chore: improved prometheus' kb article

knowledge base/prometheus.md

@@ -1,12 +1,82 @@
 # Prometheus
 
-Monitoring system that collects metrics from configured targets at given intervals, evaluates rule expressions, displays the results, and can trigger alerts when specified conditions are observed.
+Monitoring and alerting system that collects metrics from configured targets at given intervals, evaluates rule expressions, displays the results, and can trigger alerts when specified conditions are observed.<br/>
+Metrics can also be pushed using plugins, in the event hosts are behind a firewall or prohibited from opening ports by security policy.
 
 ## Table of contents <!-- omit in toc -->
 
+1. [Components](#components)
+   1. [Extras](#extras)
+1. [Configuration](#configuration)
+1. [Queries](#queries)
 1. [Further readings](#further-readings)
 1. [Sources](#sources)
 
+## Components
+
+Prometheus is composed by its **server**, the **Alertmanager** and its **exporters**.
+
+Alerting rules can be created within Prometheus, and configured to send custom alerts to _Alertmanager_.<br/>
+Alertmanager then processes and handles the alerts, including sending notifications through different mechanisms or third-party services.
+
+The _exporters_ can be libraries, processes, devices, or anything else exposing metrics so that they can be scraped by Prometheus.<br/>
+Such metrics are usually made available at the `/metrics` endpoint, which allows them to be scraped directly from Prometheus without the need of an agent.
+
+### Extras
+
+As welcomed addition, [Grafana] can be configured to use Prometheus as a backend of its in order to provide data visualization and dashboarding functions on the data it provides.
+
+## Configuration
+
+The default configuration file is at `/etc/prometheus/prometheus.yml`.
+
+```yml
+global:
+  scrape_interval: 15s
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: [ 'localhost:9090' ]
+  - job_name: nodes
+    static_configs:
+      - targets:
+          - fqdn:9100
+          - host.local:9100
+```
+
+## Queries
+
+Prometheus' query syntax is [PromQL].
+
+All data is stored as time series, each one identified by a metric name, e.g. `node_filesystem_avail_bytes` for available filesystem space.<br/>
+Metrics' names can be used in the expressions to select all of the time series with this name and produce an **instant vector**.
+
+Time series can be filtered using selectors and labels (sets of key-value pairs):
+
+```promql
+node_filesystem_avail_bytes{fstype="ext4"}
+node_filesystem_avail_bytes{fstype!="xfs"}
+```
+
+Square brackets allow to select a range of samples from the current time backwards:
+
+```promql
+node_memory_MemAvailable_bytes[5m]
+```
+
+When using time ranges, the vector returned will be a **range vector**.
+
+[Functions] can be used to build advanced queries:
+
+```promql
+100 * (1 - avg by(instance)(irate(node_cpu_seconds_total{job='node_exporter',mode='idle'}[5m])))
+```
+
+![advanced query](prometheus%20advanced%20query.png)
+
+Labels are used to filter the job and the mode. `node_cpu_seconds_total` returns a **counter**, and the irate() function calculates the **per-second rate of change** based on the last two data points of the range interval.<br/>
+To calculate the overall CPU usage, the idle mode of the metric is used. Since idle percent of a processor is the opposite of a busy processor, the irate value is subtracted from 1. To make it a percentage, it is multiplied by 100.
+
 ## Further readings
 
 - [Website]
@@ -15,11 +85,13 @@ Monitoring system that collects metrics from configured targets at given interva
 - [Node exporter]
 - [SMNP exporter]
 - [`ordaa/boinc_exporter`][ordaa/boinc_exporter]
+- [Grafana]
 
 ## Sources
 
 All the references in the [further readings] section, plus the following:
 
+- [Getting started with Prometheus]
 - [Node exporter guide]
 - [SNMP monitoring and easing it with Prometheus]
 - [`prometheus/node_exporter`][prometheus/node_exporter]
@@ -30,10 +102,12 @@ All the references in the [further readings] section, plus the following:
   -->
 
 <!-- Upstream -->
+[functions]: https://prometheus.io/docs/prometheus/latest/querying/functions/
 [github]: https://github.com/prometheus/prometheus
 [node exporter guide]: https://prometheus.io/docs/guides/node-exporter/
 [prometheus/node_exporter]: https://github.com/prometheus/node_exporter
 [prometheus/snmp_exporter]: https://github.com/prometheus/snmp_exporter
+[promql]: https://prometheus.io/docs/prometheus/latest/querying/basics/
 [website]: https://prometheus.io/
 
 <!-- In-article sections -->
@@ -48,5 +122,6 @@ All the references in the [further readings] section, plus the following:
 [docker/monitoring]: ../docker/monitoring/README.md
 
 <!-- Others -->
+[getting started with prometheus]: https://opensource.com/article/18/12/introduction-prometheus
 [ordaa/boinc_exporter]: https://gitlab.com/ordaa/boinc_exporter
 [snmp monitoring and easing it with prometheus]: https://medium.com/@openmohan/snmp-monitoring-and-easing-it-with-prometheus-b157c0a42c0c