Explain SLI, SLO, SLA, and error budgets, and how they relate.
What they are really testing: Whether you can define each precisely and, crucially, explain how an error budget turns reliability into a decision-making tool for balancing velocity and stability.
A real interview question
Explain SLI, SLO, SLA, and error budgets, and how they relate.
What most people say
drag me
“SLAs, SLOs, and SLIs are all about uptime targets you promise to customers.”
It blurs all three and misses error budgets entirely. The point is the hierarchy (measured SLI, internal SLO target, contractual SLA) and that the error budget is a decision tool, not just a number.
The follow-ups they ask next
Why set your SLO stricter than your SLA?
To have a safety margin: you want internal alarms to fire and corrective action to start before you breach the contractual SLA and owe penalties. The SLO is the early-warning target.
How does an error budget change how a team operates?
Budget remaining = freedom to ship and take risk; budget exhausted = freeze risky changes and prioritize reliability. It replaces arguing about velocity vs stability with a shared, data-driven rule.
What the interviewer is listening for
- Defines SLI/SLO/SLA distinctly + hierarchy
- Explains error budget = 1 - SLO
- Frames error budget as a velocity/stability decision tool
What sinks the answer
- Blurs the three terms
- No mention of error budgets
- Treats them as just uptime promises
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“An [SLI is a measured health signal (% successful requests)], an [SLO is the target for it (99.9% over 30 days, internal goal)], an [SLA is the contractual promise with penalties, set stricter SLO than SLA]. The [error budget is 1 - SLO, the allowed unreliability]: [spend it on velocity, and when exhausted slow risky changes and fix reliability]. It also [drives burn-rate alerting].”
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Junior
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What are the main metric types (counter, gauge, histogram), and when do you use each?
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