-process.

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Mixed model servers

To leverage the advantages of finite state machines, threads and processes, many webservers implement a mixture of all these programming techniques, trying to use the best model for each task (i.e. for serving static or dynamic content, etc.).

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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.

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Anti Overload Techniques

To partially overcome above load limits, most popular Web sites use common techniques like:

  • deploying Web cache techniques;
  • using different URLs to serve different content, i.e.: 
 static  content (i.e. http://images.example.com) and
 dynamic content (i.e. http://www.example.com),

by separate Web servers;

  • 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.
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Overload Causes

At any time web servers can be overloaded because of:

  • too much legal web traffic (i.e. thousands or even millions of clients hitting the web site in a short interval of time);
  • illegal DDoS (Distributed Denial of Service) attacks;
  • 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 (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.
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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, 503 HTTP errors are returned to clients (sometimes also unrelated 404 error may be returned);
  • TCP connections are refused or reset before any content is sent to clients.
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Historical note

Image:First Web Server.jpg 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.

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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 comprehensive list of HTTP server programs.

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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 and Windows NT. 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 are 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 provide statistics is SecuritySpace: [1] and they also provide a detail break down for each version of Web server: [2]

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See also

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External links

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

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