Growing bigger

Twenty times bigger by Sunday. There are exactly two ways any system has ever gotten bigger, and Traversal will use both before the weekend, in proportions the review argues about for a solid hour. Two ways. That is the whole menu.

The first is vertical scalingConcept · lights on your mapvertical scalingScaling up: replacing the machine with a bigger one. Simple, since no software changes, and limited: there is always a biggest machine, and you still have exactly one of it., the same machine but bigger. Scaling up. It is beautifully simple, nothing about the software changes, no architecture is revisited and no meeting is called, and it has two hard edges that no amount of money files down. There is always a biggest machine. And when you buy it, you still have one of it. One. Hold that word for the next hour.

The second is horizontal scalingConcept · lights on your maphorizontal scalingScaling out: adding more machines rather than a bigger one. No practical ceiling and spares built in, and it only works if the work can actually be split across many hands.: more machines, identical, side by side. Scaling out. No ceiling worth naming, and safety in numbers into the bargain. Its price is a condition. The work has to be splittable, and whether it is splittable turns out to be the deepest question you can ask about any component, deeper than language or vendor or age, the question Saturday night’s whole tour is secretly organised around. Spectator page loads split perfectly. The one authoritative registrations database does not.

The traffic has to find the copies

Thirty identical machines are useless if every request arrives at the first one. Spreading incoming work across the copies is load balancingConcept · lights on your mapload balancingSpreading incoming requests across many identical machines, so added machines actually absorb added load. What makes horizontal scaling work at all, and its health checks are what make a machine’s death boring., and it is what makes horizontal scaling a strategy instead of a shopping spree. It also quietly changes what a machine’s death means. With one machine, death is an outage. With thirty behind a balancer that checks their pulse, death is routine: the sick one stops receiving traffic, the other twenty-nine absorb its share, and nobody watching the race notices a thing.

Boring machine death is one of the great unsung achievements of the field. Friday is about where the thirty machines come from. First, though, the review has to face the opposite question: not what grows, but what breaks.