Web Server(unit-5)

Web Server
The term web server can mean one of two things:
A computer program that is responsible for accepting HTTP requests from clients (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.).
A computer that runs a computer program as described above.
Common features
The rack of web servers hosting the My Opera Community site on the Internet. From the top, user file storage (content of files.myopera.com), "bigma" (the master MySQL database server), and two IBM blade centers containing multi-purpose machines (Apache front ends, Apache back ends, slave MySQL database servers, load balancers, file servers, cache servers and sync masters.
Although web server programs differ in detail, they all share some basic common features.
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.
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:
Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.
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.).
HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.
Content compression to reduce the size of the responses (to lower bandwidth usage, etc.).
Virtual hosting to serve many web sites using one IP address.
Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.
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.