如何从ASG或UserData提取InstanceID用于CloudFormation资源引用？

ASG是动态伸缩资源，CloudFormation在部署阶段无法直接获取其下的实例ID（实例由ASG启动后创建），!GetAtt也无法从ASG返回实例ID，以下是几个可行的解决思路：

方案1：CloudWatch仪表盘用ASG维度动态关联实例 #

这是最简便的方案，无需硬编码实例ID。CloudWatch支持通过AutoScalingGroupName维度直接关联ASG下的所有实例，自动适配实例的伸缩变化。

示例CloudWatch Dashboard资源定义：

Resources:
  MyCloudWatchDashboard:
    Type: AWS::CloudWatch::Dashboard
    Properties:
      DashboardName: NFSDashboard
      DashboardBody: !Sub |
        {
          "widgets": [
            {
              "type": "metric",
              "x": 0,
              "y": 0,
              "width": 12,
              "height": 6,
              "properties": {
                "metrics": [
                  [ "AWS/EC2", "CPUUtilization", "AutoScalingGroupName", "${MyASG}" ]
                ],
                "period": 300,
                "stat": "Average",
                "title": "NFS Instance CPU Utilization"
              }
            }
          ]
        }
  MyASG:
    Type: AWS::AutoScaling::AutoScalingGroup
    # 你的ASG配置...

方案2：用Lambda自定义资源获取实例ID #

如果必须拿到具体实例ID（如特定监控需求），可通过Lambda自定义资源在ASG启动后获取实例ID，再传递给CloudWatch资源：

1. 创建Lambda执行角色，赋予ASG/EC2实例描述权限、CloudFormation自定义资源权限及日志权限；
2. 编写Lambda函数，通过ASG名称调用describe_auto_scaling_groups API获取实例ID，返回给CloudFormation；
3. 在CF模板中定义自定义资源并依赖ASG，最后在CloudWatch资源中引用自定义资源的输出。

示例自定义资源部分：

Resources:
  GetASGInstanceIDLambda:
    Type: AWS::Lambda::Function
    Properties:
      Runtime: python3.11
      Handler: index.lambda_handler
      Role: !GetAtt LambdaExecutionRole.Arn
      Code:
        ZipFile: |
          import boto3
          import cfnresponse

          def lambda_handler(event, context):
              asg_name = event['ResourceProperties']['ASGName']
              asg_client = boto3.client('autoscaling')
              response = asg_client.describe_auto_scaling_groups(AutoScalingGroupNames=[asg_name])
              instance_ids = [instance['InstanceId'] for instance in response['AutoScalingGroups'][0]['Instances']]
              # 单实例ASG取第一个ID，多实例场景需自行处理
              instance_id = instance_ids[0] if instance_ids else None
              cfnresponse.send(event, context, cfnresponse.SUCCESS, {'InstanceId': instance_id})

  LambdaExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
          - Effect: Allow
            Principal:
              Service: lambda.amazonaws.com
            Action: sts:AssumeRole
      Policies:
        - PolicyName: ASGInstanceAccess
          PolicyDocument:
            Version: '2012-10-17'
            Statement:
              - Effect: Allow
                Action:
                  - autoscaling:DescribeAutoScalingGroups
                  - ec2:DescribeInstances
                Resource: "*"
              - Effect: Allow
                Action: logs:CreateLogGroup
                Resource: !Sub "arn:aws:logs:${AWS::Region}:${AWS::AccountId}:*"
              - Effect: Allow
                Action:
                  - logs:CreateLogStream
                  - logs:PutLogEvents
                Resource: !Sub "arn:aws:logs:${AWS::Region}:${AWS::AccountId}:log-group:/aws/lambda/${GetASGInstanceIDLambda}:*"

  ASGInstanceIDResource:
    Type: Custom::ASGInstanceID
    Properties:
      ServiceToken: !GetAtt GetASGInstanceIDLambda.Arn
      ASGName: !Ref MyASG

  # 引用实例ID的CloudWatch告警示例
  MyCloudWatchAlarm:
    Type: AWS::CloudWatch::Alarm
    Properties:
      AlarmName: NFSInstanceHighCPU
      MetricName: CPUUtilization
      Namespace: AWS/EC2
      Dimensions:
        - Name: InstanceId
          Value: !GetAtt ASGInstanceIDResource.InstanceId
      Threshold: 80
      ComparisonOperator: GreaterThanThreshold
      # 其他配置...

方案3：改用单EC2实例（仅适用于无需伸缩的场景） #

如果NFS服务不需要自动伸缩，可直接创建EC2实例替代ASG，这样就能通过!GetAtt直接获取实例ID：

Resources:
  MyNFSInstance:
    Type: AWS::EC2::Instance
    # 你的实例配置...

  MyCloudWatchAlarm:
    Type: AWS::CloudWatch::Alarm
    Properties:
      # ...
      Dimensions:
        - Name: InstanceId
          Value: !GetAtt MyNFSInstance.InstanceId

内容的提问来源于stack exchange，提问作者user21281320