Gauges: Tracking Metrics that Go Up and Down

In addition to counters, Prometheus also supports gauges. Gauges are used to measure the current value of a metric, which can go up or down over time, in contrast to counters, which only increase.

A gauge is useful for tracking values that can change dynamically, such as the number of active connections to a server or the amount of free memory on a system, but also for tracking application-specific metrics.

In order to use it in our app, we will create two routes that add and remove numbers from a list. The gauge will track the length of the list.

We proceed by defining a gauge to track the size of a list:

# Define a gauge to track the size of a list
list_size_gauge = Gauge(
    'list_size',
    'Size of a list',
    ['list']
)

Then we create the routes (/add and /remove) to add and remove numbers from the list, respectively:

@app.route('/add/')
def add(num1):
    """ Route to add a number to my_list """
    # add num1 to my_list
    my_list.append(num1)
    # set the gauge to the length of my_list
    size = len(my_list)
    list_size_gauge.labels('my_list').set(size)
    # return a message
    return f"Added {num1} to my_list"

@app.route('/remove/')
def remove(num1):
    """ Route to remove a number from my_list """
    # remove num1 from my_list
    my_list.remove(num1)
    # set the gauge to the length of my_list
    size = len(my_list)
    list_size_gauge.labels('my_list').set(size)
    # return a message
    return f"Removed {num1} from my_list"

Run the following command to create the full app (on server1):

cat <<EOF > /prometheus-python-example/using_gauges.py
from flask import Flask, Response
from prometheus_client import (
    Gauge,
    generate_latest,
    CONTENT_TYPE_LATEST
)

# Create a Flask application
app = Flask(__name__)

# Define a gauge to track the size of a list
list_size_gauge = Gauge(
    'list_size',
    'Size of a list',
    ['list']
)

my_list = []

@app.route('/add/')
def add(num1):
    """ Route to add a number to my_list """
    # add num1 to my_list
    my_list.append(num1)
    # set the gauge to the length of my_list

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