Can you calculate active users using time series - grafana

Question

My atomist client exposes metrics on commands that are run. Each command is a metric with a username element as well a status element.
I've been scraping this data for months without resetting the counts.
My requirement is to show the number of active users over a time period. i.e 1h, 1d, 7d and 30d in Grafana.
The original query was:
count(count({Username=~".+"}) by (Username))
this is an issue because I dont clear the metrics so its always a count since inception.
I then tried this:
count(max_over_time(help_command{job=“Application
Name”,Username=~“.+“}[1w]) -
max_over_time(help_command{job=“Application name”,Username=~“.+“}[1w]
offset 1w) > 0)
which works but only for one command I have about 50 other commands that need to be added to that count.
I tried the:
"{__name__=~".+_command",job="app name"}[1w] offset 1w"
but this is obviously very expensive (timeout in browser) and has issues with integrating max_over_time which doesn't support it.
Any help, am I using the metric in the wrong way. Is there a better way to query... my only option at the moment is the count (format working above for each command)
Thanks in advance.

Answer 1

To start, I will point out a number of issues with your approach.
First, the Prometheus documentation recommends against using arbitrarily large sets of values for labels (as your usernames are). As you can see (based on your experience with the query timing out) they're not entirely wrong to advise against it.
Second, Prometheus may not be the right tool for analytics (such as active users). Partly due to the above, partly because it is inherently limited by the fact that it samples the metrics (which does not appear to be an issue in your case, but may turn out to be).
Third, you collect separate metrics per command (i.e. help_command, foo_command) instead of a single metric with the command name as label (i.e. command_usage{commmand="help"}, command_usage{commmand="foo"})
To get back to your question though, you don't need the max_over_time, you can simply write your query as:
count by(__name__)(
(
{__name__=~".+_command",job=“Application Name”}
-
{__name__=~".+_command",job=“Application name”} offset 1w
) > 0
)
This only works though because you say that whatever exports the counts never resets them. If this is simply because that exporter never restarted and when it will the counts will drop to zero, then you'd need to use increase instead of minus and you'd run into the exact same performance issues as with max_over_time.
count by(__name__)(
increase({__name__=~".+_command",job=“Application Name”}[1w]) > 0
)