Docker: A To Z + Hands On Lab! Part 2

Troubleshooting

1 – 🧾 Docker Logs (In Short)

Docker logs let you see the output (stdout/stderr) from a running container — like error messages, server output, etc.

🔹 Basic Command:

docker logs <container_name_or_id>

🔹 Examples:

docker logs my-mysql

Shows logs from the my-mysql container.

🔧 Useful Options:

-f → Follow logs (like tail -f) docker logs -f my-app
--tail → Show last N lines docker logs --tail 50 my-app
Combine both: docker logs -f --tail 100 my-app

✅ Use Case:

Debugging errors
Checking service output
Monitoring container activity

View logs on VS code terminal

View logs on Docker Desktops

2nd

exec errors Logs – Use Wimpty

You’re on Git Bash + Windows, and it’s messing up the path.

Git Bash is rewriting /bin/sh or /bin/bash into a Windows path like:

C:/Program Files/Git/usr/bin/sh

which Docker doesn’t understand.

If Not Then try this below

✅ Fix (Use winpty)

Run the command with winpty in Git Bash:

winpty docker exec -it mysql-latest sh

If that doesn’t work, try:

winpty docker exec -it mysql-latest bash

🧠 Why it works:

winpty fixes the interactive terminal handling on Windows + Git Bash.

Docker Vs VM

🧾 Docker vs Virtual Machine (VM) — In Short

Feature	Docker (Containers)	Virtual Machines (VMs)
Speed	🏎️ Fast startup (~seconds)	🐢 Slower (~minutes)
Lightweight	✅ Yes (shares OS kernel)	❌ Heavy (full OS per VM)
Resources	Uses less CPU/RAM	Uses more CPU/RAM
Isolation	Process-level isolation	Full OS-level isolation
Portability	High (same image runs anywhere)	Medium (depends on OS/Hypervisor)
Boot OS?	❌ No (uses host OS kernel)	✅ Yes (each VM has its own OS)

✅ Docker Use Case

Microservices
Dev/testing environments
Lightweight apps

✅ VM Use Case

Run different OS (e.g., Linux on Windows)
Full system simulation
Legacy apps requiring full OS

💡 TL;DR:
Docker = Lightweight, fast, app-level.
VM = Heavy, full OS, better isolation.

Note : Docker relies on the Docker Desktop application to run images and containers on Windows and macOS systems. Thats why we use this softwere on windows and MacOS

🧾 Docker Desktop = Combo of Tools + VM (on Windows/macOS)

🧠 On Windows/macOS, Docker Desktop:

Runs a lightweight VM behind the scenes using Hyper-V (Windows) or Apple Hypervisor/VirtualBox (macOS)
This VM runs Linux, where Docker Engine lives
So yes — it acts like a hypervisor to enable Docker on non-Linux systems

💡 So:

Platform	How Docker Runs
Linux	Native (no VM needed)
Windows/macOS	Via Docker Desktop → Runs Linux VM

✅ Why This Matters:

On Windows/macOS, Docker Desktop is essential to simulate a Linux environment (since Docker uses Linux kernel features like cgroups & namespaces).
Docker CLI talks to the Linux VM in the background.

Dev With Docker

Network

🧾 What is Docker Network?

Docker networking lets containers talk to each other, and optionally to the outside world (your host, internet, etc.).

🔌 Types of Docker Networks:

Network Type	Description
bridge (default)	Default network for containers; containers can talk to each other using container names.
host	Container shares the host’s network (no isolation). Faster, but less secure.
none	No networking at all (totally isolated).
custom bridge	Like bridge, but you create and control it. Enables easier name-based communication between containers.
overlay	For multi-host Docker (Swarm); allows containers across machines to communicate.

🧪 Example: Custom Network

docker network create my-net

Then run two containers:

docker run -d --name app1 --network my-net nginx
docker run -d --name app2 --network my-net alpine sleep 9999

Now app2 can ping app1:

docker exec -it app2 ping app1

✅ It works because they share the same network.

