Prometheus how "up" metrics works

Question

I'm looking for information how "up" metrics is calculated by Prometheus

up{job="<job-name>", instance="<instance-id>"}: 1 if the instance is healthy, i.e. reachable, or 0 if the scrape failed.

How Prometheus calculate when

the instance is healthy

I'm using Apache Cassandra with Prometheus and from time to time "up" metrics showing "down". However Cassandra working OK.

Answer 1

From the docs:

up{job="<job-name>", instance="<instance-id>"}: 1 if the instance is healthy, i.e. reachable, or 0 if the scrape failed.

i.e. it is a per scraper / exporter metric which means whether the exporter was available / reachable or not.