append string to variable in a grafana query?

Question

In Grafana I have a drop down for variable $topic with values "topic_A" "topic_B"

"topic_A" is selected so $topic = "topic_A"

I want to query prometheus using

function{topic=$topic}

and that works fine.

How would I implement

function{topic="$topic" + "_ERROR"}

(this fails) where what I want to query would be "topic_A_ERROR" if "topic_A" is selected.

How do I combine variable $topic and string "_ERROR" in the query?

Answer 1

UPDATE 2020-08-17:

There is a new syntax for Grafana variables, new format is to use curly braces after dollar sign:

function{topic=~"${topic}_ERROR"}

Double brackets syntax is deprecated and will be deleted soon.

Also now you can define the format of the variable, which may help to solve some spacial characters issues. Example: ${topic:raw}

Docs: https://grafana.com/docs/grafana/latest/variables/syntax/

---

If you want to include text in the middle you need to use a different syntax:

function{topic=~"[[topic]]_ERROR"}

Note not only the double brackets but also the change from = to =~. It is documented on the link at the end of my comment, basically it says:

When the Multi-value or Include all value options are enabled, Grafana converts the labels from plain text to a regex compatible string. Which means you have to use =~ instead of =.

You can check the official explanation here: https://grafana.com/docs/grafana/latest/features/datasources/prometheus/#using-variables-in-queries