mediumDesignScenarioMidSeniorStaffLead
The business says the system must be highly available. How do you turn that into a number, and what does that number cost?
4 min readUpdated 2026-08-15
Answer
Short answer
Convert the phrase into an error budget over a stated window, derived from what an outage costs the business rather than from a round number of nines. Each additional nine roughly multiplies cost, because it forces redundancy, cross-region failover and operational maturity that the previous nine did not need.
An SLO is the contract shape that makes availability measurable. A useful SLO says what users can expect, where it is measured, and how much error budget the service can spend. Without an SLO, the availability target is only a slogan; with an SLO, the business can decide whether the next nine is worth the engineering and operational cost.
Was this answer helpful?
Likely follow-ups
- Your target is 99.9% but your cloud database's SLA is 99.95%. What can you actually promise?
- Who decides when the error budget is spent, and what changes when it is?
- How does a target of 99.99% change your deployment process?
- The business wants five nines for a reporting dashboard. How do you handle that conversation?
Related questions
- You promise 99.9 percent availability and you call five services that each promise the same. What have you committed to?mediumAlso on availability and slo4 min
- A stakeholder tells you the system must be scalable. How do you turn that into a requirement you can design and test against?hardAlso on quality-attributes and slo7 min
- How would you set an SLO for a service, and what is an error budget for?mediumAlso on slo and error-budget5 min
- Your error budget burn alert pages every few hours, but half the time nobody outside the team has noticed anything. How do you tune it without simply making it quieter?hardAlso on slo5 min