🔍 Container Monitoring: Becoming a Docker Detective

The Story of the Watchful Guardian

Imagine you’re the guardian of a magical kingdom where tiny houses (containers) pop up and disappear. Your job? Keep an eye on everything! Know when a house is healthy, when it’s sick, and what happened before it vanished.

That’s Container Monitoring! It’s like being a detective who watches over all your Docker containers, making sure they’re happy and healthy.

🏠 The Five Superpowers of a Container Detective

graph TD
    A[🔍 Container Monitoring] --> B[📜 Container Logs]
    A --> C[🚚 Log Drivers]
    A --> D[🔬 Container Inspection]
    A --> E[📊 Stats & Metrics]
    A --> F[🚪 Exit Codes]
    style A fill:#667eea,color:#fff
    style B fill:#4ECDC4,color:#fff
    style C fill:#FF6B6B,color:#fff
    style D fill:#45B7D1,color:#fff
    style E fill:#96CEB4,color:#fff
    style F fill:#FFEAA7,color:#333

📜 Container Logs: The Container’s Diary

What Are Container Logs?

Think of logs like a diary. Every time your container does something, it writes it down!

• Good news? Written in the diary.
• Error happened? Written in the diary.
• Someone connected? Written in the diary.

Reading the Diary

# See all diary entries
docker logs my-container

# Watch diary entries live!
docker logs -f my-container

# Last 10 entries only
docker logs --tail 10 my-container

# Show timestamps (when it happened)
docker logs -t my-container

Real Life Example

# Your web app container's diary
docker logs web-app

# Output might look like:
# [INFO] Server started on port 3000
# [INFO] User logged in: alice
# [ERROR] Database timeout!

Why it matters: When something breaks, the logs tell you what happened!

🚚 Log Drivers: Where Does the Diary Go?

The Problem

Your container writes a diary. But where should it be stored?

• On your computer?
• Sent to a cloud service?
• Written to a special system?

Log Drivers = Delivery Trucks

Think of log drivers as different trucks that carry your diary entries to different places!

graph TD
    A[📜 Container Logs] --> B[🚚 json-file]
    A --> C[🚚 syslog]
    A --> D[🚚 journald]
    A --> E[🚚 awslogs]
    A --> F[🚚 none]
    B --> G[📁 Local File]
    C --> H[📡 Syslog Server]
    D --> I[🐧 Linux Journal]
    E --> J[☁️ AWS CloudWatch]
    F --> K[🗑️ Nowhere]
    style A fill:#667eea,color:#fff

Common Log Drivers

Setting a Log Driver

# Use syslog driver
docker run --log-driver=syslog \
  my-container

# Use json-file with size limit
docker run --log-driver=json-file \
  --log-opt max-size=10m \
  --log-opt max-file=3 \
  my-container

Pro Tip: json-file is the default. It stores logs on your machine in JSON format!

🔬 Container Inspection: X-Ray Vision

What Is It?

docker inspect is like having X-ray vision. You can see EVERYTHING about a container:

• What image it uses
• Its network settings
• Environment variables
• Mounted folders
• And much more!

The Magic Command

# See EVERYTHING
docker inspect my-container

# That's too much! Let's filter:

# Just the IP address
docker inspect -f \
  '{{.NetworkSettings.IPAddress}}' \
  my-container

# Just the state
docker inspect -f \
  '{{.State.Status}}' my-container

What You Can Find

graph TD
    A[🔬 docker inspect] --> B[🌐 Network Info]
    A --> C[💾 Mount Points]
    A --> D[⚙️ Environment Vars]
    A --> E[📦 Image Details]
    A --> F[🏃 Running State]
    style A fill:#45B7D1,color:#fff

Real Example

# Find container's IP address
docker inspect -f \
  '{{.NetworkSettings.IPAddress}}' \
  web-server

# Output: 172.17.0.2

Think of it like this: If docker logs tells you what happened, docker inspect tells you what the container IS!

