Gauge: accessing valBits

[skwiffhub]

Noob question: Is there any particular reason that valBits would not be exposed to a caller via Gauge? Seems like pretty basic counter functionality to simply return its current value.

valBits source ref: https://github.com/prometheus/client_golang/blob/master/prometheus/gauge.go#L82

[beorn7]

Generally, the various metric types are specialized on updating their value in instrumentation and then return them in a scrape. If we had methods to directly access the metric values, it would signal that those methods are useful and efficient, and generally meant to be used for regular operation.

See also https://pkg.go.dev/github.com/prometheus/[email protected]/prometheus/testutil#ToFloat64 for a helper function to access the value of a Gauge and why that's only recommended for special cases like tests.

[skwiffhub]

I see your point. However, there is the general issue of synchronization for which a quick check of valBits would provide a sanity check. For example, a Gauge that goes negative when the user intends for it to always be a positive integer. Alternatively, a Gauge that exceeds a pre-defined maximum. These are all most likely software errors, but nonetheless it would be useful to be able to access the value and re-calibrate it or log an error.

Perhaps an API with an accompanying set of disclaimers?

[beorn7]

In that case, I would recommend to manage the Gauge value on your own and expose it via https://pkg.go.dev/github.com/prometheus/[email protected]/prometheus#NewGaugeFunc

Note that you run into all kind of concurrency problems. After retrieving the value from a Gauge, another goroutine might already update it before you set it again. Furthermore, future versions of client_golang might leverage core-local variables that are only gathered together upon scrape…