Fonctions Intrinsèques AWS (Intrinsic Fn)

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Fonctions Intrinsèques AWS (Intrinsic Fn)

some common ones, see AWS links for complete list

CheatSheet

Doc Officielle

!FindInMap

La fonction intrinsèque Fn::FindInMap renvoie la valeur correspondant aux clés dans un mappage à deux niveaux déclaré dans la section Mappings.

!Split & !Select

Fn::Split

{ "Fn::Split" : [ "delimiter", "source string" ] }

{
   "Fn::Split":[
      ":",
      {
         "Fn::GetAtt":[
            "OpenSearchDeliveryStreamLogGroup",
            "Arn"
         ]
      }
   ]
}

This function will split an input string by a defined delimiter. It is usually used together with Fn::Select and I have never used it standalone.

Fn::Select

{ "Fn::Select" : [ index, listOfObjects ] }

{
   "Fn::Select":[
      "6",
      {
         "Fn::Split":[
            ":",
            {
               "Fn::GetAtt":[
                  "OpenSearchDeliveryStreamLogGroup",
                  "Arn"
               ]
            }
         ]
      }
   ]
}

This function is returning a part of a string from a list of objects. It always starts with the index 0.

I often use it together with the Parameters attribute if I need to split an ARN and only return a certain part. In this example, it only returns the name of the log group of a provided CloudWatch LogGroup ARN.

!Ref

!Sub

Eq. to Substitute:

Fn::Sub

{ "Fn::Sub" : [ String, { Var1Name: Var1Value, Var2Name: Var2Value } ] }

{
   "Fn::Sub":"arn:aws:logs:${AWS::Region}:${AWS::AccountId}:*"
}

This function replaces parts of your input string with variables. The following predefined parameters are supported:

• AWS::AccountId
• AWS::NotificationARNs
• AWS::NoValue
• AWS::Partition
• AWS::Region
• AWS::StackId
• AWS::StackName
• AWS::URLSuffix

I usually use it to construct an ARN.

!Join

Fn::Join

{ "Fn::Join" : [ "delimiter", [ comma-delimited list of values ] ] }

{
   "Fn::Join":[
      "",
      [
         {
            "Fn::GetAtt":[
               "LogBackupS3Bucket",
               "Arn"
            ]
         },
         "/*"
      ]
   ]
}

This function appends several values to a single one. This is handy if you need to create resources for policies for example. We can use it together with other functions too.

ex: TemplateURL: !Join ["/", [!Ref S3BucketUrl, 'EU-WEST-1/05-APIGatewayNoPrivateLink.yaml']]

Fn::Join with lambda source code

{
   "Type":"AWS::Lambda::Function",
   "Properties":{
      "FunctionName":"CloudWatchTransformFunction",
      "Handler":"index.lambda_handler",
      "Code":{
         "ZipFile":{
            "Fn::Join":[
               "",
               [
                  "import boto3\n",
                  "import json\n",
                  "import re\n",
                  "s3Client = boto3.client('s3')\n",
                  "s3Resource = boto3.resource('s3')\n",
                  "def lambda_handler(event, context):\n",
                  "    for event in event['Records']:\n",
                  "        bucketName = event['s3']['bucket']['name']\n",
                  "        objectName = event['s3']['object']['key']\n",
                  "        fileName = event['s3']['object']['key'].split('/')[-1]\n",
               ]
            ]
         }
      },
      "Runtime":"python3.8",
      ...
}

One special use case for this function is to add source code to a lambda function. It is better readable than a single line of code.

!GetAtt

Fn::GetAtt

{ "Fn::GetAtt" : [ "logicalNameOfResource", "attributeName" ] }

{

  "Fn::GetAtt":[

   "OpenSearchDataStream",

   "Arn"

  ]

}

This function will get the ARN or other details from a created resource. Make sure the resource exists before you reference it. You can use the DependsOn attribute to ensure this.

ex (Yml): NetworkLoadBalancerTargetGroupArn: !GetAtt LoadBalancer.Outputs.NetworkLoadBalancerTargetGroupArn