📊 Container Stats and Metrics: The Health Dashboard

What Are Stats?

Just like your phone shows battery and storage, docker stats shows you:

• CPU usage - How hard the brain is working
• Memory - How much space is used
• Network - Data flowing in and out
• Disk I/O - Reading and writing files

Watching Stats Live

# Live dashboard for ALL containers
docker stats

# Just one container
docker stats my-container

# One snapshot (no live updates)
docker stats --no-stream

What You See

CONTAINER   CPU%    MEM USAGE/LIMIT   NET I/O       BLOCK I/O
web-app     2.50%   150MB / 1GB       5MB / 2MB     10MB / 5MB
database    15.3%   500MB / 2GB       1MB / 500KB   50MB / 20MB

Understanding the Numbers

graph TD
    A[📊 docker stats] --> B[🧠 CPU Usage]
    A --> C[💾 Memory Usage]
    A --> D[🌐 Network I/O]
    A --> E[💿 Disk I/O]
    B --> F{Too High?}
    C --> G{Near Limit?}
    F --> H[⚠️ Slow App]
    G --> I[💥 Crash Risk]
    style A fill:#96CEB4,color:#fff
    style H fill:#FF6B6B,color:#fff
    style I fill:#FF6B6B,color:#fff

🚪 Exit Codes: The Final Message

What Are Exit Codes?

When a container stops, it leaves behind a number. This number tells you WHY it stopped!

Think of it like a goodbye note:

• 0 = “I finished my job perfectly! Bye!”
• Other numbers = “Something went wrong…”

Common Exit Codes

Finding the Exit Code

# See exit code of stopped container
docker inspect -f \
  '{{.State.ExitCode}}' my-container

# Or check container status
docker ps -a
# Shows "Exited (0)" or "Exited (1)"

Exit Code Detective Work

graph TD
    A[Container Stopped] --> B{Exit Code?}
    B --> C[0 = Success ✅]
    B --> D[1 = Error ❌]
    B --> E[137 = Killed 💀]
    B --> F[139 = Crash 💥]
    B --> G[143 = Stopped 🛑]
    D --> H[Check Logs!]
    E --> I[Memory Limit?]
    F --> J[Code Bug?]
    style A fill:#667eea,color:#fff
    style C fill:#4ECDC4,color:#fff
    style D fill:#FF6B6B,color:#fff
    style E fill:#FF6B6B,color:#fff
    style F fill:#FF6B6B,color:#fff
    style G fill:#FFEAA7,color:#333

Real Example

# Container crashed - why?
docker inspect -f \
  '{{.State.ExitCode}}' broken-app

# Output: 137
# Meaning: Docker killed it (probably
# ran out of memory!)

🎯 Putting It All Together

When something goes wrong, follow this detective process:

graph TD
    A[🚨 Problem!] --> B[Check Exit Code]
    B --> C[Read Logs]
    C --> D[Check Stats]
    D --> E[Inspect Container]
    E --> F[🎉 Found the Issue!]
    style A fill:#FF6B6B,color:#fff
    style F fill:#4ECDC4,color:#fff

The Detective’s Toolkit

# Step 1: What's the exit code?
docker inspect -f \
  '{{.State.ExitCode}}' my-app

# Step 2: What do the logs say?
docker logs --tail 50 my-app

# Step 3: What are the stats?
docker stats my-app --no-stream

# Step 4: Deep inspection
docker inspect my-app

🌟 Quick Reference

🚀 You’re Now a Container Detective!

You’ve learned the five superpowers:

1. 📜 Logs - Read the container’s diary
2. 🚚 Log Drivers - Choose where diaries are stored
3. 🔬 Inspection - X-ray vision for containers
4. 📊 Stats - Real-time health dashboard
5. 🚪 Exit Codes - Understand goodbye messages

With these powers, no container mystery can escape you!

Remember: When something breaks, check the exit code → read the logs → check the stats → inspect everything. You’ll find the problem every time!