min statistic for distribution summaries and timers

[randysecrist]

Quick question ...

Why doesn't https://github.com/micrometer-metrics/micrometer/blob/master/micrometer-core/src/main/java/io/micrometer/core/instrument/distribution/HistogramSnapshot.java have a min function?

[jkschneider]

Just wanted to be clear that there was a use for it before introducing. Seems like alerting on a min is a rare thing, but there is probably a real world case somewhere. What are you thinking of doing?

[jkschneider]

BTW, workaround is a P0 percentile:

@Test
void minimum() {
    DistributionSummary summary = DistributionSummary.builder("my.summary")
            .publishPercentiles(0)
            .register(new SimpleMeterRegistry());

    // dummy data
    for (int i = 1; i < 10; i++) {
        summary.record(i);
    }

    assertThat(summary.percentile(0)).isEqualTo(1);
}

Micrometer's client-side percentiles are designed to decay in the same way max does. P0 can be interpolated, but it's something.

[randysecrist]

Historical monitoring of snapshots (but not necessarily alert) a distribution summary for say ... minimum values for something like say ... request size. All the other relevant stats are there; just seems to be missing this one.

[jkschneider]

Reason I'm seeking concrete use cases is we should determine whether this is an always-on thing or an option to select. Leaning towards the latter unless there are an overwhelming number of use cases.

Request size has an interesting characteristic that it has a natural lower bound (request header size). Interestingly, it means request size essentially is never normally distributed and standard deviation is useless, but I digress...

Would you alert on min size?

[randysecrist]

My thought was to track request size only for something like POST or PUT; since as you say the natural lower bound for a GET or HEAD request would typically be just the header + query string.

In the case of using HTTP for incoming writes; I would only alert if the use case was below the expected minimum; if that is even knowable for a given use case ... and that really depends on the application imo.

[jkschneider]

Sounds like default off with an option to turn it on would be OK then?

[randysecrist]

Ya, I think that would work. Thanks!

[jkschneider]

This would be useful for the StackDriver Range aggregate as well.

[jkschneider]

When implemented, fix min statistic on Azure App Insights implementation as well. #441.

[martel]

This would also be useful while reporting to Cloudwatch where you could use StatisticSets:
https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_StatisticSet.html

Instead of emitting each value separately in the reporter:

micrometer/implementations/micrometer-registry-cloudwatch/src/main/java/io/micrometer/cloudwatch/CloudWatchMeterRegistry.java

Lines 200 to 207 in b604d1d

	Stream.Builder<MetricDatum> metrics = Stream.builder();
	metrics.add(metricDatum(summary.getId(), "sum", summary.totalAmount()));
	long count = summary.count();
	metrics.add(metricDatum(summary.getId(), "count", StandardUnit.Count, count));
	if (count > 0) {
	metrics.add(metricDatum(summary.getId(), "avg", summary.mean()));
	metrics.add(metricDatum(summary.getId(), "max", summary.max()));
	}

[thrxde]

to enable the adjustments for Cloudwatch. i added min value to micrometer. on branch 1.3.x
pullrequest