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System Design -- Finite State Machines

4: Adsense

Hello fellow developers! I'm working a project where I can share info with
the public again so I figured I would restart my developer journal.

I've been tasked with re-architecting a large project. The project grew
'organically' and has evolved into a large tangled mess. At this point it is
impossible to fix anything or add any new functionality; without causing ripples
across the entire system.

More often than not these ripples cause bugs.
Due to the design of the system, unit testing is not possible. The testing
process has decayed into a QA engineer throwing whatever playout combos
that they can think of at the system. Typically just a list of the last bugs
that were marked fixed.

No point in laying blame; I'm going to put on the engineering armor and slay
the bad design dragon.

I'm not going to get into too much detail about the current project
architecture. Lets just leave it at there being state machines at the low level
that control the visual elements. Above is a gateway into the Abyss.

The state machine architecture needs to be propagated through the rest
of the system so that its always clear what each component is doing.

So, What is a finite state machine?

When I was taught about state machines, the example used was that of
a cola vending machine:

Cola State Machine

In this example, the vending machine charges .55 for a soda... [Not a very
friendly vending machine, It does not provide change, and doesn't give you an
option of what soda you want.]

You can add any number of Quarters, Dimes, and Nickels to it. The goal is to
have the final state reach .55, so that it dispenses your drink and resets.

The possible states for the machine are each .05 cent up to .55. The granularity
of the .05 comes from the fact that it is legal to buy a cola with just Nickels.

As you add money, you can jump from any lower state to a higher one, just trace
the lines associated with the Quarters, Dimes, and Nickels as input.

Example: you could add a Dime to be at state .10; then a Quarter to jump to
0.35; then a Nickel to jump to .40; etc...

The next state only depends on the current state and an input.

Depending on the project, you may have any number of state machines running at
the same time.

The same could apply to the vending machine: if there is a LCD display showing
advertisements then it could be doing its state transitions as you were adding
money. Same to be said if the refrigeration were controlled via state.

In my project; I have a state machine that is controlling commercials, another
that toggles between sponsor logos, network logos, and the clock, another that
controls interruption overlays, and another that controls headers.

All of these systems are designed so that they that only need to know the
minimal state of each other system to do their job.

Using state machines makes postmortem debugging easier as well.
If you get a crash dump, and it has your state variables--then you
can backtrace to figure out what it would have taken to get the system into
such a state.

Okay, back to work.
Next entry will be on the evils of coupling, and how to start fixing it.

Nov 06 2010 01:56 PM
Good post, state machines are so important.

If state can be avoided it's even better :)

For example instead of the vending machine keeping track of the amount of bottles contained and updating this as it is refilled or emptied, it could just check if there was a bottle there when someone presses the button to buy it. One less thing to go wrong.

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