🎯 Use Cases:

Link app and database containers
Control traffic between services
Enable service discovery by container name

DB Setup

Setting up multiple containers via terminal gets messy—port bindings, environment variables, custom names—it’s easy to make mistakes.

Setting up mongo and mongo-express via docker command in terminal
(complicated + not recommended)

type this in your terminal to setup mongo database

docker  run -d \
-p27017:27017 \
--name mongo \
--network mongo-network \
-e MONGO_INITDB_ROOT_USERNAME: admin \
-e MONGO_INITDB_ROOT_PASSWORD: qwerty \
mongo

DOCKER COMPOSE (recommended method ) .yaml

yet another markup language = yaml

Why Use Docker Compose?

Docker Compose solves this by letting you define everything in a single .yml file. It’s a tool for managing multi-container apps in a structured, easy-to-read way.

✅ Clean and organized
✅ Easy to edit and reuse
✅ No more long terminal commands

Just run docker-compose up and you’re good to go!

or run commands in image shown!

Docker command Vs Docker Compose

✅ `.yaml` vs `.yml`

There is no difference between .yaml and .yml — both are valid YAML file extensions.

📝 Why two options?

.yml existed because early systems (like Windows) had issues with 4-letter extensions.
.yaml is the official extension (as per YAML spec).

👍 Use either:

.yaml is preferred in modern tools and docs.
But Docker, GitHub Actions, etc., support both.

✅ Step-by-Step Verification of MongoDB via Mongo Express

1. Start Your Containers

Make sure you’re in the directory where your docker-compose.yml file is, then run:

docker-compose up -d

This will start:

mongo (your database)
mongo-express (your web UI)

2. Visit Mongo Express in Browser

Go to:
👉 http://localhost:8081

You should see something like this:

🔒 Login Prompt

Mongo Express will auto-login if the environment variables are correct.

If it prompts for username/password, it uses the values from your docker-compose.yml:

ME_CONFIG_MONGODB_ADMINUSERNAME: admin
ME_CONFIG_MONGODB_ADMINPASSWORD: qwerty

So use:

Username: admin
Password: qwerty or pass

📸 Sample Screenshots (Visual Guide)

1. Mongo Express Dashboard

You should see:

Your MongoDB server connected
List of databases (e.g., admin, local)
Options to view collections, insert documents, etc.

2. If Login Fails

If it fails to connect:

Make sure mongo and mongo-express are in the same Docker network (which they are by default in docker-compose)
Double-check your docker-compose.yml indentation (YAML is picky!)
Ensure you’re using correct ports: 8081 on your host, mapped to 8081 in the container

Fix This : http://localhost:5050/getUsers

its for to verify database entries

🛑 To Stop & Remove Containers from a YAML File

From the same directory where your docker-compose.yml lives, run:

docker-compose down

This will:

Stop all containers started with docker-compose up
Remove those containers
But it won’t delete volumes or images unless you ask it to

Optional Flags:

🔥 To Also Remove Volumes (databases/data):

docker-compose down -v

🧼 To Also Remove Images:

docker-compose down --rmi all

📁 If You’re Asking How to “Download” a YAML File

If you meant download a sample YAML file, here’s a simple one you can copy and save as docker-compose.yml:

version: "3.8"
services:
  mongo:
    image: mongo
    ports:
      - "27017:27017"
    environment:
      MONGO_INITDB_ROOT_USERNAME: admin
      MONGO_INITDB_ROOT_PASSWORD: qwerty

  mongo-express:
    image: mongo-express
    ports:
      - "8081:8081"
    environment:
      ME_CONFIG_MONGODB_ADMINUSERNAME: admin
      ME_CONFIG_MONGODB_ADMINPASSWORD: qwerty
      ME_CONFIG_MONGODB_URL: mongodb://admin:qwerty@mongo:27017/

Save this in your project folder as docker-compose.yml.