We looked previously at the counter, how does the Prometheus gauge work?
The Prometheus gauge is essentially the same simple idea as gauges in other monitoring systems. Gauges can go up and down over time, and scrapes take a snapshot of the current value. There's no potential for messing around with moving averages or resets, as there is with counters.
Where Prometheus differs from other other monitoring systems is the API for gauges in instrumentation. The Prometheus Gauge API covers all the gauge uses cases in one place. It has inc, dec, and set methods, plus various utility functions for common uses such as setting the current time. The client library takes care of all the bookkeeping and thread safety for you. There's usually also some way to do a callback such as set_function in Python, setChild in Java, and Go has GaugeFunc.
By contrast the popular Dropwizard metric library offers a number of different gauge classes, primarily based on what processing you'd like it to perform before exposing the gauge. There are two classes I'd consider core, as they don't do any additional processing. The Dropwizard Gauge has a callback-based API. The Dropwizard Counter has inc and dec methods, which actually makes it a gauge in Prometheus terms. The uses cases for other Dropwizard gauge types are handled differently in Prometheus, the Dropwizard Meter would be a Prometheus Counter combined with the processing done with the PromQL rate function for example.
There is one other difference with many other monitoring and instrumentation systems. As is true throughout Prometheus, full 64bit floating point values are supported.
PromQL offers a number of functions and operators to deal with gauges. In fact except for the handful of functions that are for use solely with counters (such as rate), all functions can be sanely used with a gauge. Functions of note include deriv to calculate the value's rate of change using simple linear regression, and predict_linear which takes that a step further and projects into the future. Aggregation across series is possible with operators like max, min and avg and aggregation across time is possible with functions such as max_over_time, min_over_time and avg_over_time.
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