SLA — Service Level Agreement#
Your team has an internal target. But what do you promise your customers?
That promise is your SLA.
What it is#
An SLA is an external contract between you and your customers.
It says — here's what we guarantee, and here's what happens if we break that guarantee.
- SLO — internal, your team's target, no consequences if breached other than fixing the problem
- SLA — external, customer-facing, breach it and you owe money, credits, or face legal action
A Real Example — AWS S3 SLA#
"We guarantee 99.9% availability. If we drop below that, you get a service credit of 10% of your monthly bill. If we drop below 99%, you get 30% back."
That's an SLA. A written promise with real financial consequences.
SLOs are always stricter than SLAs#
This is intentional. The gap between them is your safety buffer.
| Target | Who sees it | Consequence if breached | |
|---|---|---|---|
| SLO | 99.9% availability | Internal team only | Team gets alerted, engineers fix it |
| SLA | 99.5% availability | Customers, contracts | Refunds, credits, legal action |
If your SLA promises 99.5% availability — your internal SLO is 99.9%.
Your team gets alerted at 99.9% and fixes the problem before it ever reaches 99.5% — before you owe anyone a refund.
The safety buffer in plain English
SLO is the alarm. SLA is the cliff edge. You fix the problem when the alarm goes off — not when you fall off the cliff.
SLAs are not just for big companies#
Any time you make a reliability promise to a customer, you have an SLA — even informally:
| Scenario | Implicit SLA |
|---|---|
| Startup telling customers "99.9% uptime" on their pricing page | That's an SLA |
| AWS, GCP, Azure publishing availability guarantees | That's an SLA |
| Your company's API docs saying "responses under 500ms" | That's an SLA |
| An enterprise contract with explicit uptime and penalty clauses | That's an SLA |
Real world SLAs#
| Company | Service | SLA |
|---|---|---|
| AWS S3 | Object storage | 99.9% availability, credits if breached |
| Google Cloud SQL | Managed database | 99.95% availability |
| Twilio | SMS API | 99.95% uptime |
| Stripe | Payments API | 99.99% uptime |