The New Upstart State Machine

Awhile ago I started working on describing a new "service state machine" for Upstart. This would be a departure from the event driven model, and offer a richer mechanism to describe what should be running at any given time. I think I more or less have the design down.

Now if only I could explain it to someone else :)

This series of posts will attempt to explain to mere mortals the abstract workings of this new wonder. Previous descriptions have proven a bit verbose and mathy, so I'm going to take it very slow this time. It may get a bit too easy, but I'm going to take baby steps no matter what.

I should probably note beforehand that the objective here is to develop a body of logic that can describe services, and then from that produce a program that will govern them. This post and ones after will not describe how the state machine should be implemented but how it should behave. For the curious, implementing it has been somewhat crudely and quite inefficiently prototyped here.

So, first we need to define a state. A state consists of 1) a name, 2) a series of properties.

mystate(crunchy: yes, decaf: no, flavor: immitation_beef)

Also, a state is either up or down. Pay attention to those colors, as I will use them consistently here.

There's a lot of comparing of states that will go on shortly, so it is important that we understand what makes states equal.

mystate(crunchy: yes) != mystate(crunchy: no)
mystate(crunchy: yes) != thystate(crunchy: yes)
mystate(crunchy: yes) == mystate(crunchy: yes)
mystate(crunchy: yes, chewy: yes) == mystate(chewy: yes, crunchy: yes)
mystate(crunchy: yes) == mystate(crunchy: yes)

That last one is important. A state being up or down does not effect its identity for the purposes of comparison.

Now we get into a bit of a mathy idea. Its not hugely complex, but its not entirely intuitive from a computer standpoint. Suppose I were to list a few states inside the state machine:

mystate(crunchy: yes)
mystate(crunchy: no, cheesy: yes)
mystate(crunchy: no)
thystate()
thystate(chewy: perhaps)
thystate(cheesy: thursdays)

You know that there are states called mystate and thystate. Depending on the context in which this list appeared, it might be safe to assume that a state called frystate doesn't make sense. For any particular state machine, there are only a few possible valid names states can have.

That doesn't seem too rough, but as is so often the case, what isn't said is what's tricky. The list of states above would have the same meaning if you appended the following:

mystate(crunchy: maybe)
mystate(crunchy: level_1)
mystate(crunchy: level_2)
mystate(crunchy: level_3)
mystate(crunchy: yes, threebears: eleventeen, number: threeve)
...
mystate(complete_works_of_shakespeare: ...)

The long and short of it is if you see mystate(anything) you can assume that there exists a mystate with any possible set of parameters you can imagine. The corollary to this is that there are an infinite number of states in any state machine.