Unable to setup alerts based on expression

[iamyogeshg]

@kylebrandt I am trying to setup alerts based on exact same use case as OP (#31471), however I can't still setup alerts on Math expression and it shows error as can be seen in screenshot:

Now if i try to setup alert on reduced expression D, it seems to be not throwing any error, however I am not certain if this is correct way to configure for this use case. Any help would be highly appreciated. Thanks!

Originally posted by @iamyogeshg in #31471 (comment)

[kylebrandt]

Alerting works on numbers and not time series type data. So if $A and $B are time series data, that math expression will also provide time series data. So reducing is required like when you select $D.

The other option, depending on your data and logic, is to reduce $A and $B with additional operations, and then use math operation on those reduced inputs. Then the math operation will output numbers instead of time series and can be selected as the condition.

[iamyogeshg]

Thanks @kylebrandt.

[hlior]

the reduce didn't work for me.
i use a new query that in it i reduce a from b in one line:

(sum by (projectId) (rate(events_total{projectId='XXX'}[5m]))) - (sum by (projectId) (rate(events_total{projectId='XXX'}[24h])))

[Richard-Payne]

@kylebrandt I'm not sure I follow your comment there.

If I create a query from my Cloudwatch database like this:

SELECT SUM(BlockedRequests) FROM "AWS/WAFV2"

I get a time series plot. I can configure an alert on this expression.
But if I then add an math Expression which is simply this:

$A

I still get a time series plot, and I can still configure an alert.

However, if I add a second query:

SELECT SUM(CountedRequests) FROM "AWS/WAFV2"

and a math expression:

100*($A/$B)

And try to alert on the expression then I get an error:

The datasource does not support alerting queries

Expression C still visualises as a time series.

Could you explain how what you were saying about time series/numbers in relation to alerting applies here? Could you, or anyone else, give some guidance on how to accomplish alerting on one time series as a percentage of another?