majiayu000 Container Debugging
Overview
Container debugging focuses on issues within Docker/Kubernetes environments including resource constraints, networking, and application runtime problems.
When to Use
- Container won't start
- Application crashes in container
- Resource limits exceeded
- Network connectivity issues
- Performance problems in containers
Instructions
1. Docker Debugging Basics
# Check container status
docker ps -a
docker inspect <container-id>
docker stats <container-id>
# View container logs
docker logs <container-id>
docker logs --follow <container-id> # Real-time
docker logs --tail 100 <container-id> # Last 100 lines
# Connect to running container
docker exec -it <container-id> /bin/bash
docker exec -it <container-id> sh
# Inspect container details
docker inspect <container-id> | grep -A 5 "State"
docker inspect <container-id> | grep -E "Memory|Cpu"
# Check container processes
docker top <container-id>
# View resource usage
docker stats <container-id>
# Shows: CPU%, Memory usage, Network I/O
# Copy files from container
docker cp <container-id>:/path/to/file /local/path
# View image layers
docker history <image-name>
docker inspect <image-name>
2. Common Container Issues
Issue: Container Won't Start
Diagnosis:
1. docker logs <container-id>
2. Check exit code: docker inspect (ExitCode)
3. Verify image exists: docker images
4. Check entrypoint: docker inspect --format='{{.Config.Entrypoint}}'
Common Exit Codes:
0: Normal exit
1: General application error
127: Command not found
128+N: Terminated by signal N
137: Out of memory (SIGKILL)
139: Segmentation fault
Solutions:
- Fix application error
- Ensure required files exist
- Check executable permissions
- Verify working directory
---
Issue: Out of Memory
Symptoms: Exit code 137 (SIGKILL)
Debug:
docker stats <container-id>
# Check Memory usage vs limit
Solution:
docker run -m 512m <image>
# Increase memory limit
docker inspect (MemoryLimit)
# Check current limit
---
Issue: Port Already in Use
Error: "bind: address already in use"
Debug:
docker ps # Check running containers
netstat -tlnp | grep 8080 # Check port usage
Solution:
docker run -p 8081:8080 <image>
# Use different host port
---
Issue: Network Issues
Symptom: Cannot reach other containers
Debug:
docker network ls
docker inspect <container-id> | grep IPAddress
docker exec <container-id> ping <other-container>
Solution:
docker network create app-network
docker run --network app-network <image>
3. Container Optimization
Resource Limits:
Set in docker-compose:
version: '3'
services:
app:
image: myapp
environment:
- NODE_ENV=production
resources:
limits:
cpus: '1.0'
memory: 512M
reservations:
cpus: '0.5'
memory: 256M
Limits: Maximum resources
Reservations: Guaranteed resources
---
Multi-Stage Builds:
FROM node:16 AS builder
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build
FROM node:16-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY package*.json ./
RUN npm install --production
EXPOSE 3000
CMD ["node", "dist/index.js"]
Result: 1GB → 200MB image size
4. Debugging Checklist
Container Issues:
[ ] Container starts without error
[ ] Ports mapped correctly
[ ] Logs show no errors
[ ] Environment variables set
[ ] Volumes mounted correctly
[ ] Network connectivity works
[ ] Resource limits appropriate
[ ] Permissions correct
[ ] Dependencies installed
[ ] Entrypoint working
Kubernetes Issues:
[ ] Pod running (not Pending/CrashLoop)
[ ] All containers started
[ ] Readiness probes passing
[ ] Liveness probes passing
[ ] Resource requests/limits set
[ ] Network policies allow traffic
[ ] Secrets/ConfigMaps available
[ ] Logs show no errors
Tools:
docker:
- logs
- stats
- inspect
- exec
docker-compose:
- logs
- ps
- config
kubectl (Kubernetes):
- logs
- describe pod
- get events
- port-forward
Key Points
- Check logs first:
docker logs <container> - Understand exit codes (137=OOM, 127=not found)
- Use resource limits appropriately
- Network containers on same network
- Multi-stage builds reduce image size
- Monitor resource usage with stats
- Port mappings: host:container
- Exec into running containers for debugging
- Update base images regularly
- Include health checks in containers