PHP Tutorial: The Allegory of The Factory Pattern

If you've read the About This Site page, you'll know that instead of pursuing my passion for computers, I got sidetracked into the arts in college. So when it came time for me to learn and explain the Factory Pattern, I thought it might be fun to express it as an allegory...

Henry had a company that made these new fangled things called automobiles. Now automobiles were made up of a lot of parts, and one of the primary parts they needed was axles. Each axle needed to implement two methods: attachWheels() and turn().

Steven had a company that made axles. He supplied axles to Henry's assembly line. When Henry first designed his automobile, he thought an axle from a solid piece of hickory would do a really good job. So he printed up a bunch of ordering forms that specified new HickoryWoodAxle() as the part to be used.

One day, over drinks at the country club, Steven was talking with Andrew. Andrew told him "I can supply you with birch at half the cost of hickory and it will be just as strong." Knowing it would be wrong to change the wood without approval, Steven called Henry, who was busy trying to hit rubber frogs into a copper spitoon with a croquet mallet. "Yeah, yeah, whatever," Henry said.

Steven went into production immediately, making axles out of birch. Feeling it would be dishonest to call them HickoryWoodAxle(), he changed the part ID to BirchWoodAxle(). Henry, however, had caught a ship to Bora Bora where he was under the delusion that he was headlining a three-act musical tribute to molybdenum in the basement of an opium den, and forgot to inform his plant to change the order form.

The next time Henry's plant requested a HickoryWoodAxle(), the shipping manager at Steven's plant looked in inventory and said "we don't have any of those." He refused to send Henry's plant a HickoryWoodAxle(), and Henry's plant had to shut down. If only they'd known to order a BirchWoodAxle(), they could have kept production plugging along.

Henry's stockholders met in secret and sent a team of elite Jesuit ninjas to extract Henry from the steppes of Mongolia where he'd become the alpha male in a herd of Yak. When he got back, he called Steven and chewed him out. Steven argued back that Henry should have told his procurement staff of the change. The argument got so heated that they met and had a forty-seven-hour thumb wrestling competition to settle it, but that ended in a draw.

Still, during that competition, they got to talking. "Why does it have to be so difficult," Henry asked. "All I want is an axle. All it has to do is attachWheels() and turn(). I don't care what material you use."

As their thumbs darted back and forth in an intricate dance better explained by a color commentator, their lawyers drew up a contract, defining how Steven and Henry would interface. At Steven's factory, if they got in an order for a new axle(), so long as it could attachWheels() and turn(), that would be good enough. And that way Steven could change his tooling, his materials, or even ship the production overseas. So long as Henry got an axle() that behaved as expected, he'd be happy.

So the Factory pattern, as I understand it, is one of the most basic implementations of Loose Coupling. That's where too much knowledge is a dangerous thing. Through the order forms, Henry's assembly line was coded to request a HickoryWoodAxle() object. Every time Steven's plant changed the material, and thus the name of the product class, Henry's assembly line would have to change its order form or the whole system could break down.

So they negotiated an interface...

interface IAxle {
	function attachWheels();
	function turn();
}

Using that interface as a guide, Steven could create his axle factory that would send an Axle, every time someone sent it a buy order.

class Axle implements IAxle {
	private $_material = "ceramic";
	
	public function attachWheels(){
		return "Wheels attached";
	}
	
	public function turn(){
		return "Turning";
	}

	public static function buy(){
		return new Axle();
		}
}

Then all you'd need to do is buy an Axle and you can attach Wheels or set it turning.

$axle = Axle::buy();
echo "{$axle->attachWheels()}<br>";
echo "{$axle->turn()}<br>";

And there you have it. If you like my idea of pattern allegories, please speak up in the comments. It will encourage me to do more. If you don't, then you're a philistine.