Service Level Hierarchy: SLI, SLO, SLA¶

Reliability requires three distinct layers: what you measure (SLI), what you target internally (SLO), and what you promise externally (SLA) - conflating these leads to either broken promises or unnecessary conservatism.

Tension Addressed¶

Availability vs. Reliability - SLOs formalize the chosen trade-off point

Recognizable As¶

Any system distinguishing measurement, target, and commitment; internal quality gates vs external contracts; success rate definitions tied to business outcomes

Canonical Sources¶

Google SRE Book, Chapters 4-5 (2016); Betsy Beyer et al., "Site Reliability Engineering"

Transfers To¶

Quality assurance (defect rate targets), Manufacturing (Six Sigma), Contract law (warranties and guarantees)

Sub-concepts: - SLI (Indicator): The specific measurement (e.g., request latency at p99) - SLO (Objective): Internal target with consequences (e.g., 99.9% success) - SLA (Agreement): External contract with penalties (e.g., 99.5% uptime guaranteed)