📘 Understanding Prometheus in a Simple Way-Part 3 (For DevOps Beginners)

 In our previous blogs, we have already covered:

Today, we will move one step ahead and learn Prometheus, which will collect the metrics generated by cAdvisor.

🔍 What Is Prometheus?

Before understanding Prometheus, let’s talk about how data is stored in databases.

✔️ Relational Databases (MySQL, PostgreSQL)

Data is stored in rows and columns — like an Excel sheet.

✔️ Prometheus (TSDB — Time Series Database)

Prometheus does NOT store data in tables.
 It stores data in Time Series format.

⏱️ What is Time Series Data? (TSDB Explained in Simple Words)

Time Series means:

Data that changes with time and is stored along with timestamps.

Imagine a graph of a YouTube channel’s growth:

  • In 2024, you create the channel → output is low
  • In 2025, views/subscribers increase → output goes up
  • Time goes forward, and output keeps changing

Vertical axis = output
 Horizontal axis = time

This type of continuously changing data is time series data, and a database that stores it is called TSDB.

👉 Prometheus = Time Series Database (TSDB)
 👉 It stores metrics with timestamps.

📡 How Metrics Flow in Our Setup

Here is the flow:

cAdvisor  →  Prometheus  →  TSDB (Prometheus internal storage)
  • cAdvisor collects metrics from Docker containers
  • Prometheus scrapes/collects those metrics
  • Prometheus stores them inside its TSDB
  • We can query these metrics using PromQL

Project Link: https://github.com/Raeesqazi/observability-for-devops

⚙️ Prometheus Installation (Docker Compose)

Below is the configuration we use inside docker-compose.yml:

prometheus:
  image: prom/prometheus:latest
  container_name: Prometheus
  ports:
    - 9090:9090
  command:
    - --config.file=/etc/prometheus/prometheus.yml
  volumes:
    - ./prometheus.yml:/etc/prometheus/prometheus.yml:ro
  depends_on:
    - cadvisor
  networks:
    - monitoring

Explanation (Easy Words):

image: prom/prometheus:latest Pull the latest Prometheus image

ports: 9090:9090 Prometheus runs on port 9090

command: --config.file=... Prometheus always needs a config file volumes: Copy our local prometheus.yml into the container

depends_on: cadvisor Start Prometheus only after cAdvisor

networks: Both run on the monitoring network

📄 Understanding prometheus.yml File

This file defines HOW Prometheus will scrape the metrics.

# my global config
global:
  scrape_interval: 15s # Set the scrape interval to every 15 seconds. Default is every 1 minute.
  evaluation_interval: 15s # Evaluate rules every 15 seconds. The default is every 1 minute.
  # scrape_timeout is set to the global default (10s).

# Alertmanager configuration
alerting:
  alertmanagers:
    - static_configs:
        - targets:
          # - alertmanager:9093

# Load rules once and periodically evaluate them according to the global 'evaluation_interval'.
rule_files:
  # - "first_rules.yml"
  # - "second_rules.yml"

# A scrape configuration containing exactly one endpoint to scrape:
# Here it's Prometheus itself.
scrape_configs:
  # The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.
  - job_name: "prometheus"

    # metrics_path defaults to '/metrics'
    # scheme defaults to 'http'.

    static_configs:
      - targets: ["localhost:9090"]
       # The label name is added as a label `label_name=<label_value>` to any timeseries scraped from this config.
        labels:
          app: "prometheus"

Simple Explanation:

🔹 scrape_interval: 15s

Prometheus will collect metrics every 15 seconds.

🔹 scrape_configs:

Here you tell Prometheus which target to scrape.

🔹 targets: ["localhost:9090"]

Prometheus will collect its own metrics.

Later, we will add:

cadvisor:8080

So Prometheus can collect container metrics from cAdvisor.

🛠 Practical Work (Commands You Use)

1. Download Default Prometheus Config

wget https://raw.githubusercontent.com/prometheus/prometheus/main/documentation/examples/prometheus.yml

2. Edit docker-compose.yml

vim docker-compose.yml

(Add Prometheus service as shown earlier)
 Save file → :wq

3. Restart Docker Compose

docker compose down
docker compose up -d

You will now see 4 containers running:

  1. Redis
  2. Notes-App
  3. cAdvisor
  4. Prometheus

Check running containers:

docker ps

🌐 Access Prometheus

  1. Go to AWS EC2 → Security Groups
  2. Open inbound port 9090
  3. Enter in browser:
http://<YOUR-EC2-IP>:9090

You will see the Prometheus dashboard 🎉.

Prometheus is now live and collecting metrics!


🙏 Final Words

I hope this blog helped you understand Prometheus in a simple and clear way.
 In upcoming blogs, we will connect cAdvisor → Prometheus → Grafana to build a complete monitoring dashboard.

Please share and subscribe to support the content!
 Stay tuned for the next part.

🌐 Online References


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