AWS ECS

Amazon Elastic Container Service (ECS) is a fully managed container orchestration service. Run containers on AWS Fargate (serverless) or EC2 instances.

Table of Contents

• Core Concepts
• Common Patterns
• CLI Reference
• Best Practices
• Troubleshooting
• References

Core Concepts

Cluster

Logical grouping of tasks or services. Can contain Fargate tasks, EC2 instances, or both.

Task Definition

Blueprint for your application. Defines containers, resources, networking, and IAM roles.

Task

Running instance of a task definition. Can run standalone or as part of a service.

Service

Maintains desired count of tasks. Handles deployments, load balancing, and auto scaling.

Launch Types

Common Patterns

Create a Fargate Cluster

AWS CLI:

# Create cluster
aws ecs create-cluster --cluster-name my-cluster

# With capacity providers
aws ecs create-cluster \
  --cluster-name my-cluster \
  --capacity-providers FARGATE FARGATE_SPOT \
  --default-capacity-provider-strategy \
    capacityProvider=FARGATE,weight=1 \
    capacityProvider=FARGATE_SPOT,weight=1

Register Task Definition

cat > task-definition.json << 'EOF'
{
  "family": "web-app",
  "networkMode": "awsvpc",
  "requiresCompatibilities": ["FARGATE"],
  "cpu": "256",
  "memory": "512",
  "executionRoleArn": "arn:aws:iam::123456789012:role/ecsTaskExecutionRole",
  "taskRoleArn": "arn:aws:iam::123456789012:role/ecsTaskRole",
  "containerDefinitions": [
    {
      "name": "web",
      "image": "123456789012.dkr.ecr.us-east-1.amazonaws.com/my-app:latest",
      "portMappings": [
        {
          "containerPort": 8080,
          "protocol": "tcp"
        }
      ],
      "environment": [
        {"name": "NODE_ENV", "value": "production"}
      ],
      "secrets": [
        {
          "name": "DB_PASSWORD",
          "valueFrom": "arn:aws:secretsmanager:us-east-1:123456789012:secret:db-password"
        }
      ],
      "logConfiguration": {
        "logDriver": "awslogs",
        "options": {
          "awslogs-group": "/ecs/web-app",
          "awslogs-region": "us-east-1",
          "awslogs-stream-prefix": "ecs",
          "mode": "non-blocking",
          "max-buffer-size": "25m"
        }
      },
      "healthCheck": {
        "command": ["CMD-SHELL", "curl -f http://localhost:8080/health || exit 1"],
        "interval": 30,
        "timeout": 5,
        "retries": 3,
        "startPeriod": 60
      }
    }
  ]
}
EOF

aws ecs register-task-definition --cli-input-json file://task-definition.json

Create Service with Load Balancer

aws ecs create-service \
  --cluster my-cluster \
  --service-name web-service \
  --task-definition web-app:1 \
  --desired-count 2 \
  --launch-type FARGATE \
  --network-configuration "awsvpcConfiguration={
    subnets=[subnet-12345678,subnet-87654321],
    securityGroups=[sg-12345678],
    assignPublicIp=DISABLED
  }" \
  --load-balancers "targetGroupArn=arn:aws:elasticloadbalancing:us-east-1:123456789012:targetgroup/web-tg/1234567890123456,containerName=web,containerPort=8080" \
  --health-check-grace-period-seconds 60 \
  --deployment-configuration "deploymentCircuitBreaker={enable=true,rollback=true}"

Run Standalone Task

aws ecs run-task \
  --cluster my-cluster \
  --task-definition my-batch-job:1 \
  --launch-type FARGATE \
  --network-configuration "awsvpcConfiguration={
    subnets=[subnet-12345678],
    securityGroups=[sg-12345678],
    assignPublicIp=ENABLED
  }"

Update Service (Deploy New Image)

# Register new task definition with updated image
aws ecs register-task-definition --cli-input-json file://task-definition.json

# Update service to use new version
aws ecs update-service \
  --cluster my-cluster \
  --service web-service \
  --task-definition web-app:2 \
  --force-new-deployment

Fargate Spot with SQS-Based Scaling

Use FARGATE_SPOT for batch/queue workloads to cut costs ~70%. Always include a fallback to regular FARGATE.

# Create service with Spot + fallback
aws ecs create-service \
  --cluster batch-cluster \
  --service-name queue-processor \
  --task-definition my-processor:1 \
  --desired-count 0 \
  --capacity-provider-strategy \
    capacityProvider=FARGATE_SPOT,weight=4,base=0 \
    capacityProvider=FARGATE,weight=1,base=1 \
  --network-configuration "awsvpcConfiguration={
    subnets=[subnet-12345678],
    securityGroups=[sg-12345678],
    assignPublicIp=DISABLED
  }"

# Register scalable target (scale to zero when queue empty)
aws application-autoscaling register-scalable-target \
  --service-namespace ecs \
  --resource-id service/batch-cluster/queue-processor \
  --scalable-dimension ecs:service:DesiredCount \
  --min-capacity 0 \
  --max-capacity 20

# Scale-out alarm: messages > 100
aws cloudwatch put-metric-alarm \
  --alarm-name queue-scale-out \
  --metric-name ApproximateNumberOfMessagesVisible \
  --namespace AWS/SQS \
  --dimensions Name=QueueName,Value=my-queue \
  --statistic Average \
  --period 60 \
  --evaluation-periods 1 \
  --threshold 100 \
  --comparison-operator GreaterThanThreshold \
  --alarm-actions <scale-out-policy-arn>

