mirror of
https://github.com/prometheus/prometheus.git
synced 2024-11-14 09:34:05 -08:00
138 lines
5.4 KiB
Markdown
138 lines
5.4 KiB
Markdown
# Introduction
|
|
|
|
Prometheus is in the business of doing several things:
|
|
|
|
* exposing metrics,
|
|
* collecting metrics,
|
|
* performing computational rules upon them and generating synthetic series,
|
|
* triggering events based upon rule conditions, and
|
|
* metric series archival and deletion.
|
|
|
|
The focus of this guide is oriented exclusively toward exposing metrics with
|
|
an eye toward computational rules and event handling in the Prometheus core.
|
|
|
|
After reading this guide, you will understand the following:
|
|
|
|
* what types of Prometheus metrics are available and how they work,
|
|
* how you should embed these metrics in your applications,
|
|
* what metadata you attach to them, and
|
|
* what to instrument.
|
|
|
|
# Metric Types
|
|
|
|
Prometheus offers three core metric types:
|
|
|
|
* counters,
|
|
* gauges, and
|
|
* summaries.
|
|
|
|
Each is tailored toward a different purpose, so it is important to use the
|
|
right one for the right job.
|
|
|
|
## Counter
|
|
|
|
A _Counter_ tracks addition or subtraction of values from itself. It is used
|
|
to represent state that you control and is not external.
|
|
|
|
The following are good examples or applications of counters:
|
|
|
|
* pedometers: devices that track the number of steps you take,
|
|
* tickers: devices that measure the number of times an event has occurred,
|
|
* censuses: processes to count to number of individuals of a population, and
|
|
* _your_ bank account ledger: the sum of all transactions conducted on your
|
|
liquid assets.
|
|
|
|
The key thing to note about _Counters_ is that they are best used when you are
|
|
directly receiving the events that mutate their values and mutating the
|
|
underlying values that represent their individual state.
|
|
|
|
A good example is of an elevator that tracks how many times it has been used in
|
|
its duty cycle. Each time someone hits the button to dispatch it, the elevator
|
|
can update its tally; and this is a tally it keeps!
|
|
|
|
See also the [counter Javadoc](https://github.com/prometheus/client_java/blob/master/client/src/main/java/io/prometheus/client/metrics/Counter.java)
|
|
in Prometheus' Java client library for further documentation and concrete code
|
|
examples.
|
|
|
|
## Gauges
|
|
|
|
A _Gauge_ reports instantaneous values based on external state. This is
|
|
primarily state that you do not control directly but rather that you depend on
|
|
or can inspect.
|
|
|
|
The following are good examples or applications of gauges:
|
|
|
|
* thermometer,
|
|
* barometer,
|
|
* altimeter, and
|
|
* total system resource utilization.
|
|
|
|
Unlike a _Counter_, you don't have direct access to the events that brought the
|
|
_Gauge_ to its value today. This is a white-box and black-box distinction.
|
|
|
|
See also the [gauge Javadoc](https://github.com/prometheus/client_java/blob/master/client/src/main/java/io/prometheus/client/metrics/Gauge.java)
|
|
in Prometheus' Java client library for further documentation and concrete code
|
|
examples.
|
|
|
|
## Which to use: Counter or Gauge?
|
|
Knowing whether to use a _Counter_ or a _Gauge_ can be tough. Let's perform a
|
|
practical thought exercise. Below is an image of an automobile's instrument
|
|
dashboard:
|
|
|
|
|
|
|
|
What do you see in this photo? Take a minute to make a list. (Don't worry;
|
|
I'll wait for you.)
|
|
|
|
From left to right and top to bottom, there are the following instruments:
|
|
|
|
* Tachometer: it measures the revolutions per minute of the engine;
|
|
* Clock: it measures the time;
|
|
* Thermometer: it measures the temperature outside the vehicle;
|
|
* Odometer: they measure the total miles the vehicle has traveled as well as
|
|
how far it has traveled on its last trip;
|
|
* Fuel Gauge: it measures how much fuel is in the tank;
|
|
* Speedometer: it measures how fast we are going.
|
|
|
|
Great! Look at this list and now think about the definitions above for
|
|
_Counter_ and _Gauge_. Can you identify what instruments are what? (Again,
|
|
I'll wait for you.)
|
|
|
|
* Tachometer: Counter. Tricky. The vehicle's sensors observe how quickly
|
|
the flywheel rotates and update the instrument's value on frequent
|
|
intervals by counting revolutions.
|
|
* Clock: Gauge. It is an external value, and we cannot watch the clock's
|
|
internal events.
|
|
* Thermometer: Gauge. It measures an external value, and there is no way to
|
|
track thermodynamic events that influence the outside temperature.
|
|
* Odometer: Counter. It measures distances by the number of times the
|
|
wheels have turned, which itself is based on an event.
|
|
* Fuel Gauge: Gauge. Who would have guessed? It uses a floating ballast and
|
|
a rheostat and is not event-based.
|
|
* Speedmeter: Counter. Tricky. It measures wheel rotation (events) over
|
|
time.
|
|
|
|
## Summaries
|
|
|
|
_Summary_ is a unique metric type, and one you have probably not used before.
|
|
It samples events over sliding windows of time and provides instantaneous
|
|
insight into their distributions, frequencies, and sums.
|
|
|
|
Practically speaking, a summary collects event values and reports the
|
|
following information:
|
|
|
|
* streaming quantile values of the entire observed population,
|
|
* the total sum of all observed values, and
|
|
* the count of events that have been observed.
|
|
|
|
This is quite convenient, for if you are interested in tracking latencies of an
|
|
operation in real time, you get three types of information reported for free
|
|
with one metric.
|
|
|
|
See also the [summary Javadoc](https://github.com/prometheus/client_java/blob/master/client/src/main/java/io/prometheus/client/metrics/Summary.java)
|
|
in Prometheus' Java client library for further documentation and concrete code
|
|
examples.
|
|
|
|
# Acknowledgements
|
|
Robert Bray: Vehicle Instrument Panel
|