Rolling Updates

The Docker Compose Wrapper provides a powerful rolling update mechanism that ensures zero-downtime deployments.

How Rolling Updates Work

The rolling update process follows these steps:

1. Scale Up: Double the number of replicas
2. Wait for New Containers: Ensure new containers are running
3. Remove Old Containers: Stop and remove old containers
4. Scale Down: Return to the desired number of replicas

Configuration

Configure rolling updates in your values file:

services:
  web:
    replicas: 3
    rollingUpdate:
      replicas: 3        # Number of replicas to maintain
      retryCount: 10     # Number of attempts to wait for new containers
      retryInterval: 30  # Time between retry attempts in seconds

Usage

Basic Rolling Update

dcw rolling-update web

Rolling Update with Environment

dcw rolling-update -e prod web

Custom Configuration

dcw rolling-update --replicas 3 --retry-count 10 --retry-interval 30 web

Process Details

1. Scale Up

The service is scaled to double the desired replicas:

docker compose up -d --scale web=6

2. Wait for New Containers

The system waits for new containers to start:

// Wait for new containers
for i := 0; i < retryCount; i++ {
    time.Sleep(time.Duration(retryInterval) * time.Second)
    // Check if new containers are running
}

3. Remove Old Containers

Old containers are stopped and removed:

docker stop <old-container-id>
docker rm <old-container-id>

4. Scale Down

The service is scaled back to the desired number of replicas:

docker compose up -d --scale web=3

Best Practices

1. Replica Count
   • Set appropriate replica counts for your workload
   • Consider resource constraints
   • Plan for peak loads
2. Retry Configuration
   • Set reasonable retry counts and intervals
   • Consider container startup time
   • Account for network latency
3. Monitoring
   • Monitor container health during updates
   • Watch for resource usage spikes
   • Check application logs
4. Rollback Plan
   • Have a rollback strategy ready
   • Test rollback procedures
   • Keep previous versions available

Common Scenarios

High Availability Setup

services:
  web:
    replicas: 3
    rollingUpdate:
      replicas: 3
      retryCount: 15
      retryInterval: 20

Development Environment

services:
  web:
    replicas: 1
    rollingUpdate:
      replicas: 2
      retryCount: 5
      retryInterval: 10

Production Environment

services:
  web:
    replicas: 5
    rollingUpdate:
      replicas: 5
      retryCount: 20
      retryInterval: 30

Troubleshooting

Common Issues

1. Container Startup Failures
   • Check container logs
   • Verify resource constraints
   • Review environment variables
2. Update Timeouts
   • Increase retry count
   • Adjust retry interval
   • Check network connectivity
3. Resource Exhaustion
   • Monitor system resources
   • Adjust replica counts
   • Consider resource limits

Debug Commands

# Check container status
docker ps -a

# View container logs
docker logs <container-id>

# Check service status
dcw ps

# View service logs
dcw logs web

Advanced Topics

Custom Health Checks

Add health checks to your service configuration:

services:
  web:
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
      interval: 30s
      timeout: 10s
      retries: 3

Graceful Shutdown

Configure graceful shutdown periods:

services:
  web:
    stop_grace_period: 30s

Resource Limits

Set resource constraints:

services:
  web:
    deploy:
      resources:
        limits:
          cpus: '0.50'
          memory: 512M
        reservations:
          cpus: '0.25'
          memory: 256M