映射的默认值 AWS CloudFormation

Question

What would be a good strategy to have a default value on mappings?

IE

I have a parameter called country

Based on that country I reference a DNS using mappings

"Mappings" : {
   "DNS":{
     "us" : {"dns" : "mypage.us.com", "ttl" : "600"},
     "mx" : {"dns" : "mypage.default.com", "ttl" : "300"},
     "ar" : {"dns" : "mypage.default.com", "ttl" : "300"},
     "br" : {"dns" : "mypage.default.com", "ttl" : "300"}
   }
}

If us it's been mapped:

{ "Fn::FindInMap" : [ "DNS", { "Ref" : "country" }, "dns" ]}

I get "mypage.us.com" for the other countries I've created a huge list of countries with a default value mypage.default.com , in the future, this values will be changing and we will be adding more countries, is there a better approach to this?

Answer 1

The only way I was able to do this was to chain Fn::If statements instead of using the map. I tried using a combination of Fn::If and Fn::FindInMap but Fn::FindInMap will always raise an error it if can't find the mapping.

Therefore the only solution for me was to resort to using something like the following (for me it was setting ecs memory based on instance type):

Conditions:
  IsT2Micro: !Equals [!Ref InstanceType, "t2.micro"]
  IsT2Small: !Equals [!Ref InstanceType, "t2.small"]
  ...

taskdefinition:
    Type: AWS::ECS::TaskDefinition
    Properties:
      ...
      Memory: !If [ IsT2Micro, 900, !If [ IsT2Small, 1900, !Ref "AWS::NoValue"]]

Answer 2

To elaborate on Steve Smith's answer.

Cloudformation always expects a valid mapping, even behind a missed logic gate.

You can combine !Sub and !If for a fair amount of flexibility though.

For example, we do this for dynamic staging ECS Env vars:

Parameters:

  Env:
    Type: String

  Branch:
    Type: String

  DevelopUrl:
    Type: String
    Default: "develop.example.com"

  MasterUrl:
    Type: String
    Default: "master.example.com"

  ...(ECS Resource)
          Environment:
            
            - !If
              - IsStaging
              - {
                  Name: SOME_CALLBACK_URL,
                  Value: !Sub [
                    "https://${Url}/some-callback-endpoint",
                    { Url: !If [ IsDevelop, !Ref DevelopUrl, !If [ IsMaster, !Ref MasterUrl, !GetAtt MyLoadBalancer.DNSName ] ] }
                  ]
                }
              - !Ref "AWS::NoValue"

Answer 3

Cloud Formation helps create AWS resources once in the beginning of their life. You can also do updates with it, but I think in your case it sounds like you'll be better off building your DNS config logic into your application. Maybe create a Database table in DynamoDB with the mapping data. You could pass the Country value into the servers as an environment variable, and have them query the DynamoDB table on launch based on their environment variable.

Alternatively, you can have Cloud Formation invoke a Lambda function when it launches a new stack to query DynamoDB to get the value of the DNS config based on the country so you don't have to keep modifying your stack JSON every time there's a new entry and don't have to change your application.

Answer 4

In your mapping, add a default entry:

"Mappings" : {
   "DNS":{
     "us" : {"dns" : "mypage.us.com", "ttl" : "600"},
     "mx" : {"dns" : "mypage.mx.com", "ttl" : "300"},
     "default" : {"dns" : "mypage.default.com", "ttl" : "300"}
   }
}

Then create a condition (YAML):

Conditions:
  HasSpecialDNS: !Or:
    - !Equals [!Ref country, "us"]
    - !Equals [!Ref country, "mx"]

Then change the 2nd parameter of FindInMap to:

{ "Fn::FindInMap" : [ "DNS", { "Fn::If": ["HasSpecialDNS", {"Ref" : "country"}, "default" ]}, "dns" ]}

Or YAML:

Fn::FindInMap:
  - DNS
  - !If ["HasSpecialDNS", !Ref country, "default" ]
  - "dns"