Notify users when incident happens and show your service uptime to your customers via your public status page & various integration channels StatusEntry provides.
The reason why we are able to calculate uptime metrics in single digit milliseconds is using a cache layer instead of hitting the database every time and making the same calculations.
We used event driven architecture using Amazon SNS to publish events and other services can subscribe to them to do what they need to do when that event happens. This allowed us to preserve highly cohesive and lowly coupled services and it also allowed us to reduce each request's completion time using an asynchronous flow.