Serverless-application-model: The event source and function provided mapping already exists. Please update or delete the existing mapping with UUID

[MatteoGioioso]

Description:
I am trying to implement Dynamodb streams and a trigger for lambda

 myFunction:
    Type: AWS::Serverless::Function
    Properties:
      Policies:
        - AWSLambdaBasicExecutionRole
        - DynamoDBCrudPolicy:
            TableName: !Ref TableName
        - DynamoDBStreamReadPolicy:
            TableName: !Ref TableName
            StreamName:
              !Select
                - 2
                - !Split
                  -  "/"
                  - Fn::ImportValue:
                      !Sub ${TableName}-streams-arn
      CodeUri: bin/
      Handler: myFunctions
      Events:
        dynamodb:
          Type: DynamoDB
          Properties:
            Stream:
              Fn::ImportValue: !Sub ${TableName}-streams-arn
            StartingPosition: LATEST
            BatchSize: 10

The first deployment was successful, however after adding the DynamoDBStreamReadPolicy and changed StartingPosition from TRIM_HORIZON to LATEST I got the following error:

The event source arn (" arn:aws:dynamodb:.../stream/... ") and function (" myFunction-...") provided mapping already exists. Please update or delete the existing mapping with UUID xxxx-xxxx-xxx-xxx (Service: AWSLambda; Status Code: 409; Error Code: ResourceConflictException; Request ID: xxxx-xxxxxxx-xxxx-xxx)

Observed result:
Error

Expected result:
Probably error, but at this point I did not find any explanation

PS: as my understanding once the event mapping is created must be replaced or deleted? If yes why it says "Please update or delete"? If now I want to change the mapping configuration do I need to remove it, deploy, add it again and redeploy?

Thanks

[elembie]

+1

[shearn89]

For future Googlers: I had this issue, and it was because there was a manually-created trigger hanging around that wasn't deleted with my stack. Using the CLI to delete by UUID, or going to SQS -> the queue -> Lambda triggers should have let me find it.

[MatteoGioioso]

Ok, this is a long standing bug with Cloudformation.
Similar happening with Kinesis and it was already spotted in 2018 on the serverless framework.
This is a minimal reproducible case with kinesis: https://github.com/MatteoGioioso/sls-event-source-mapping-bug
You just need to change startingPosition from TRIM_HORIZON to LATESTto trigger the bug.

[cli402]

Hit the issue same day trying to update the kinesis source mapping settings.