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Bypassing Prometheus SDK push model

Gautier DI FOLCO February 07, 2024 [Software engineering] #software engineering #design #monitoring #lgtm-stack #haskell

I have talked in an earlier log that I have set up business monitoring which has led to interesting conversations with the stakeholders.

I have used prometheus-client which is a great start providing Counter/Gauge/Summaries/Histograms but they are push-based, which means: you declare them, and you act on them (increasing/decreasing/etc.) so they are ready when they are requested.

I have tried to set them on a stateful element (for reference, we receive requests, which have a step associated to them, there are ~20 different steps, a typical request going through 30-40 steps, the goal was to have counter) as a Gauge (increasing and decreasing floating point number).

The status changes were event sourced, so I thought: each time a status change occurs, decrease the previous status' Gauge and increase the new status' Gauge.

I have tried really hard, but I was ending up with negative Gauge and incorrect counts. More over, I had to maintain a double-accounting piece of code to have coherent counters (unsuccessfully).

So, I ended up creating a custom Metric which does the count once:

registerMetrics :: IORef (Map.Map RequestId RequestState) -> IO ()
registerMetrics requestsRef =
  void $ register $ Metric $ return ((), mkSampleGroups)
  mkSampleGroups :: IO [SampleGroup]
  mkSampleGroups = do
    requests <- readIORef requestsRef
    let samples =
          map (\(labels, counts) -> Sample "requests" labels (BS.fromString $ show $ int2Float counts)) $
            Map.toList $
              foldl (flip $ Map.alter (Just . maybe 1 (+ 1))) Map.empty $
                map mkLabels $
                  Map.toList requests
      [ SampleGroup
          (Info "requests" "requests")

Which made my Metric only relying on my state, and not on a temporal state.