One of the things our Prometheus environment has is a notion of different sorts of alerts, and in particular of less important alerts that should go to a subset of people (ie, me). There are various reasons for this, including that the alert is in testing , or it concerns a subsystem that only I should have to care about, or that it fires too often for other people (for example, a reboot notification for a machine we routinely reboot).

For historical reasons, there are at least four different ways that this can be done in our Prometheus environment:

• a special label can be attached to the Prometheus alert rule , which is appropriate if the alert rule itself is in testing or otherwise is low priority.

• a special label can be attached to targets in a scrape configuration , although this has some side effects that can be less than ideal. This affects all alerts that trigger based on metrics from, for example, the Prometheus host agent (for that host).

• our Prometheus configuration itself can apply alert relabeling to add the special label for everything from a specific host, as indicated by a "host" label that we add . This is useful if we have so many exporters being scraped from a particular host, or if I want to keep metric continuity (ie, the metrics not changing their label set) when a host moves into production.

• our Alertmanager configuration can specifically route certain alerts about certain machines to the 'less important alerts' destination.

The drawback of these assorted approaches is that now there are at least three places to check and possibly to update when a host moves from being a testing host into being a production host. A further drawback is some of these (the first two) are used a lot more often than others of these (the last two). When you have multiple things, some of which are infrequently used, and fallible humans have to remember to check them all, you can guess what can happen next.

And that is the simple version of why alerts about one of our fileservers wouldn't have gone to everyone here for about the past year.

How I discovered the problem was that I got an alert about one of the fileserver's Prometheus exporters restarting, and decided that I should update the alert configuration to make it so that alerts about this service restarting only went to me. As I was in the process of doing this, I realized that the alert already had only gone to me, despite there being no explicit configuration in the alert rule or the scrape configuration. This set me on an expedition into the depths of everything else, where I turned up an obsolete bit in our general Prometheus configuration.

On the positive side, now I've audited our Prometheus and Alertmanager configurations for any other things that shouldn't be there. On the negative side, I'm now not completely sure that there isn't a fifth place that's downgrading (some) alerts about (some) hosts.