Learn/Availability
MONITORING & OBSERVABILITY

Availability

The proportion of time a system is operational and accessible.

Availability = Uptime / (Uptime + Downtime)

The proportion of time a system is operational and accessible.

## The Quest for "Nines" **Availability** is usually expressed as a percentage of time that a service is working. "The Nines" refer to the number of 9s in that percentage. ### The Cost of Nines Each additional "9" costs exponentially more money to achieve. * **99% (Two Nines)**: Good for internal tools. Down 3.6 days/year. (Cheap). * **99.9% (Three Nines)**: Industry Standard. Down 8.8 hours/year. (Affordable). * **99.99% (Four Nines)**: High Availability. Down 52 mins/year. (Expensive). * **99.999% (Five Nines)**: Telco/Medical Grade. Down 5 mins/year. (Extremely Expensive). ### Availability vs. Reliability * A car that breaks down every day but is fixed in 1 minute has high **Availability** (it runs most of the time) but low **Reliability** (it breaks often). * A car that never breaks down is both.

ExThe Region Outage

"A company hosted their app in a single AWS region (us-east-1). The region went down for 4 hours."

Impact
Availability dropped to 99.5% for the year, violating customer contracts.
Resolution
They moved to a Multi-Region Active-Active architecture to achieve 99.99% availability, costing 2x more in infrastructure.

Why Availability Matters

Availability directly impacts revenue and user experience. Every minute of downtime costs money.

High availability (99.9%+) requires redundancy and careful architecture.

Common Pitfalls

Chasing Five Nines
Aiming for 99.999% unavailability when your ISP only gives you 99.9%. You cannot be more available than your dependencies.

Industry Benchmarks

Five Nines< 5 min/year
99.999%
Four Nines< 53 min/year
99.99%
Three Nines< 8.8 hours/year
99.9%
Two Nines< 3.7 days/year
99%

Related Terms

ReliabilitySLOSLAMTBFMTTR

Frequently Asked Questions

Is 99.9% good enough?
For most SaaS products, yes. Users generally tolerate ~45 minutes of maintenance per month.
Does scheduled maintenance count as downtime?
It depends on your user contract (SLA). Usually, yes, because the user cannot use the service.

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