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Docker Events & Health Checks

+15 XP on completion

#Docker Events & Health Checks

After this lesson you'll know:

how to monitor the daemon with docker events
how healthchecks auto-heal containers
what docker inspect reveals about container state
how to automate monitoring

#docker events — the daemon live feed

The Docker daemon emits events for every action. You can watch them in real time:

# All events live
docker events

# Filter for specific events
docker events --filter 'type=container' --filter 'event=start'
docker events --filter 'event=die' --filter 'event=oom'

# As JSON for scripting
docker events --format '{{json .}}'

Start a new container in a second terminal — can you see the events in the first terminal?

#Healthchecks — the self-healing mechanism

A Healthcheck regularly tests if a container is healthy. Docker marks the container as healthy or unhealthy — and Swarm automatically replaces unhealthy containers:

# In your Dockerfile (permanent)
FROM node:20-alpine
HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
  CMD curl -f http://localhost:3000/health || exit 1
CMD ["node", "server.js"]

# Or at container start
docker run \
  --health-cmd='curl -f http://localhost/health || exit 1' \
  --health-interval=10s \
  --health-retries=3 \
  --health-start-period=15s \
  nginx

# Check status
docker inspect --format='{{.State.Health.Status}}' container-name
# → healthy / unhealthy / starting

#docker inspect — the Swiss Army Knife

docker inspect gives you all details about a container, image, volume, or network as JSON:

# Container details
docker inspect container-name

# Network config only
docker inspect --format='{{json .NetworkSettings}}' container-name

# Mount status
docker inspect --format='{{json .Mounts}}' container-name

# State (running, exit code, restart count)
docker inspect --format='{{json .State}}' container-name

# Health history
docker inspect --format='{{json .State.Health}}' container-name

#Automation & Alerting

With Events + Healthchecks + a few lines of scripting, you can monitor your containers:

# Monitor all container exits
while read -r event; do
  container=$(echo "$event" | grep -oP 'name=K[^)]+')
  status=$(echo "$event" | grep -oP 'exitCode=K[0-9]+')
  if [ "$status" != "0" ]; then
    echo "⚠️ Container $container crashed (Exit code: $status)" |      mail -s "Docker Alert" [email protected]
  fi
done < <(docker events --filter 'event=die' --filter 'type=container')

For production, use proper tools like Watchtower (auto-updates), Diun (image notifications), or a full monitoring platform. But a simple script is a great start.

#✋ Try it out

# Terminal 1:
docker events --filter 'event=die'

# Terminal 2:
docker run alpine sh -c "exit 1"
# → In Terminal 1 you'll see the event!

# Try out a healthcheck
docker run -d --name healthtest --health-cmd='exit 0' --health-interval=2s alpine sleep 30
sleep 5
docker inspect --format='{{.State.Health.Status}}' healthtest
# → healthy
docker rm -f healthtest

#📌 Summary

docker events shows all daemon actions in real time
Healthchecks let Docker auto-detect broken containers
docker inspect provides all container details
Events + Healthchecks + scripting = simple monitoring

Monitoring Challenge

Test your knowledge with a quick quiz!

4 questions · +40 XP

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