Side by side
SLAvsSLO
What is the difference between an SLA and an SLO?
Updated 2 min read7 differences
In short
An SLO is an internal target, like 99.9% of requests succeeding in 30 days; an SLA promises customers a service level, usually with credits if it is missed.
SLA
Service Level Agreement
An SLA (service level agreement) is a provider's commitment to customers about the level of service, such as 99.9% uptime, and what happens if it isn't met.
Read the page on SLASLO
Service Level Objective
An SLO is a measurable reliability target for a service, such as 99.9% of requests succeeding over 30 days, that tells a team how reliable is reliable enough.
Read the page on SLOSLA and SLO compared
| Aspect | SLA | SLO |
|---|---|---|
| What it is | A contract with customers | An internal reliability target |
| Audience | Customers, sales and legal | Engineers and product teams |
| When missed | Credits, refunds or other penalties | The error budget is spent and reliability work comes first |
| Typical value | Looser, such as 99.5% | Stricter, such as 99.9% |
| Based on | Measurements defined in the contract | SLIs such as success rate, latency and availability |
| Changes | Rarely, through contract updates | Often, as the team learns |
| Needed for | Services that promise customers a level | Any service people rely on |
The difference, explained
A service level indicator (SLI) measures something users care about, such as the share of requests that succeed or that answer within 300 milliseconds. A service level objective (SLO) sets a target for it: 99.9% of requests succeed over 30 days. A service level agreement (SLA) is a promise to customers, written into a contract, with consequences such as service credits when it is broken.
The two are set differently on purpose. An SLA is usually looser than the SLO behind it, for example an SLA of 99.5% against an SLO of 99.9%, so the team is warned and acts long before the company owes anyone money. SLOs are for engineers and change as the team learns about the service; SLAs are for customers and lawyers and change rarely.
SLOs also drive everyday decisions through the error budget: a 99.9% SLO allows 0.1% of requests to fail, about 43 minutes of full downtime in 30 days. While budget is left, the team can ship quickly; when it runs out, the focus turns to reliability. The practice comes from Site Reliability Engineering, as described by Google.
Not every service has an SLA, but every important one should have SLOs; internal services and free products usually have only SLOs. And 100% is never the right target: it can't be met, and chasing it slows everything else down.
Which one should you use?
Choose SLA when…
- Customers pay for the service and need a formal promise.
- Sales or legal need terms they can put in a contract.
- You are ready to back the promise with credits or refunds.
Choose SLO when…
- You want to know whether the service is reliable enough before users complain.
- The team needs a rule for balancing new features against reliability work.
- The service is internal or free, with no contract to sign.
An SLA and the SLO behind it
# SLA: the promise in the customer contract (summary)
service: Orders API
availability: 99.5% # measured monthly
service_credit:
below_99_5: 10% # of the monthly fee
below_99_0: 25%# SLO: the internal target behind it
service: Orders API
sli: successful requests / all requests
objective: 99.9%
window: 30d
error_budget: 0.1% # about 43 minutes of full downtime
page_when: the budget burns 2x faster than allowedReaders ask
What is an SLI?
A service level indicator is the measurement an SLO is set on, such as the percentage of successful requests or of requests faster than 300 milliseconds.
Why not promise customers the same number as the SLO?
Because the SLO is where the team starts reacting. A looser SLA leaves room to notice and fix problems before the contract is broken.
How much downtime does 99.9% allow?
About 43 minutes in a 30-day month, or roughly 8 hours and 46 minutes in a year. Each extra nine cuts that tenfold: 99.99% allows about 4 minutes a month.