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Portal | Level: L2: Operations | Topics: Postmortems & SLOs | Domain: DevOps & Tooling

Postmortem & SLO/SLI - Skill Check

Mental model (bottom-up)

SLIs measure user experience. SLOs set targets. Error budgets create objective tradeoffs between reliability and velocity. Postmortems turn incidents into organizational learning.

Visual stack

[SLA             ]  business contract (99.5% or refund)
|
[SLO             ]  engineering target (99.9%)
|
[Error Budget    ]  1 - SLO = allowed failure (0.1% = 43 min/month)
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[SLI             ]  actual measurement (99.2% this month)

Glossary

  • SLI - Service Level Indicator; metric measuring quality (e.g., success rate)
  • SLO - Service Level Objective; target for an SLI (e.g., 99.9%)
  • SLA - Service Level Agreement; contract with consequences
  • error budget - allowed unreliability (1 - SLO)
  • burn rate - how fast you're consuming error budget
  • blameless postmortem - focuses on systems, not individuals

Core questions (easy -> hard)

  • What's the difference between SLI, SLO, and SLA?
  • SLI: measurement. SLO: target. SLA: contract. SLA < SLO < theoretical max.
  • What is an error budget?
  • 1 - SLO. For 99.9%: 0.1% = 43.2 min/month of allowed downtime.
  • What makes a good postmortem?
  • Blameless, specific root cause, timeline with UTC, owned action items, runbook link.
  • Why use percentiles instead of averages for latency?
  • Averages hide tail. p50=50ms + p99=5s = avg looks fine but 1% of users suffer.
  • What is a burn rate alert?
  • Alerts when error budget is consumed too fast. 14.4x = all budget in 1 hour.
  • What happens when error budget is exhausted?
  • Error budget policy: feature freeze, focus on reliability, mandatory postmortem.

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Prerequisites

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