Justifying Every Box in the Drawing With the Number That Forced It

A final drawing is a transcription, not a design. By the time it is drawn, every

decision in it has already been made by arithmetic, and the drawing's only job

is to hold those decisions where both of you can see them. That is also what

makes it defensible. Asked why a component is there, you are not being asked

for an opinion about that component. You are being asked for a number, and

either you have it or the box was decoration.

The brief is a link shortener with click analytics, and the estimates were

settled before anything was drawn. There are 100,000,000 new links a day, and

since a day is 86,400 seconds that is $100{,}000{,}000 / 86{,}400 = 1{,}157$ a

second, rounded up to 1,200 writes a second. A link is followed about 100 times

over its life, so redirects run at $1{,}200 \times 100 = 120{,}000$ a second at

the mean. Peak is three times the mean, giving $3 \times 1{,}200 = 3{,}600$

writes and $3 \times 120{,}000 = 360{,}000$ reads a second. A link row is 500

bytes, so storage grows by $100{,}000{,}000 \times 500 = 50$ GB a day and

$50 \times 365 = 18{,}250$ GB a year, which is 18 TB. Every redirect also

records a click event of 100 bytes, so clicks arrive at the same 360,000 a

second at the peak and occupy $10{,}000{,}000{,}000 \times 100 = 1$ TB a day,

or 365 TB a year. The read to write ratio is 100 to 1.

Against that, one machine does 10,000 reads a second from memory, 5,000 writes

a second to a store that goes to disk, and holds 1 TB of usable disk. Those

three numbers and the six above are the whole of what the drawing has to

answer to.