🐳 Day 12: Docker Restart Policy – Master It Like a Pro! 🚀

Welcome to this video on Docker Restart Policies! Whether you're a Docker newbie or an intermediate user, this guide will help you understand, practice, and master restart policies to build resilient containers. Let’s dive in! 💡

🎬 Video Demonstration

📘 What is a Docker Restart Policy?

Docker restart policies help you automatically restart containers under certain conditions (like after failure, reboot, etc.) — useful for keeping your services up without manual intervention. Understanding Docker restart policies is crucial for maintaining container uptime and ensuring that services recover from failures automatically.

🔁 Available Restart Policies

📌 Docker Restart Policies (Deep Dive)

Note: For unless-stopped: ✅* It restarts only if it was running before the host reboot or daemon restart.
If you stopped the container manually (docker stop), it will not restart on reboot or daemon restart. on-failure, ✅** indicates the container restarts only if the app exits with a non-zero exit code.

🔧 How to Set Restart Policy

You can set the restart policy using the --restart flag while running a container:

docker run --restart=<policy> [OPTIONS] IMAGE [COMMAND] [ARG...]

🎯 Practice Examples – Let’s Try Them Out!

1. 🚫 no – Default Behavior (No Restart)

This policy is the default setting. With the no restart policy, Docker does nothing if a container stops or crashes.

# Run container
docker run -itd --name no-restart ibraransaridocker/restart-policy-demo
# Above command is similar to below command
docker run -itd --name no-restart --restart=no ibraransaridocker/restart-policy-demo

2. 🔁 always – Restart Forever

When you set a container’s restart policy to always, Docker attempts to restart the container every time it stops, or If it is manually stopped, regardless of the reason. it is restarted only when Docker daemon restarts.

# 📌 Stop it and watch it restart:
docker run -itd --name always --restart=always ibraransaridocker/restart-policy-demo
docker stop always
docker ps -a | grep always
docker rm -f always

# 📌 Kill it and watch it restart:
docker run -itd --name always --restart=always ibraransaridocker/restart-policy-demo
docker kill always 
docker ps -a | grep always
docker rm -f always

# 📌 App crash (`exit`) and watch it restart:
docker run -itd --name always --restart=always ibraransaridocker/restart-policy-demo
docker exec always kill 1
docker inspect always --format='{{.RestartCount}}'
docker ps -a | grep always
docker rm -f always

# 📌 Daemon Restart
docker run -itd --name always --restart=always ibraransaridocker/restart-policy-demo
docker ps -a | grep always
sudo systemctl restart docker
docker ps -a | grep always
docker rm -f always

# 📌 Even after reboot:
docker run -itd --name always --restart=always ibraransaridocker/restart-policy-demo
docker ps -a | grep always
sudo init 6
docker ps -a | grep always
docker rm -f always

3. ⚙️ unless-stopped – Persistent Until You Stop It

The unless-stopped policy tells Docker to keep restarting the container until a developer forcibly stops it. It's similar to always, but it respects intentional stops. it is not restarted even after Docker daemon restarts.

# Run Container
docker run -itd --name unless --restart=unless-stopped ibraransaridocker/restart-policy-demo

# ✔️ This will restart unless you explicitly stop it with:
docker stop test-unless

Then restart your system — it won’t restart again unless you start it manually.

4. ❌ on-failure – Conditional Restart Based on Exit Code

The on-failure policy instructs Docker to restart the container only if it exits with an error code (non-zero exit status). Restart the container if it exits due to an error.

# Run container
docker run -itd --name failure --restart=always ibraransaridocker/restart-policy-demo

# 📌 App crash (`exit`) and watch it restart:
docker exec failure kill 1
docker ps -a | grep failure

# 📌 It will try to restart 3 times, then stop.
Check restart count:
docker inspect test-failure --format='{{.RestartCount}}'

📝 Container’s Restart Policy in Production(Check and update Policy)

# You can inspect any container’s restart policy with:
docker inspect <container_name> --format='{{.HostConfig.RestartPolicy.Name}}'

# Check all containers restart policy
docker inspect --format '{{.Name}} Restart: {{ .HostConfig.RestartPolicy.Name }}' $(docker ps -aq)

# Reset restart  single container
docker update --restart=no <container_name_or_id>

# Reset restart  All container
docker update --restart=no $(docker ps -q)

# Set all restart always
docker update --restart=always $(docker ps -aq)

🧼 Clean Up (After Practice)

docker rm -f $(docker ps -aq)

Choosing the Right Restart Policy

Here are some considerations for choosing the right restart policy:

• Development vs. Production: In a development environment, you might prefer no to debug issues, while in production, always or on-failure could be more appropriate.

• Stateless vs. Stateful Containers: Stateless containers can often use always, but stateful containers might need a more nuanced approach like on-failure.

• Dependent Services: For containers that depend on other services, on-failure ensures they don't restart in a loop if a dependency is unavailable.

✅ Best Practices

• Use always for critical services that should never go down.

• Use unless-stopped for services you want to auto-restart, but still be able to manually shut down.

• Use on-failure for short-lived jobs or scripts that may fail and need retry.

• Avoid no unless you're doing one-off or testing containers.

🔗 Additional Resources

• 📘 Docker Policy