Uptime

Also called Availability, Downtime, Nines

The proportion of time a service is working. Small-sounding percentages hide large amounts of downtime.

What it means

Uptime is the percentage of a period during which a service is available. It is usually quoted in nines: 99.9% is three nines.

The arithmetic is worth knowing. 99% allows about 3.65 days of downtime a year. 99.9% allows about 8.8 hours. 99.99% allows about 53 minutes.

Why it matters

The difference between 99% and 99.9% sounds trivial and is a factor of ten in outage time.

It also determines what an SLA is actually promising, and what the service credits are worth when it is missed.

What it looks like in practice

Read the exclusions. Most availability commitments exclude planned maintenance, which can be a significant share of the downtime you experience.

Check how it is measured and by whom. A supplier measuring its own availability from inside its own network is not measuring your experience.

Monitor independently for anything you depend on. External monitoring is inexpensive.

What to watch out for

Service credits as the sole remedy, capped at a fraction of a month's fee. If an outage costs you far more than that, the SLA is not the protection you thought.

Uptime measured on the platform while the specific feature you need is unavailable.

Where to get proper advice

Suppliers usually publish a status page with history. Read a year of it before signing.