CloudFormation中基于Number参数的条件始终无法评估为true问题排查

问题描述 #

我尝试通过CloudFormation（CFN）的Number参数创建多台EC2实例，配置了如下参数与条件：

Parameters:
  NumberOfInstances:
    Type: Number
    MinValue: 1
    MaxValue: 10
    Default: 1
    Description: >
      How many Edge Nodes do you want to launch in this Stack? (Min = 1, Max = 10)

Conditions:
  MoreThan1:
    !Or [!Equals [NumberOfInstances, 2], !Equals [NumberOfInstances, 3], !Equals [NumberOfInstances, 4], !Equals [NumberOfInstances, 5], !Equals [NumberOfInstances, 6], !Equals [NumberOfInstances, 7], !Equals [NumberOfInstances, 8], !Equals [NumberOfInstances, 9], !Equals [NumberOfInstances, 10] ]
  MoreThan2:
    !Or [!Equals [NumberOfInstances, 3], !Equals [NumberOfInstances, 4], !Equals [NumberOfInstances, 5], !Equals [NumberOfInstances, 6], !Equals [NumberOfInstances, 7], !Equals [NumberOfInstances, 8], !Equals [NumberOfInstances, 9], !Equals [NumberOfInstances, 10] ]
  MoreThan3:
    !Or [!Equals [NumberOfInstances, 4], !Equals [NumberOfInstances, 5], !Equals [NumberOfInstances, 6], !Equals [NumberOfInstances, 7], !Equals [NumberOfInstances, 8], !Equals [NumberOfInstances, 9], !Equals [NumberOfInstances, 10] ]

资源段中第一台实例始终创建，当NumberOfInstances大于1时使用不同名称格式：

MyInstance01:
    Type: 'AWS::EC2::Instance'
    Properties:
      Tags:
        - Key: Name
          Value: !If [MoreThan1, !Sub '${AWS::StackName}-001',!Ref AWS::StackName ]

第二台实例配置：

MyInstance02:
    Condition: MoreThan1
    Type: 'AWS::EC2::Instance'
    Properties:
      Tags:
        - Key: Name
          Value: !Sub '${AWS::StackName}-002'

但无论NumberOfInstances设为大于1的任何值，MoreThan1条件始终不评估为true：第一台实例名称标签未正确设置，第二台实例也无法创建。后续简化条件后问题依旧：

Conditions:
  TwoOrMoreInstances: !Or [!Equals [NumberOfInstances, 2], !Condition ThreeOrMoreInstances]
  ThreeOrMoreInstances: !Or [!Equals [NumberOfInstances, 3], !Condition FourOrMoreInstances]
  FourOrMoreInstances: !Or [!Equals [NumberOfInstances, 4], !Condition FiveOrMoreInstances]
  FiveOrMoreInstances: !Or [!Equals [NumberOfInstances, 5], !Condition SixOrMoreInstances]
  SixOrMoreInstances: !Or [!Equals [NumberOfInstances, 6], !Condition SevenOrMoreInstances]
  SevenOrMoreInstances: !Or [!Equals [NumberOfInstances, 7], !Condition EightOrMoreInstances]
  EightOrMoreInstances: !Or [!Equals [NumberOfInstances, 8], !Condition NineOrMoreInstances]
  NineOrMoreInstances: !Or [!Equals [NumberOfInstances, 9], !Condition TenOrMoreInstances]
  TenOrMoreInstances: !Equals [NumberOfInstances, 10] # Not actually ten OR MORE, but keeping same format makes easier to extend in future

对应资源段调整后问题仍存在。

错误原因 #

CloudFormation中，Number类型的参数是数值类型，但你在!Equals里直接使用的数字（比如2、3）属于字符串字面量。CFN的类型比较是严格的，数值和字符串永远不相等，因此所有条件判断结果都是false。

解决方案 #

有两种可行的修正方式：

方式1：将数值参数转为字符串后比较 #

通过!Sub把NumberOfInstances参数转换为字符串，再和字符串形式的数字比较：

Conditions:
  MoreThan1:
    !Or 
      - !Equals [!Sub '${NumberOfInstances}', '2']
      - !Equals [!Sub '${NumberOfInstances}', '3']
      - !Equals [!Sub '${NumberOfInstances}', '4']
      - !Equals [!Sub '${NumberOfInstances}', '5']
      - !Equals [!Sub '${NumberOfInstances}', '6']
      - !Equals [!Sub '${NumberOfInstances}', '7']
      - !Equals [!Sub '${NumberOfInstances}', '8']
      - !Equals [!Sub '${NumberOfInstances}', '9']
      - !Equals [!Sub '${NumberOfInstances}', '10']

方式2：使用数值比较函数（更简洁推荐） #

利用CFN的!GreaterThan、!LessThan等数值比较函数，直接判断数值大小，无需逐个列举：

Parameters:
  NumberOfInstances:
    Type: Number
    MinValue: 1
    MaxValue: 10
    Default: 1
    Description: >
      要在栈中启动多少台边缘节点？(最小值=1，最大值=10)

Conditions:
  MoreThan1: !GreaterThan [!Ref NumberOfInstances, 1]
  MoreThan2: !GreaterThan [!Ref NumberOfInstances, 2]
  MoreThan3: !GreaterThan [!Ref NumberOfInstances, 3]
  # 以此类推，或者直接用TwoOrMoreInstances: !GreaterThan [!Ref NumberOfInstances, 1]

这种写法不仅更简洁，还避免了类型不匹配的问题，同时更易扩展（比如后续调整MaxValue时无需修改条件）。

内容的提问来源于stack exchange，提问作者Fat Monk