How can I use `!GetAtt` in serverless.yml default value operator?

Question

I have serverless.yml which is to deploy a dynamodb and a lambda function. The lambda function needs to reference the dynamodb table stream arn.

functions:
  onStreamHandler:
    handler: ...
    name: ...
    events:
      - stream:
        type: dynamodb
        arn: ${opt:dbStreamArn, !GetAtt EntityTable.StreamArn}

In the arn: field, I'd like to specify if dbStreamArn is not specified from command line parameter, get the value from EntityTable.StreamArn. But I got this error:

  Serverless Error ---------------------------------------
 
  Missing "arn" property for stream event in function "onStreamHandler" The correct syntax is: stream: <StreamArn> OR an object with an "arn" property. Please check the docs for more info.

what is the right way to reference the arn value?

Answer 1

Could be because cloud formation is trying to create your lambda before your dynamodb table.

To fix this, add dependsOn to your lambda function e.g:

functions:
  onStreamHandler:
    handler: ...
    name: ...
    events:
      - stream:
        type: dynamodb
        arn: ${opt:dbStreamArn, !GetAtt EntityTable.StreamArn}
    dependsOn:
      - EntityTable

This will deploy the function once the the EntityTable has been deployed. !GetAtt EntityTable.StreamArn should then resolve correctly.