Tech Leaderism

99, 99.9, 99.99 and 99.999% SLA

The difference between availability percentages:

- 99% = 3.65 days downtime/year

- 99.9% = 8.77 hours downtime/year

- 99.99% = 52.6 minutes downtime/year

- 99.999% = 5.26 minutes downtime/year

Each additional "9" demands exponentially more investment. Redundant systems, automated failover, distributed architectures, continuous monitoring and substantially higher infrastructure costs.

The question isn't "how many nines should we target?" It's "what does downtime actually cost us?"

A payment processing system going down for even five minutes can mean millions in lost transactions and immediate regulatory scrutiny. An e-commerce platform during Black Friday can't afford 52 minutes of downtime. Healthcare systems managing patient records need near-constant availability because lives may depend on access to critical information.

But an internal HR portal experiencing 8 hours of downtime spread across a year is inconvenient, not catastrophic. A corporate blog being down for half a day won't damage the business. Development and staging environments can tolerate even lower availability without meaningful impact.

The right SLA aligns availability with actual business impact. A critical platform at 99.9% is underserving users. An internal tool at 99.999% is probably burning money on unnecessary infrastructure.