What is the difference between availability and durability?

Availability is whether the system answers right now; durability is whether the data still exists eventually. S3 makes this vivid: eleven nines of durability but a 99.9 percent availability SLA. An outage can make your objects unreachable for hours without losing a single byte. You buy availability with redundant serving paths and durability with redundant storage and backups.

Does any product actually need five nines?

A few do: emergency services, telephony, core payment networks, air traffic systems. Most consumer and SaaS products do not, because their clients, ISPs, and devices fail more often than a four nines backend does. If the user's wifi provides two nines, paying for five on the server is spending money below the noise floor.

Why does my system fail more often than the SLA math predicts?

Almost always correlated failure. The parallel formula assumes copies fail independently, but real replicas share deploy pipelines, configs, certificates, DNS, and cloud regions. A bad deploy or an expired certificate takes out every copy at once, which no amount of multiplication predicted. Config pushes and cert expiry belong in your failure model alongside hardware.

Numbers to Knowchapter 3 of 4 · 3 lessons

Availability Nines

How many nines → how much downtime you're allowed per year.

← All chapters View in cheatsheet

Your checkout calls five services: auth, catalogue, pricing, payments, email. Each team promises 99.9 percent, and every team hits it.

Multiply the five together and checkout is available 99.5 percent of the time. That is 43 hours down a year, not the 8 hours everybody in the room believes they signed up for.

Nobody missed their number. The arithmetic of chaining did it, and the same arithmetic run the other way is why two unreliable machines can be more dependable than one excellent one.

Lessons

3 in this chapter

What Each Nine Allows and CostsEvery added nine cuts downtime 10x and raises engineering cost far more than 10x.3 min
Availability in ChainsSerial dependencies multiply, so your system is less available than its worst component.2 min
The Math of RedundancyParallel copies multiply failure probabilities, turning 99.9 into 99.9999 with one extra box.3 min