# Scale-in alarm: queue empty for 3 periods (conservative to avoid flapping)
aws cloudwatch put-metric-alarm \
  --alarm-name queue-scale-in \
  --metric-name ApproximateNumberOfMessagesVisible \
  --namespace AWS/SQS \
  --dimensions Name=QueueName,Value=my-queue \
  --statistic Average \
  --period 60 \
  --evaluation-periods 3 \
  --threshold 0 \
  --comparison-operator LessThanOrEqualToThreshold \
  --alarm-actions <scale-in-policy-arn>

Fargate Spot interruption handling: Spot tasks receive a SIGTERM 2 minutes before termination. Catch it in your application for graceful shutdown. For SQS consumers, call ChangeMessageVisibility on in-flight messages so they return to the queue rather than timing out.

Auto Scaling

# Register scalable target
aws application-autoscaling register-scalable-target \
  --service-namespace ecs \
  --resource-id service/my-cluster/web-service \
  --scalable-dimension ecs:service:DesiredCount \
  --min-capacity 2 \
  --max-capacity 10

# Target tracking policy
aws application-autoscaling put-scaling-policy \
  --service-namespace ecs \
  --resource-id service/my-cluster/web-service \
  --scalable-dimension ecs:service:DesiredCount \
  --policy-name cpu-target-tracking \
  --policy-type TargetTrackingScaling \
  --target-tracking-scaling-policy-configuration '{
    "TargetValue": 70.0,
    "PredefinedMetricSpecification": {
      "PredefinedMetricType": "ECSServiceAverageCPUUtilization"
    },
    "ScaleOutCooldown": 60,
    "ScaleInCooldown": 120
  }'

CLI Reference

Cluster Management

Task Definitions

Services

Tasks

Best Practices

Security

• Use task roles for AWS API access (not access keys)
• Use execution roles for ECR/Secrets access
• Store secrets in Secrets Manager or Parameter Store
• Use private subnets with NAT gateway
• Enable CloudTrail for API auditing

Performance

• Right-size CPU/memory — monitor and adjust
• Use Fargate Spot for fault-tolerant workloads (70% savings)
• Enable container insights for monitoring
• Use service discovery for internal communication

Reliability

• Deploy across multiple AZs
• Configure health checks properly
• Set appropriate deregistration delay
• Use circuit breaker for deployments

aws ecs update-service \
  --cluster my-cluster \
  --service web-service \
  --deployment-configuration '{
    "deploymentCircuitBreaker": {
      "enable": true,
      "rollback": true
    }
  }'

Cost Optimization

• Use Fargate Spot for batch workloads
• Right-size task resources
• Scale to zero when not needed
• Use capacity providers for mixed Fargate/Spot

Troubleshooting

Task Fails to Start

Check:

# View stopped tasks
aws ecs describe-tasks \
  --cluster my-cluster \
  --tasks $(aws ecs list-tasks --cluster my-cluster --desired-status STOPPED --query 'taskArns[0]' --output text)

Common causes:

• Image not found (ECR permissions)
• Secrets access denied
• Network configuration (subnets, security groups)
• Resource limits exceeded

Container Keeps Restarting

Debug:

# Check CloudWatch logs
aws logs get-log-events \
  --log-group-name /ecs/web-app \
  --log-stream-name "ecs/web/abc123"

# Check task details
aws ecs describe-tasks \
  --cluster my-cluster \
  --tasks task-arn \
  --query 'tasks[0].containers[0].{reason:reason,exitCode:exitCode}'

Causes:

• Health check failing
• Application crashing
• Out of memory

Live Debugging with ECS Exec

Connect directly to a running container without SSH. Requires enableExecuteCommand: true on the service and the SSM agent in your container image (included in most base images).

# Enable on existing service
aws ecs update-service \
  --cluster my-cluster \
  --service web-service \
  --enable-execute-command

# Get a shell in a running task
TASK_ARN=$(aws ecs list-tasks --cluster my-cluster --service-name web-service \
  --query 'taskArns[0]' --output text)

aws ecs execute-command \
  --cluster my-cluster \
  --task $TASK_ARN \
  --container web \
  --interactive \
  --command "/bin/sh"

Requirements: Task role must have ssmmessages:CreateControlChannel, ssmmessages:CreateDataChannel, ssmmessages:OpenControlChannel, ssmmessages:OpenDataChannel permissions.

Service Stuck Deploying

# Check deployment status
aws ecs describe-services \
  --cluster my-cluster \
  --services web-service \
  --query 'services[0].deployments'

# Check events
aws ecs describe-services \
  --cluster my-cluster \
  --services web-service \
  --query 'services[0].events[:5]'

Causes:

• Health check failing on new tasks
• Not enough capacity
• Target group health checks failing

Cannot Pull Image from ECR

Check execution role has:

{
  "Effect": "Allow",
  "Action": [
    "ecr:GetAuthorizationToken",
    "ecr:BatchCheckLayerAvailability",
    "ecr:GetDownloadUrlForLayer",
    "ecr:BatchGetImage"
  ],
  "Resource": "*"
}

Also check:

• VPC endpoint for ECR (if private subnet)
• NAT gateway (if private subnet)
• Security group allows HTTPS outbound

References

• ECS Developer Guide
• ECS API Reference
• ECS CLI Reference
• boto3 ECS