RE: the meaning of stateless

Hi Brian,

Markus came closest when he distinguished "application state" from "resource state". But even that doesn't quite go far enough until you define what an "application" is and understand what "state" is.

Lots of web developers think an "application" is "the code I wrote in Python or Ruby that runs on the server." But that is worlds apart from the way Roy uses it. He uses it in its original sense of "accomplishing a goal with something". Building a house is an application of (among other things) boards, nails, and a hammer.

Lots of web developers think that "state" means "data in my database". But that is worlds apart from the way Roy uses it. He uses it in its original sense of "the current node in a state machine." Let's say you supply boards and nails. If my application is "building a house", the "state" of my application is not how many boards and nails you have. It is not even how many boards and nails I have. It is how much of my house has been built. The state is advanced by building walls and assembling them together. Those have sub-states, some of which involve buying more boards and nails.

The interaction between me and you is "stateless" if I can buy boards and nails from you without you knowing anything about how much of my house has been built, or even if my application of what you provide is "building a house". If I have to talk to the same sales rep every time then it's not stateless. If I've bought 100 board feet and you pre-order another 900 board feet from your vendor because I'm building a house then it's not stateless. If you keep 1000 board feet on hand because building a house is a very common operation, then it might be stateless, because your business doesn't know anything about my application.

Now let's say you not only provide boards and nails, but also hammers, and plans, and labor, and installation. In fact, the house is built by you on your property. But I get to pick out the floorplan and curtain colors. The application has not changed. The state machine has not changed. The methods used to advance the application from one state to the next have not changed. What has changed?

    1) Who does which activity to advance the state machine. Most of it used to be me. Now most of it is you.
    2) Where the current node of the state machine is kept; that is, how much of my house has been built. That information used to be kept on my property. Now it is kept on your property.

The interaction between me and you can no longer be "stateless" because you know how much of my house has been built. If I ask you to change the floorplan or the curtain colors, first you have to authenticate me, and then you have to go to the property to inspect the state to see if you can comply with the request. That's two scalability problems you didn't have before. It doesn't mean you're not raking in the dough. But I'm no longer advancing the application state except at two points; in many cases, I'm not even doing that--I'm configuring the application state machine instead.


Robert Brewer
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