Service Level Objective
A target reliability threshold for a service, typically expressed as a percentage over a time period.
A target reliability threshold for a service, typically expressed as a percentage over a time period.
## The "Reliability Goal" **SLO (Service Level Objective)** is the target you aim for. It is the dividing line between "Happy Users" and "Sad Users." ### The Three S's of SRE 1. **SLI (Indicator)**: The number you measure (e.g., Latency). 2. **SLO (Objective)**: The target you set (e.g., Latency < 200ms). 3. **SLA (Agreement)**: The punishment if you miss it (e.g., Refund). ### Choosing an SLO **Do not aim for 100%.** * 100% is impossible (physics, ISP outages). * 100% is too expensive. * 100% freezes innovation. Aim for **99.9%** (the "Three Nines"), which allows for ~43 minutes of downtime per month.
ExThe Checkout Service
"An e-commerce site set an SLO of 99.95% availability for their Checkout API."
Why SLO Matters
SLOs align engineering and business on what "good enough" reliability means.
Without SLOs, you can't have error budgets. Every incident becomes an emergency.
The Formula
SLO = (Successful Requests / Total Requests) × 100%