Docker Container Lifecycle (Create, Run, Stop, Exit)

In production, one of the most common alerts is:

“Container stopped unexpectedly”

For beginners, this feels like an error. For DevOps engineers, this is normal behavior.

Let’s understand the Docker container lifecycle using real-world scenarios.

🔄 Container States Explained (Practically)

1️⃣ Created

The container exists but hasn’t started yet.

Real project scenario:

• CI/CD pipeline creates a container
• Deployment step fails before starting it

docker create nginx
docker ps -a

You’ll see the container in Created state.

2️⃣ Running

The container’s main process is running.

Real project scenario:

• Application is serving traffic
• Health checks are passing

docker run -d nginx
docker ps

📌 Important rule A container lives only as long as its main process runs.

3️⃣ Stopped

The container was stopped gracefully.

Real project scenario:

• Deployment rollout
• Maintenance window
• Scaling down services

docker stop <container_id>

Docker sends SIGTERM and waits before stopping the container.

This allows:

• Graceful shutdown
• Request completion
• Cleanup tasks

4️⃣ Exited

The container’s process ended or crashed.

Real project scenarios:

• App crashed due to missing environment variables
• Wrong CMD or ENTRYPOINT
• Port already in use
• App finished its job (batch job)

docker ps -a

Example output:

Exited (0)   → normal exit
Exited (1)   → error exit

📌 Exited ≠ always failure

❓ Why Containers Stop Unexpectedly (Real Reasons)

In real systems, containers stop because:

• Main process crashed
• App finished execution
• Config error
• Dependency missing
• OOM (out of memory)

Docker is doing exactly what it’s designed to do.

🛠️ Debugging Stopped Containers (Real DevOps Flow)

When a container stops:

Step 1: Check logs

docker logs <container_id>

Step 2: Check exit code

docker inspect <container_id>

Step 3: Verify

• CMD / ENTRYPOINT
• Environment variables
• Resource limits (CPU / memory)

This is the daily debugging flow in real projects.

⚔️ Docker stop vs Docker kill (Very Important)

docker stop

• Sends SIGTERM
• Graceful shutdown
• Used in deployments

docker kill

• Sends SIGKILL
• Immediate termination
• Used when container is unresponsive

Real rule:

Always try stop first. Use kill only as last option.

🎯 Interview Alignment

Question: Why does a Docker container stop?

Strong answer: A Docker container runs as long as its main process is running. When the process exits or crashes, the container stops. This is expected behavior and logs help identify the reason.

🧠 Day 5 Key Takeaways

• Container states are normal
• Exited containers are expected
• Logs explain why a container stopped
• stop ≠ kill
• Lifecycle knowledge is critical in production

👉 Next (Day 6): Dockerfile Basics — FROM, RUN, COPY, CMD (with real mistakes)