What server allows thousands of simultaneous users and perform several task everyday?

Answer 1

A server is an application (software), not a machine. There are many different types of server applications, including e-mail servers, print servers, file servers, DNS servers, web servers, ftp servers and so on. Each server application is dedicated to a single task, so there is no such thing as a server that performs several tasks.

In organisations with many clients requiring access to a variety of services, the servers will typically be split across several machines. The DNS server is by far the most important server and will typically be handled by a single machine dedicated solely to the task of translating domain names into IP addresses. In this sense we may refer to the machine as being "the server", however the server is the application, not the machine.

The machine itself does not need to be particularly powerful because the biggest bottleneck in any client-server application is not the raw speed of the machine running the server but the available bandwidth between the server and its clients. As the number of clients increases, the speed of delivery drops off substantially, so the server spends most of its time simply prioritising data packets so each client is dealt with fairly. For instance, a file server typically delivers files of varying sizes, but will generally prioritise smaller files over larger files. In this way, no single client ends up hogging all the bandwidth downloading large video files at the expense of those downloading a 1 KB text file.

In order to prevent backlogs at the server end, there needs to be sufficient bandwidth available to meet client demands, with a suitable overhead to deal with peak demand. A small network server might use just one network interface card, but a larger network will require several independent interfaces. To support "thousands" of simultaneous users, the server itself may be split across several dedicated machines, creating a "data centre". Many large organisations have buildings dedicated purely to housing data centres, known as "data farms", and the largest organisations have farms all across the world. These are often known as content distribution networks, or CDNs.

You don't need a CDN or indeed a powerful machine in order to connect a mere thousand users simultaneously, but what you will need is bandwidth -- and lots of it.