At that scale, a complete outage is unlikely. I have services which haven't gone down _at all_ for longer than a year. But we lose requests every now and then -- during a deploy, or due to a bug. So we've moved from a time-based view of outage to a request-based view.

This helps, too, as it lets us build out services to be more reliable in combination, rather than less reliable. With retries and fail-over, an outage in an entire region may not necessarily result in any user requests failing.

For scale, pre-pandemic our published figures claimed >100M MAU.

I find https://andrewaylett.github.io/multi-burn-rate-calculator/ helpful for visualising error rates -- largely cribbed from the project it's forked from :) but with the tweakables switched around and the time between alert and error budget exhaustion in the tooltip.

It's worth noting that we only evaluate our alerts at most once a minute.