Question

Your team runs a checkout service with a 99.9% monthly availability SLO. The checkout service depends on a downstream payments API that another team operates and that has a published SLO of 99.5% availability. During your reliability review, an engineer proposes that your team can safely commit to a 99.95% SLO because your own code has been very stable this quarter. What is the most accurate reliability principle to apply here?

Accepted Answer

A service's achievable SLO is generally capped by the reliability of its critical dependencies, so a 99.95% target is unrealistic while the payments API only promises 99.5%.

Answer

Because your own code is stable, you can commit to 99.95% and rely on retries to fully compensate for any downstream failures.

Answer

You should set your SLO to match the payments API exactly at 99.5%, since dependencies and dependents must always share identical SLO targets.

Answer

SLOs are internal targets only, so the downstream dependency's SLO has no bearing on what your team can promise to users.