When the deadline for registering to vote in the UK EU referendum approached there were issues with the online registration system. It stopped working due to the high numbers of people trying to register all at once. The system failed to scale and fell over. By scale, I mean that the hardware was no longer powerful enough to service all of the requests made of the software it was running and it was unable to become more powerful.

There are two main ways to scale computer hardware, vertically and horizontally. Most software can scale vertically, regardless of how it’s designed. To scale horizontally special design considerations must be taken into account.

Vertical Scaling
Imagine you’ve got 1000 people to move from point A to point B 10 miles away and a car which can hold 5 people and travels at an average speed of 60 miles an hour. That means it takes 10 minutes to get the car once from point A to point B. Ignoring the return journey and including the driver in the number of people moved, it would take 200 trips, at 10 minutes each, which is 33 hours. That’s pretty slow.

If we use the same car, but with a more powerful engine which can travel at an average speed of 120 miles an hour, it now takes 5 minutes to get the car once from point A to point B and a total time of 16.5 hours. That’s already a good improvement.

If you use 4 minibuses the time comes down to 1 hour.  If we continue to increase the number of minibuses we can bring the time down significantly.

This is an example of vertical scaling. By increasing the processing power (engine) and the memory (number of people the vehicle can hold) in a computer we can increase how quickly it responds to users. However, you can only increase processing power and memory to a point. There is a threshold where it becomes impractical to scale further and another where it is no longer cost effective.

Horizontal Scaling
Imagine you’ve got the same 1000 people to move from point A to point B and two minibuses which hold 20 people each and travel at an average speed of 120 miles an hour. Both minibuses take 5 minutes to get one from point A to point B. Ignoring the return journey and including the driver in the number of people moved, it would take 25 trips, at 5 minutes each, which is 2 hours.

If you use 4 minibuses the time comes down to 1 hour. If we continue to increase the number of minibuses we can bring the time down significantly.

This is an example of horizontal scaling. By increasing the number of computers which are working together in parallel we can increase how quickly the overall system responds to users.

Words by Paul