Should my SLO be the same as my SLA?

No, the SLO should be stricter. The gap between them is your reaction time: you want internal alarms firing, and fixes underway, well before the contractual threshold where credits start accruing. A common pattern is an internal SLO one notch tighter than the external SLA, like 99.95 internal against 99.9 contractual.

We haven't missed our SLO in a year. Is that good?

It deserves investigation rather than celebration. Either the target is too loose to describe what users need, or you are overspending on reliability that could fund features, or dependents are quietly building on a level of service you never promised. Google's answer to the last problem was planned outages for Chubby, deliberately spending the excess so nobody could depend on it.

Why not just target 100 percent availability?

Because a 100 percent target means an error budget of zero, which forbids deploys, migrations, maintenance, and any risk at all, forever. Users also cannot tell 100 percent from 99.99 over a month; their own wifi fails more often than that. Every nine multiplies cost, so the right target is the cheapest one users genuinely cannot distinguish from perfect.

Reliabilitychapter 4 of 5 · 3 lessons

SLA / SLO / SLI

SLI measures reality, SLO sets the target, SLA is the promise to customers.

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Three acronyms one letter apart, and mixing them up costs you either money or credibility. One is what you measure, one is the target you hold yourself to internally, and one is the promise you make to customers with penalties attached.

Get the relationships right and a lot of hard things start resolving themselves mechanically: alerting, prioritisation, the standing argument between product and infrastructure. Get them wrong and it has a signature, where sales promises a number engineering never agreed to and everybody finds out in the same meeting, months later, when the credits come due.

Lessons

3 in this chapter

SLI, SLO, SLA: Measure, Target, PromiseThe SLI measures reality, the SLO sets your target, the SLA is the contract.3 min
Error BudgetsThe gap between your SLO and perfection is a budget you are allowed to spend.2 min
Choosing Good SLOsTarget what users feel, set it below perfection, and keep the list short.3 min