Clients and servers

What are these?

One way to divide up the world of computing is into "servers" and "clients."   Formally, the terms refer to software: servers are software applications (programs) that carry out tasks for client applications.  Now the terms are also commonly used to refer to the hardware on which that software runs.

For example, Web server software is responsible for processing Web content, for presentation to client software called a Web browser (e.g., Internet Explorer, Firefox, Opera or Safari).  Email server software (such as Microsoft Exchange) processes email messages for presentation to client email software (such as Microsoft Outlook, Apple Mail, Eudora or Entourage). 

The server software runs on server computers, usually located in a protected datacenter.  Client software runs on personal computers (PCs), or portable computing devices like a PDA or smart cellphone.  The public Internet, a private wired or wireless network, or the cell phone network provides the connections among them.

This arrangement is actually a venerable one (by computing standards).  In the early days, centrally located computers called "mainframes" interacted over communications links with "dumb terminals."  The terminals were "dumb" because they could do essentially nothing on their own -- they had a keyboard and a video display, but no significant processor "brain."  Today's PCs are obviously much more capable, but still must rely on servers for tasks.

Why do you need to know this?

If all is going well, the interaction between server and client is seamless.  You don't need to know whether the server or client is performing a particular task.  But if there is a failure of some sort, it may be helpful to have a basic understanding of what work is being done by each component in your particular situation.  At the very least, it'll help you with translating the explanations our technicians give you.

For example, under some email protocols your messages are immediately downloaded and stored on your PC by your email client software; in others, they remain on the email server.   A communications problem does not affect your access to old messages in the first situation, but it does in the latter.  A hard drive failure in the first situation may be catastrophic for your email collection, but not in the latter.

As that example shows, one of the most important things for you to understand in a client-server world is where your data files are being stored, so you know whether you have the responsibility for keeping backup copies.  More generally, it affects how much you need to worry about data security for your computer or personal computing device. 

Confusing the categories

Some of our services blur the distinction between clients and servers:

The Citrix Applications Portal allows you to use a Web browser to access software applications like those of Microsoft Office, that normally must be installed as clients on your PC.  The software is actually running on the Citrix server, interacting with you over your network connection, mimicking the function of installed client software. 

We set up your network files -- personal home directory and shared file directories -- to be accessed in the same manner as the files on your computer's local hard drive.  The files are actually stored on file server computers in our datacenter, made accessible to you over your network connection. 

The advantage of server-based applications is that you do not need to install your own copy of the software; and, as noted, with server-stored files you are not dependent on the security or reliability of your PC (particularly its hard drive). 

The disadvantage is the dependence on a secure high-speed connection between the servers and your system.  In a workplace environment, that's usually not a problem.  For remote use, such as from home, you'll need a broadband connection for most of these services.

Note that even services that are entirely "server-based" may require some kind of minimal software installation on your client device, such as a plug-in for your Internet browser.  The first time you use the service, you will be prompted for permission to do this install.  As always, you should only allow installations of software from trusted sources.

What's coming next

As more and more services migrate to the Web, browsers like Internet Explorer have become a sort of "universal client" that allows access to everything.   As another example of this -- and another service that blurs the categories between server and client -- we offer a "Web access" version of Outlook that runs on a server computer rather than your PC.  It allows you to access your email using just an Internet browser rather than a copy of Outlook installed on your PC.

If you can use servers to run your software and store all your files, for what do you need the processing power and storage capacity of a PC?  Indeed, it has been proposed by some that PCs should become essentially like the dumb terminals of old -- no local storage, minimal processing power, reliant on the server for what is needed.  Don't call them dumb, though.  The polite label for such down-scaled PCs is "thin client."

Learn more

Servers (Wikipedia)
What the varieties do, with links to technical detail on the various branches of the family

Client-server model (Wikipedia)
More on the architecture that weaves together clients and servers