Remove sum from Summary

[felixbarny]

Just summing all snapshot values does not work with dropwizard metrics, because the values array does not hold all values but only a sample of recent values.

[brian-brazil]

@dopuskh3

I could have sworn there was a way to get this data.

[dopuskh3]

Removing sum will make prometheus histogram useless. Don't we have a way to get this value elsewhere?

[brian-brazil]

We still have the quantiles, so it just stops us getting the average.

[felixbarny]

Unfortunately, there actually isn't because the Timer only contains data from approximately the last 5 minutes.

One (very bad) estimation would be timer.getSnapshot().getMean() * timer.getCount(). But this value is equally incorrect as the mean is only representative for the last 5 minutes.

Do some prometheus query depend on the sum? Which and why?

[dopuskh3]

My understanding is that it will work with uniform reservoirs (http://metrics.dropwizard.io/3.1.0/manual/core/#uniform-reservoirs) but not with exponentially decaying reservoirs (defaults for histogram and timer in dropwizard). In this case (uniform reservoir), getValues(1.0) should work.

[felixbarny]

getValue(1.0) returns the max value (100th percentile). Summing the values returned by getValues() also does not work for the UniformReservoir, because it only holds 1028 representative values. This would work for UniformReservoirs: timer.getSnapshot().getMean() * timer.getCount(). But like you said, the default is the ExponentiallyDecayingReservoir where it is not possible to get the sum.

[brian-brazil]

So shall we go ahead and remove this then?

[felixbarny]

I can't say how this would affect prometheus histogram queries if there is no sum but the current sum value is just plain wrong.