Thread: how to use of web server

Web Server

The term web server or webserver can mean one of two things:

1. A computer program that is liable for accepting HTTP requests from customers (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.



Contents

1 Common features
2 Origin of returned content
3 Path translation
4 Load limits
5 Kernel-mode and user-mode web servers
5.1 Overload causes
5.2 Overload symptoms
5.3 Anti-overload techniques
6 Historical notes

Common features

A standard 19" Rack of servers as seen from the front.
Although web server programs differ in detail, they all share some basic common

features.

1.HTTP: each web server program operates by accepting HTTP requests from the client, and given that an HTTP response to the client. The HTTP reply usually consists of an HTML or XHTML 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 reply which may include some custom HTML or text messages to better explain the problem to end users.

2.Logging: Frequently web servers have also the ability 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 also implement the following features:

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,ColdFusion, 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 zip 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 come 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 great deal 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.

Load limits

A web server (program) has defined load limits, because it can grip only a limited number of concurrent client connections as per IP address (and TCP port) and it can serve only a certain maximum number of requests per second depending on:
• It’s 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 also implement into the OS kernel, or in user space. The kernel web server will classically work former because, as part of the system, it is straightly use all the hardware resources it needs, such as:
• non-paged memory;
• CPU time-slices;
• Network adapter’s buffers.
As a result, the only way for a user-mode web server to match kernel-mode performances is to raise the excellence 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:

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:
• 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.example.com

o Example Web Page

• 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 ease and efficient of early technologies used to surf and swap data through the World Wide Web helped to port them to many dissimilar operating systems and spread their use among lots of different social group 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.