SLO vs SLA vs SLI: The Three-Letter Confusion, Resolved

The three terms ARE different. Conflating them produces postmortems with the wrong remediation.

Definitions

SLI: a measurement (request success rate, p99 latency).

SLO: a target on the SLI (99.9% success rate over 30 days).

SLA: a contract guaranteeing the SLO with consequences (5% credit if missed).

Relationships

• SLI is the metric. SLO is the goal. SLA is the legal-and-money version of the SLO.
• You can have all three; usually only the SLO ships internally; the SLA goes to customers in the contract.

How to tell which you have

If a number is in PromQL: SLI.

If a number is in a Confluence page that engineers read: SLO.

If a number is in a customer contract: SLA.

Why teams confuse them

Vendor marketing teams say ‘5 nines SLA’ when they mean SLO. Internal teams adopt the misuse.

Cleanest approach: write down your three numbers explicitly with the labels. Avoid the slang.

Antipatterns

• Treating SLA as the only number. Misses internal targets.
• SLOs that nobody negotiates. Decoration.
• SLIs that do not match user experience. Wrong metric.

What to do this week

Three moves. (1) Apply the pattern to your most-impactful service. (2) Measure adherence for 30 days. (3) Rewrite the policy or the SLO if the gap is durable.