Web server

1. HTTP: every web server program operates by accepting HTTP requests from the

client, and providing an HTTP response to the client. The HTTP response usually

consists of an HTML document, but can also be a raw file, an image, or some other

type of document (defined by MIME-types). If some error is found in client request or

while trying to serve it, 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 these files.

In practice many web servers implement the following features also:

1. Authentication, optional authorization request (request of user name and

password) before allowing access to some or all kind of resources.
2. Handling of static content (file content recorded in server's filesystem(s)) and

dynamic content by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI,

JSP, PHP, ASP, ASP.NET, Server API such as NSAPI, ISAPI, etc.).
3. HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the

server on the standard port 443 instead of usual port 80.
4. Content compression (i.e. by gzip encoding) to reduce the size of the responses

(to lower bandwidth usage, etc.).
5. Virtual hosting to serve many web sites using one IP address.
6. Large file support to be able to serve files whose size is greater than 2 GB on 32

bit OS.
7. 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 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

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.mrfweb.we.bs/index.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.mrfweb.we.bs

The web server on www.mrfweb.we.bs 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 60,000, 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.

Overload causes
A daily graph of a web server's load, indicating a spike in the load early in the day.

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:

* managing network traffic, by using:
o Firewalls to block unwanted traffic coming from bad IP sources or having bad

patterns;
o HTTP traffic managers to drop, redirect or rewrite requests having bad HTTP

patterns;
o 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.:
o

http://images.mrfweb.we.bs/
o

http://www.mrfweb.we.bs/

* 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
The world's first web server.

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.

The following years are recent history which has seen an exponential growth of the

number of web sites and servers.

Market structure

Given below is a list of top Web server software vendors published in a Netcraft survey

in September 2008.
Vendor Product Web Sites Hosted Percent
Apache Apache 91,068,713 50.24%
Microsoft IIS 62,364,634 34.4%
Google GWS 10,072,687 5.56%
lighttpd lighttpd 3,095,928 1.71%
nginx nginx 2,562,554 1.41%
Oversee Oversee 1,938,953 1.07%
Others - 10,174,366 5.61%
Total - 181,277,835 100.00%


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