Availability & The Nines

### What is Availability?

Availability is the percentage of time a system is operational and accessible. It is heavily tied to Service Level Agreements (SLAs).

### Understanding "The Nines"

Availability is commonly measured in "nines": - **99% (Two Nines):** ~3.65 days of downtime per year. - **99.9% (Three Nines):** ~8.77 hours of downtime per year. - **99.99% (Four Nines):** ~52.6 minutes of downtime per year. - **99.999% (Five Nines):** ~5.26 minutes of downtime per year.

Achieving higher nines becomes exponentially more difficult and expensive, requiring automated failovers, multi-region deployments, and zero-downtime deployment strategies. Often, five nines is overkill unless you are building life-critical systems.