CloudFormation创建ECS服务时DesiredCount>1导致服务无法稳定的原因

Let’s walk through the most likely reasons why your ECS service can’t spin up that second task, along with actionable ways to dig into each issue:

• Resource Quota Limits
  AWS enforces account-level quotas for ECS resources, and hitting one could block your second task. For example:

  • If you’re using Fargate, you might be hitting your On-Demand vCPU or memory quota.
  • For EC2 launch type, you could have exhausted your EC2 instance count quota, or the instance type you’re using has limited availability in your region.
  • How to check: Head to the AWS Service Quotas console and search for "ECS" to verify your limits. You can also run aws service-quotas list-service-quotas --service-code ecs in the CLI. Check the ECS console’s Stopped Tasks tab—if you see errors like RESOURCE:MEMORY or RESOURCE:CPU, that’s a clear sign you’re hitting resource limits.

• Insufficient Resources on Container Instances (EC2 Launch Type)
  If your ECS cluster runs on EC2 instances, the existing instance might not have enough leftover CPU or memory to host the second task. Even if you have multiple instances, ECS might not be scheduling the task elsewhere due to placement constraints (like affinity rules) or instance availability.

  • How to check: In the ECS console, go to your cluster’s Container Instances tab and look at the "Remaining CPU" and "Remaining Memory" values. Compare these to the CPU/memory requirements defined in your task definition. You can also SSH into the instance and run docker stats to see real-time resource usage.

• Network Configuration Issues
  Network problems often fly under the radar but can prevent tasks from starting entirely:

  • Fargate Subnet IP Exhaustion: If your subnet has no available private IP addresses left, Fargate can’t assign one to the second task. Check your VPC console’s subnet details to see how many IPs are in use.
  • Security Group Restrictions: Make sure your task’s security group allows outbound HTTPS (443) traffic—this is required to pull container images from ECR or Docker Hub. If your task relies on other services (like a database), ensure inbound/outbound rules for those ports are open too.
  • VPC DNS Failures: If your VPC doesn’t have DNS support enabled, or the DNS server can’t resolve ECR/Docker Hub domains, your task will fail to pull the image. Verify your VPC’s DNS settings (enable "DNS hostnames" and "DNS resolution") in the VPC console.

• Task Definition or Image Permissions/Issues
  Even if the first task works, subtle issues can block the second:

  • Image Pull Failures: Check the Stopped Tasks tab for CannotPullContainerError—this could mean the image doesn’t exist, or your task execution role lacks permissions to pull it. Ensure the role has ecr:GetDownloadUrlForLayer, ecr:BatchGetImage, and ecr:BatchCheckLayerAvailability permissions for ECR images.
  • Volume or Secret Access Issues: If your task uses EFS volumes or Secrets Manager secrets, the second task might fail to access these due to misconfigured IAM permissions or resource unavailability. Check CloudWatch Logs for the task (if it starts briefly) to see specific error messages.

• CloudFormation Deployment Configuration
  Your ECS service’s deployment settings in CloudFormation might be causing the hang:

  • MinimumHealthyPercent Too High: If this is set to 100% (the default), ECS requires all existing tasks to stay healthy while scaling out. If the second task can’t start, the service will wait indefinitely, leading to CloudFormation’s CREATE_IN_PROGRESS state. Try lowering this to 50% to allow scaling even if some tasks aren’t ready yet.
  • HealthCheckGracePeriodSeconds Too Short: If your task takes time to initialize (e.g., loading dependencies), a short grace period might mark it as unhealthy before it’s ready, leading to termination. Increase this value to give your task enough time to start up.

• Check CloudFormation and ECS Event Logs
  Don’t forget to dig into CloudFormation’s event history—look for any error messages related to the ECS service or task creation. Also, check the ECS service’s Events tab in the console; it will log details about task scheduling attempts and failures.

内容的提问来源于stack exchange，提问作者dr34m3r