PromQL Condition within query

Question

Good day, i have an existing PromQL query which is working ok within Grafana, the query is supposed to show the success rate in percent for a custom http metric.

1 - 
sum by(api_name)
(
    increase(demo_http_requests_seconds_count{error="true", api_name="Demo_API", consumer="Demo_Consumer"}[$__range])
)
/ 
sum by(api_name)
(
    increase(demo_http_requests_seconds_count{api_name="Demo_API", consumer="Demo_Consumer"}[$__range])
)

My problem is that the failed http metric (indicated with error="true") returns sometimes no result and in turn the result does not show anything in Grafana.

Is it possible, for example to create a condition around the query to say that if no errors returned that the success rate will be 1 (100%) ?

Answer 1

Try the following PromQL query:

sum by(api_name)
(
  increase(demo_http_requests_seconds_count{
    error!="true", api_name="Demo_API", consumer="Demo_Consumer"
  }[$__range])
)
/
sum by(api_name)
(
  increase(demo_http_requests_seconds_count{
    api_name="Demo_API", consumer="Demo_Consumer"
  }[$__range])
)
or
sum by(api_name) (
  demo_http_requests_seconds_count{
    api_name="Demo_API", consumer="Demo_Consumer"
  }
) * 0

The query contains or ... part, which adds 0 value to the initial query results if they do not contain some api_name label value.

P.S. Note that increase() function in Prometheus may return unexpected results for slow-changing counters. See this issue and this question for details.