Refer to existing Cognito User Pool with ARN instead of pool name

Use case description

When using an existing Cognito User Pool and adding trigger for lambda to this user pool, the existing way to do it is to :

• define the lambda trigger in serverless.yml, thus creating the corresponding User Pool with given name :

customMessage:
    handler: customMessage.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool
          trigger: CustomMessage

• use the auto-generated resource name - CognitoUserPoolMyUserPool - to incrementally configure the user pool in native cloudformation files :

resources:
  Resources:
    CognitoUserPoolMyUserPool:
      Type: AWS::Cognito::UserPool

When using an existing pool, the only way to refer to it is using its name

functions:
  users:
    handler: users.handler
    events:
      - cognitoUserPool:
          pool: legacy-user-pool
          trigger: CustomMessage
          existing: true

However, if you did not specify any name for your user pool (this param is optional), you need to manually retrieve generated name from cognito and paste it in your serverless.yml. (there is no way from cloudformation to retrieve a generated user pool name, only logicalId, Arn, ProviderName and ProviderUrl -> https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-cognito-userpool.html#aws-resource-cognito-userpool-return-values).

Proposed solution

Accept Arn of existing user pool as trigger for lambda :

functions:
  users:
    handler: users.handler
    events:
      - cognitoUserPool:
          poolArn: !Ref MyUserPool
          trigger: CustomMessage

Would be happy to dev this feature if accepted 😃

[tommedema]

any update on this?

[medikoo]

@medikoo, would you be OK to have both solutions implemented,

@fredericbarthelet definitely! Still let’s cover that with two distinct PR’s