SLI, SLO, and SLA Definitions

Explain the difference between SLI, SLO, and SLA with examples.

SREintermediate

// interview question

Explain the difference between SLI, SLO, and SLA with examples.

Sample answer

SLI (Service Level Indicator) is a metric measuring service behavior (e.g., latency, error rate). SLO (Service Level Objective) is an internal target for that metric (e.g., 99.9% availability). SLA (Service Level Agreement) is an external contract with customers specifying consequences for missing targets. Example: SLI = request latency, SLO = 95% of requests < 200ms, SLA = credits if monthly availability drops below 99.5%.

Why this matters

These concepts form the foundation of reliability engineering. SLIs help measure user experience, SLOs balance reliability with feature velocity, and SLAs formalize business commitments. Without clear SLOs, teams either over-engineer for unnecessary reliability or ship too fast and burn out on incidents.

Code examples

SLO definition example

yaml

# Service Level Objectives for API service
slos:
  availability:
    description: Service should be available
    indicator: (successful_requests / total_requests) * 100
    target: 99.9%
    window: 30d

  latency:
    description: API should respond quickly
    indicator: p95_response_time_ms
    target: 95% of requests < 200ms
    window: 30d

  error_budget: 0.1%  # 43.2 minutes/month of allowed downtime

SLI measurement in Prometheus

promql

# Availability SLI
sum(rate(http_requests_total{status!~"5.."}[5m]))
  /
sum(rate(http_requests_total[5m]))

# Latency SLI (P95)
histogram_quantile(0.95,
  sum(rate(http_request_duration_seconds_bucket[5m])) by (le)
)

# Error budget remaining
1 - (
  (1 - availability_sli) / (1 - availability_target)
)

Common mistakes to avoid

Setting SLOs at 100% (impossible and prevents any changes)
Confusing SLOs (internal targets) with SLAs (customer contracts)
Not tracking error budget, making it meaningless

Likely follow-ups

How do you handle situations when you're close to exhausting your error budget?
What's the difference between availability and reliability?
How do you choose appropriate SLO targets?

Answer out loud first, then check yourself against the model answer.

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