Filter vector based new label introduced by `label_replace`

Question

In Prometheus I'm trying to merge multiple http request lines into groups using label_replace.

http_requests_total{account_id="124",handler="AAAAAController"...}
http_requests_total{account_id="125",handler="BBBBBController"...}
http_requests_total{account_id="126",handler="CCCCCController"...}
http_requests_total{account_id="123",handler="XXXXXController"...}

The query I wrote is :

label_replace(http_requests_total, "class", "$1", "handler", "([a-zA-Z0-9]+)Controller.*") .

This works correctly and adds the class label to vector :"AAAA","BBBB" etc. At this point I would like to remove certain classes such as empty and BBBB.

How can I further filter the vector using {class~="BBBBB"} :

label_replace(http_requests_total, "class", "", "handler", "([a-zA-Z0-9]+)Controller.*"){class~="BBBBB"}

Prometheus shows an error when I try to do so.

Answer 1

The filtering for this particular question can be done via the inner series selector before applying label_replace. For example, the following query returns only series with class=~"BBBBB":

label_replace(
  http_requests_total{handler=~"BBBBBController.*"},
  "class",
  "$1",
  "handler",
  "([a-zA-Z0-9]+)Controller.*"
)

P.S. While Prometheus doesn't provide the ability to post-filter time series by label values after they are selected by series selector, this can be done in VictoriaMetrics - Prometheus-like system I work on. See label_match and label_mismatch functions.