05. Recording Rules

Time series queries can quickly become quite complicated to remember and type using the Expression Browser in the default Prometheus User Interface.

Example query

node_memory_MemFree_bytes

node_memory_MemFree_bytes / node_memory_MemTotal_bytes

100 - (100 * node_memory_MemFree_bytes / node_memory_MemTotal_bytes)

Rather than remembering and typing this query every time, we can create a recording rule that will run at a chosen interval and make the data available as a time series.

Recording Rule Example 1

cd into the /usr/local/bin/prometheus folder

cd /usr/local/bin/prometheus Create a new file called prometheus_rules.yml

sudo vi prometheus_rules.yml Add our test expression as a recording rule

     - name: custom_rules
       rules:
         - record: node_memory_MemFree_percent
           expr: 100 - (100 * node_memory_MemFree_bytes / node_memory_MemTotal_bytes)

Save it and we can now verify the syntax is ok.

Prometheus now comes with a tool called Promtool which you can use to check your rules files and other things.

We will check our rules file is ok.

./promtool check rules prometheus_rules.yml

The response should contain SUCCESS otherwise there was a problem with your prometheus_rules.yml file.

Now lets add the prometheus_rules.yml reference to the prometheus.yml rule_files section.

     - "prometheus_rules.yml"

and restart the prometheus service.

sudo service prometheus restart
sudo service prometheus status

Refresh the Prometheus user interface and check the drop down and you will be able to see node_memory_MemFree_percent

Recording Rule Example 2

Lets do another more complicated example.

Update our prometheus_rules.yml file with,

groups:
  - name: custom_rules
    rules:
      - record: node_memory_MemFree_percent
        expr: 100 - (100 * node_memory_MemFree_bytes / node_memory_MemTotal_bytes)

      - record: node_filesystem_free_percent
        expr: 100 * node_filesystem_free_bytes{mountpoint="/"} / node_filesystem_size_bytes{mountpoint="/"}

Check it with the promtool

./promtool check rules prometheus_rules.yml

If all is ok, restart the prometheus service.

sudo service prometheus restart
sudo service prometheus status

Refresh the Prometheus user interface and check the drop down for node_filesystem_free_percent