Stateless merely means that any given request has nothing to do with the previous, or the next request. This enables the world wide web, as web servers do not need to keep track of who is receiving data, nor how much they have: they get a request, and ship data to the requestor.
It is up to the requestor (often a web browser) to handle the incoming data.
If not every part of a web page, for example, is sent, the browser will display what it can.
This is analogous to a creditor sending you a bill (request), and you sending a check back to them – once the bill has been sent, the creditor knows nothing about the state of the bill until he receives a payment. Likewise, once the check is dropped in the mail, the payor knows nothing about his bill until the check clears his bank.
Why is this important? Because of an oft-repeated “request for enhancement” to the product I use on a daily basis. When the implementors of Opsware SAS were picking how a user should communicate with the system, they picked to run everything over http(s). They chose to utilize http because it’s commonplace, well-understood, and easy to work with.
One of the things about statelessness is that you cannot know how many people are using a given web page at the same time. Google cannot tell anyone how many people are actually looking at www.google.com at this moment. They can tell you how many loaded it,and how many just pressed “Search”, but they can’t know what percentage of the loaders promptly went elsewhere – either to a different page, or a different room in their home.
One [partial] way around the statelessness of http is to utilize cookies or session data – but that merely adds a check layer to the interaction, it does not provide true “statefulness”.
Several times during my time in Support at Opsware (and after HP’s acquistion), I would have a customer who was looking for the ability to determine who was logged-in at any given time (in similar fashion to running `w` or `who` or `finger` on a Linux/Unix system). This could be important to know whether a user is “doing something” before doing an application restart.
However, since communication is all done via http, there can be no state known in the tool. Once you load a web page, it is being viewed/rendered on your local machine in your web browser – the server could be shut off, your network connection removed, or any of a host of other simulations of restarting the application. And your browser would be none-the-wiser, nor should it be: it has the data it requested/received, and you’re doing something with it.
This carries over to the product I work with. Jobs might be scheduled by a user to run every day at 0200 – but he doesn’t need to be actively logged-in to have them run. Likewise, someone may have logged-in, but is not “doing” anything currently (maybe they’re at lunch).
Another case of why technical intricacies matter 🙂