Strictly increasing monotonicity

Skyline specifically handles positive, strictly increasing monotonic timeseries. These are metrics that are integral (an incrementing count) in Graphite and converts the values to their derivative products.

In terms of visualisation Graphite has the nonNegativeDerivative function, which converts an integral or incrementing count to a derivative by calculating the delta between subsequent datapoints. The function ignores datapoints that trend down. With Skyline’s new functionality this is a requirement for metrics that only increase over time and then at some point reset to 0 (e.g. interface packets counts, varnish or haproxy backend connections, etc).

By default, Skyline now identifies and converts monotonically increasing timeseries to their derivative products through most of the Skyline analysis apps (Analyzer, Mirage and Ionosphere).

Unless you tell Skyline not to. You may have some monotonically increasing metric that you do not want to be forced to convert to its derivative product just because it is more convenient for Skyline to analyse, be taught and learn the derivatives. You may have a metric that does not reset and want analysed the way it used to be analysed in the old days as you feel it worked, that is OK, you can, using settings.NON_DERIVATIVE_MONOTONIC_METRICS

However, Skyline will automatically classify metrics into derivative_metrics (strictly positive monotonic metrics) and non_derivative_metrics. This process has been automated so that new metrics of this nature will be identified and processed via their derivative if a new namespace of this nature should to added without the requirement for each namespace pattern to be declared in the settings.

Further metrics identified as derivative_metrics continue to be identified as such even if their metric changes from a strictly increasing monotonic nature, like gets reset to zero. Skyline sets a Redis z.derivative_metric key at an expiry time of settings.FULL_DURATION to handle counter resets. However there are some edge case that will not become strictly increasing again until say a mysql.commands.create_table is run. So on very slowly changing metrics a derivative metric may be analyzed as a non_derivative_metric until such a point as it reverts to a strictly increasing monotonic nature again. However seeing as it would just revert for a period back to how these types of metrics were handled in Skyline for years now, which were not wrong, the method was just not conducive to profiling and learning, its deemed a caveat, rather than a bug as if any metrics of this extreme nature can be handled via SKIP or using settings.NON_DERIVATIVE_MONOTONIC_METRICS

Really why is this important?

If you are still reading…

With the original Analyzer/Mirage, single time resolution and 3-sigma based algorithms, these types of metrics with strictly increasing monotonicity that reset always triggered 3-sigma and alerts (false positives). That is not to say that they were not effective once the metric had settings.FULL_DURATION seconds of timeseries data, while it was sending the incrementing data 3-sigma would work, until it reset :) So it worked most of time and falsely some of the time. Not ideal.

Widening the range of the time resolutions that Skyline operates in with Mirage and Ionosphere requires that this class of metrics be converted otherwise they are limited in terms of the the amount of seasonal analysis, profiling and learning that can be done on this type of timeseries data.

So Skyline now analyses all monotonic metrics via their derivatives as these work in terms of more historical analysis, features calculations and comparison, and learning patterns due to the step nature of the data and the variability of the steps, in terms of profiling features these type of data are almost like snowflakes. The derivatives of these strictly increasing metrics are the kind of patterns that lend themselves to Skyline’s timeseries comparison and analysis methods.

The Skyline nonNegativeDerivative is based on part of the Graphite render function nonNegativeDerivative at: