Chasing the Fourth Nine

Leadership wants your service at four nines, but every request makes a blocking call to an internal profile service whose own objective is three nines. What does that imply, and what actually helps?

The cap does not apply, because the two objectives are measured independently and only errors your own service generates count against your SLO rather than dependency failures

You are capped near three nines while the call is required, so the fix is scaling out more replicas of your own service to absorb the shortfall

You are capped near three nines while the call is required, so the fix is removing it from the critical path via caching, defaults, or asynchronous handling

Retries with exponential backoff and jitter on the profile call will recover the missing nine, since dependency failures are brief transients rather than sustained outages